Persea americana

Avocado, alligator pear


Native to Central America, with carbon dating evidence showing they were used as a food there more than 9,000 years ago. There are 3 races, West Indian, Guatamalan and Mexican. These inter-cross readily so there are many natural hybrids in addition to those produced through breeding.


It is a very adaptable sub-tropical tropical species that grows within 30o of the Equator; Perth is 32o S. Most varieties are sensitive to water stress and frosts, with very little tolerance of poor drainage. Generally, well-distributed rainfall of 1250-1750mm pa is optimal. Most growing areas have dry periods when irrigation is essential. West Indian cvs are the least cold sensitive and Mexican the most, with Guatamalan intermediate.

Plant Description

Trees can be tall, 15-18m, or spreading with height controlled by pruning. It appears evergreen, but some lose their leaves at flowering with rapid replacement. Leaves are spirally arranged, dark green and elliptic-ovate, 20-25cm long and 10-13cm wide. Growth occurs as flushes on a 30-60 day cycle with roots alternating with shoots. The species evolved in regions with copious water supply and consequently it does not have an efficient root system for water uptake. Avocados have few or no root hairs.


Lauraceae Family. Relatives are bay trees, cinnamon and sassafras


Provided there is good drainage, a wide range is tolerated such as sandy loams or volcanic and calcareous soils, with pH 5-7 acceptable. Avocados perform poorly where there is saline water or soils.


Grafting or budding is widely used, as cuttings and air layering have been problematic. Rootstocks for grafting have traditionally been seedling-grown, but efforts to reduce root rot problems have led to special cvs and inter-stocks being used in high risk areas. Known cv root stocks are increasingly used to control harvest time variation.


The Mexican and Guatamalan varieties are most suitable for south west WA. The harvesting period amongst these can cover several months of the year. Some that are readily sourced and given in the order of harvest time, include Shephard, Bacon, Fuerte, Sharwill, Wurtz (dwarf), Hass (the main cv worldwide) and Reed. Rootstock breeding for root rot resistance has so far been only partially successful.

Flowering and Pollination

The small yellow-green flowers are borne on multi-branched axillary panicles, terminating in a vegetative shoot bud. Each tree can produce 1-2 million flowers but fruit set is only a small proportion of this. Flowers are perfect with 9 stamens and a single pistil with one ovule. Nectaries attract bees and other insects for pollination, but self- or wind-pollination also occurs. Flowering exhibits protogynous, diurnally synchronous dichogamy, with flowers belonging to 2 types called A and B. The As function as females on one morning and then males the following afternoon, while the Bs operate as females in the afternoon and males the next morning. This behaviour encourages cross-pollination, but molecular studies have shown considerable selfing; this is enhanced by cool weather during flowering, meaning that in their appropriate environments many cvs are reasonably self-fertile. Nevertheless, the historical practice has been to have a mix of As and Bs. There is usually considerable fruit drop and alternate bearing is common.


Young plants should preferably be protected with shade screens for the first 1-2 years. Fertilizer use will depend on soil fertility. Provision of N is the main nutrient that has shown predictable effects on yield provided the others are sufficient. In alkaline soils Fe, Zn and other micronutrient deficiencies may occur and will need correcting, often with foliar spraying. Too much N predisposes to vegetative rather than reproductive growth.

Wind Tolerance

This is poor and needs to be addressed with windbreaks and pruning.


To control size and ease of harvesting and spraying, young plants should be trained to an open vase form to enhance light penetration throughout the canopy. Lateral branches are more fruitful than verticals. In late Autumn, pruning is best done at branch bud rings to encourage multiple shoots and flowering. The trunk and branches are very sensitive to sun burn, so if protective canopy cover is impaired, the tree should be shaded or the bark painted with a white water-based paint. After fruit set, vegetative growth competes with fruit for nutrients. You can help this by pruning out non-fruiting young branches.

The Fruit

It is a large single-seeded berry, 200g-1.5kg. The skin of some cvs is thin and in others several mm thick. Skin colour can be green through to red and various shades of dark purple. The yellow-green to bright yellow flesh has a buttery consistency. Similarly to the olive, it is unusual in storing most of its energy reserves as lipids (7-40% of fresh weight) rather than carbohydrates. Like the olive, the majority of these are unsaturated and very healthy. Avocados also have good levels of Ca and vitamin A. The flesh in most cvs will slowly brown (oxidise) once cut open.

Fruit Production and Harvesting

Seedlings may take 5-15 years to start cropping and fruit quality and yield is variable, while for grafted plants this period may be only 3-4 years. Yield will steadily increase over the next several years. Fruit should be clipped from the tree and not simply pulled off. Although hard, they still bruise easily. Avocados can be stored on the tree and will ripen indoors in 1-2 weeks if not picked pre-maturely. However if this storage practice is overdone it delays replenishment of tree reserves for the following season and will contribute to the alternate bearing habit. Once activated it is difficult to rectify, but the standard practice on thinning fruit in ‘on’ years can help. With green-skinned cvs, it is not easy to assess when they are mature and ready for picking, but often the skin loses its shine and the stem develops a yellowish colour. Mature but not soft-ripe fruit can be stored in a refrigerator for up to a month and ripe fruit much less. The ideal storage temperature is 5-7°C as below this may cause chill injury.

Fruit Uses

Most people eat them by spooning out the flesh and also as slices in a fruit salad or sandwiches (smashed avocados), but they’re also used to make guacamole spread. Oxidation can be slowed by drizzling with lemon or lime juice. Avocados should not be cooked or heated excessively, but they can be used for uncooked dressings, drinks and desserts such as ice cream and cheesecake.

Pests and Diseases

The major disease problem for avocado worldwide is root rot caused by Phytophthora; it can kill whole orchards of mature trees. No one method of control is sufficient, but purchased stock should be disease-free, care taken to avoid use of any infected grafting material, trees should be planted in well-drained soil, good levels of competitive soil microbes with high organic levels maintained, and periodic use of phosphite sprays or trunk injection may be necessary. Stem end rot can degrade fruit that has not been cut from the tree. Scale and thrips may sometimes be found, but generally pests are not a major problem in the south west of WA.


Mature trees grow well here with our porous soil, which minimises root rot problems that can be much more prevalent elsewhere. Avocados are luxury fruits that are expensive, but if you manage a tree appropriately you can enjoy many dozens of fruit per year when fully grown.

More Information

The A’ and B’s of avocado flowering

The avocado (Persea americana) is a member of the Lauraceae family which ranks amongst the oldest flowering plants in the fossil record. Archaeological sites in Mexico document avocado consumption as far back as 7-8000 BC, with domestic selection of favourable plants beginning in 4000-2800 BC. From these ancient times our efforts to maximise the quantity and quality of edible produce have been ever-increasing. However for much of recorded history, practices have been based simply on harvesting wild plants or selection/preservation of randomly produced superior trees, through to vegetative propagation to assist in continuation of good product.

Real advances over most of this lengthy period were very slow because of a lack of understanding of the underlying biology, and it was also compromised with many practices becoming enshrined in unfounded traditional or culturally-based management techniques. More reliable evidence concerning species behaviour and improvement had to wait largely for the emergence of controlled scientifically-based studies in the last hundred years or so. Horticulture (the study of various crop producing plants) and pomology (the study of fruit producing species) are both now well-established disciplines that have made and continue to make profound contributions, including with avocados, resulting in many-fold increases in yields, quality, improved disease resistance etc – all desperately needed with a world population that continues to increase. Nevertheless, while many properties of avocado fruit trees have now been well characterised, understanding its complicated flowering behaviour has remained enigmatic with much work still needed to be done.

Cross fertilisation in plants confers hybrid vigour and the genetic means to adapt to changes in the environment over evolutionary time. The way in which avocado achieves this, first documented in the 1920s, is by diurnally synchronous protogynous dichogamy of the flowers which are perfect, ie have both male and female organs. Flowers bloom over several weeks depending on climate, and they open and close twice, with dichogamy indicating male and female organs are functional at different times; protogynous indicates the female receptive parts become effective before male, and synchronous indicates that A timing is operative to fertilise B, and B to fertilise A.

The A and B designation refers to the two flower types; in A flowers (eg with the Hass cv) the female organ is receptive in the morning followed by the male the next afternoon, and in the B type (eg Fuerte), the female is receptive in the afternoon and the male the following morning. Under ideal conditions the overlap between male and female stages then predisposes to cross-pollination, fertilisation, and fruit set between the A and B types, with fruit then proceeding on to maturity. In the absence of convincing evidence that this inter-mixing of cultivar types was necessary for effective crop yields, it nevertheless seemed a logical strategy and was largely practised throughout most of the growing regions in the world from the 1920s onwards. Since then the Hass cv has come to dominate world trade and fetches by far the greatest price, so intermixing plantations with B type pollinators, even if at low ratios, carries a financial cost for growers and complicates management. How beneficial or even necessary is this intermixing, particularly given avocados are now grown in many sub-tropical and tropical regions worldwide that are quite different from avocado origins in Mexico and Central America? It’s a flexible species and in Australia can be grown from 17 to 35° S (40° in NZ), ranging from semi-arid winter rainfall conditions to the humid tropics with summer rain.

The avocado is notorious in having profuse flowering (often in the millions) with less than 0.1% of these resulting in harvestable fruit, even with the best cultivars and optimal environmental and cultural conditions. The important questions are – how much of this profound loss is due to inadequate pollination, pollinizer effectiveness, pollen viability, lack or insufficiently rapid pollen tube growth through the style, ovule fertilisation, seed abortion and dysfunctional regulation of plant hormones, insufficient carbohydrate reserves or sunlight, prevailing temperature and humidity history, moisture stress, day length, type of inflorescences, time and duration of anthesis etc? Another unwanted feature of avocado behaviour, especially for Hass, is the tendency for alternate bearing with on and off years, meaning that studies on the influence of varying factors on yield can not be meaningfully interpreted in a single cropping year.

As non-commercial growers, where can we obtain reliable information on the latest findings and recommendations? There are many sources and the most common and easiest are personal contacts, popular media programmes, magazines, internet sites and the like. The quality of these can vary enormously. Many are composed by people with little fundamental understanding of the species, and they very often derive content heavily plagiarised from other comparable sources, all perhaps originally stemming from an unreliable source in the first place. The sheer repetition of a similar story all too easily then leads to the perception that with statements from so many sources they must indeed be true. Others may be hyped up by Nurseries just wanting to sell product, or industry organisations sponsoring studies which are dressed up to give an air of respectability but have to be viewed critically because of possible conflict of interest bias. At a much more reliable and objective level than these we have various public agencies such as Agriculture Departments that may produce fact sheets for the public, in addition to what is usually their principal ‘raison d’etre’ of supporting commercial growers and their associated industries with commercially important species. Often these reports (eg conference papers, patents, yearbooks, annual reports, grower association newsletters etc) may not have been subjected to sufficient and clearly independent peer review, and the underlying science and/or citations for supporting work may not be stated – reasonably enough if intended only for the public. But whenever the underlying science/citations aren’t given, the statements and opinions expressed can not be verified. However, an obvious advantage is that content may be more specifically related to local growing regions rather than what applies in other countries. At the highest and most reliable level of evidence sources we have international peerreviewed studies conducted by experienced researchers working in prestigious universities and institutes with access to the latest technology, fully documenting their methodology and references sources, and published in the best scientific journals.

So jumping over the most common low level information sources, what has the WA Ag Dept advised regarding synchronous dichogamy in avocado orchards and the need for cross-pollination? The following commentary illustrates the vexed nature of the need for A and B intercropping.

In 2001 (Bulletin 4484, Avocado Culture in Western Australia) it was stated:

The mean daily temperature during flowering affects how strictly this pattern is displayed – the higher the temperature the more pronounced it is. Type B varieties are further influenced by temperature, the female organs displaying a very low receptivity to pollen at average temperatures below 18°C during flowering. As a result, type B varieties are less suited to colder climatic regions than type A varieties. Even though avocados are self-fertile, their unusual flowering habit means that you can increase their fruit causing crosspollination. However under the climatic conditions of the main commercial regions in WA, satisfactory crops can be set on most varieties without the need for cross pollination – provided sufficient pollination vectors are present and active. It is current commercial practice not to plant pollinators when planting a block of the main commercial variety Hass.

An update in January 2017 advised that content is under review and may be out of date. It stated many of the  factors mentioned above can affect yield, including cross-pollination. They again stated it’s common practice in the south west of WA to have whole plantations consisting only of monocultures, but advise that much more research needs to be done on the importance of cross pollination influencing final yield, with a prime requirement being that there is sufficient overlap in complementary flowering times.

Now let’s consider some of the peer-reviewed international literature in chronological order over the last 30 years to give some idea of developments in understanding. Mexico is by far the largest producer in the world, but most of the research has been conducted in the US (major growing regions of Florida and California) with significant contributions from Spain, Australia, South Africa and Israel.

In a 1986 study (Avocado Flowering, Horticultural Reviews, volume 8) it was concluded that:

The events encompassing the induction, flowering, pollination, fertilisation, and post-fertilisation processes are extremely complex. Much of the published information is anecdotal with conclusions sometimes based on results obtained under poorly controlled conditions. The domestic honeybee could not have been involved, since it was not introduced to the new world until the arrival of European colonists. Furthermore, the habits of honeybees are simply not in concert with those of the avocado. Clearly however the honeybee is an effective pollinator of avocado in many areas of the world, but other insects may be as important in pollen transfer in some areas. It is plausible that if cross-pollination does not occur, the avocado has evolved a fail-safe system of self-pollination dependent on environmental conditions.

A 1990 study in Israel (HortScience, 25, 471-473) using enzyme analysis of pollinizer flower type on selfing and outcrossing of fruit in Fuerte (B type) provided evidence re the above conclusion of ‘plausibility’ of self-pollination, as Fuerte was capable of producing fruit itself. Different pollinizer cvs had a larger effect on both fruit and seed weight than whether pollininzers were type A or B. A 1994 study in Florida (J Amer Soc Hort Sci, 119, 1200-1207) found that most pollinations in type A and B flowers occurred not during the first opening (ie when female organs became receptive) but during the second male period, as stigmas remained viable and receptive during this second stage. Under the favourable temperatures in Florida of 27-31°C max and 20°C min with humidity averaging 80%, stigmas had not desiccated by the time of the second opening. In a second aspect of the study they found that excluding larger insects such as bees had no effect on pollination rate during the male phase. After testing a number of cvs with similar results, they concluded that the primary means of pollination of commercial cvs in Florida was during the second, male stage of opening.

A 2000 study (Scientia Horticulturae, 86, 135-149) in California over 4 years of cropping using DNA markers investigated the rate of out-crossing in the yield of Hass with Bacon, Fuerte and Zutano as B type pollinizers; in recent years it’s been common practice there to plant Hass in monocultures. Two climatic regions with average monthly precipitation and temperatures during bloom of 20mm, max 20°C and min 7°C min, and 70mm and 25°C max and 11°C min respectively were considered. Fuerte was the most effective pollinizer in both climates, and outcrossing rate generally was dependent on climatic region and separation of maternal and pollinizer trees. There was only a weak correlation between yield and outcrossing, and most of this variation appeared due to other causes such as yearly climatic variations.

They suggested management techniques such as careful control of water and fertilizer regimes may be as effective as mixed plantings in increasing yields, and these are much less expensive to implement. Self-fertilisation was responsible for a substantial fraction of fruit set, and low yield in California (as elsewhere round the world, with average avocado yields estimated to be only about one third of photosynthetic capacity) was not due to lack of B type pollen sources.

In South East Spain, weather conditions during bloom often allow brief overlaps between female and male phases in a reduced proportion of flowers, and this may explain why growers continue using solid orchards of ‘Hass’ with no pollinizers. A 2007 study there (Proc VI World Avocado Congress) considered possible pollination vectors. They compared pollen adhesion and resulting fruit set in control flowers freely exposed, to those produced in flowers bagged with either micro perforated plastic (allowing wind and small insects to enter) or with tissue paper (excluding all kinds of insects). Their results showed that only the activity of honey and bumble bees rather than wind allowed fruit set. The activity of thrips caused certain levels of pollen deposition on the stigmas and an initial swelling of avocado flower ovary, however fruitlets soon abscised. The flowers enclosed in paper bags had no pollen grains on the stigmas nor set any fruit. An introduction of commercial hives was therefore recommended to achieve effective avocado pollination in their region.

Another study from California (J. Amer. Soc. Hort. Sci. (2008) 133, 648–652) considered the impact of outcrossing on yields of Hass, which is characterized by excessive flower and fruit abscission resulting in extremely low fruit set; could low outcrossing rates have contributed to low yields? The authors hypothesized that self-fertilized flowers and resulting fruit might abscise at a much higher rate than fruit that were the product of outcrossing, even though significant relationships between outcrossing rates and yields have only been established in a few avocado studies.

The objective of their research was to investigate the importance of outcrossing on yield in a commercial Hass orchard containing Bacon, an effective pollinizer of Hass. Microsatellite markers were used (ie paternity analyses) to determine the rate of outcrossing of fruit persisting through to harvest. Experiments were conducted during sequential on- and off-crop years. They found during both years that outcrossing rates were not related to yield or alternate bearing. They concluded that outcrossing was not the primary factor affecting flower and fruit persistence, and ultimately yield. Similar conclusions were drawn by another research group in Florida (Scientia Horticulturae (2009) 120, 360–366) where complementary flower types were intermixed and they found that a significant portion of the progeny were the result of crosses between like, and not complementary flowering types.

Another southern Spanish study on Hass in 2011 (Ann Appl Biol 158, 354–361) using microsatellite DNA markers considered the effect of separation distance on outcrossing and yield for two successive years between Hass and Fuerte as pollinizer. Outcrossing rate ranged from 74 to 31% and showed a significant decrease as distance increased, but 13 years of data showed no significant differences in Hass yield as distance to the pollen donor trees increased. They suggested that, under their growing conditions, fruit drop in avocado was determined by fertilisation date (later showing increased abscission) and not by embryo genetic composition Finally, a more recent Californian study (Scientia Horticulturae (2016) 199 32–40) considered a number of factors over a two year period that could affect the pollination and fertilization rates of abscising flowers, the viability of developing ovules, and plant hormone concentrations of abscising versus persisting fruit through the first and second fruit drop in a Hass monoculture.

Pollination, pollen germination, pollen tube growth and ovule viability of abscising flowers were determined, and in the first drop, cellular deterioration of developing ovules of abscising and persisting fruit was visualized. Abscisic acid (ABA), indole-3-acetic acid (IAA, an auxin), and isopentenyl-adenine (IPA, a cytokinin) concentrations were determined during the on-crop year only. On average ≥ 70% of abscising Hass flowers were pollinated, but due to the failure of the pollen grains to germinate and produce pollen tubes, fertilization never occurred and the ovule deteriorated. Fruit abscission during 1st and 2nd fruit drop was due primarily to a lack of fertilization. Fruit abscission was associated initially with deterioration of the nucellus in the developing ovule in early drop, and subsequently with deterioration of the seed coat in the later drop; both occurred more frequently in abscising than persisting fruit. Abscising fruit had greater ABA concentrations during 1st and 2nd drop compared to persisting fruit, and IAA and IPA were greater in abscising fruit during 2nd drop, but not early drop or late fruit drop. Only 25% of the total number of fruit that abscised per crop year were fertilized. The authors suggested these results provided strong evidence that the majority of flowers and fruit of selfed ‘Hass’ avocados abscise due to a lack of pollen germination and subsequent fertilization (not pollination). In addition, they suggested that ABA accumulation is related to seed abortion and reduced fruit growth in abscising fruit.

Where does this snapshot of some of the more recent scientific literature leave us with managing our own avocado plants to get good yields? The following points are a summary of most of the stages and factors involved (mentioned in the introductory comments) without giving excerpts of all the additional supporting literature. Although many are somewhat peripheral to our central question of the importance of cross-pollination, they nevertheless all feed in to our ultimate goal of fruit quality and yield, and without recognising their effects can be significant villains contributing to low yield rather than assigning it all to lack of cross pollination.

  • In most of the studies conducted worldwide with Mediterranean climates similar to ours, it’s been found that cross  pollination is not mandatory for good yield. The more tropical and humid climates of Florida, sth east Asia, central  America etc present quite different environments and favoured race (West Indian rather than Guatemalan,  Mexican or their hybrids) so that some of their management strategies are different to ours. Results reported in  southern California and other locales with cooler, drier climates are more relevant to us. Many of the familiar cvs  in WA originate from California.
  • Temperature and humidity have a major effect on the text-book dichogamous flowering habit of the species, with  lower temperatures leading to considerable overlap between male and female open stages (later and more prolonged  opening of female), meaning plants can produce adequate crops with self-pollination. Higher humidity also  predisposes to selfing with stigmas remaining receptive and viable through to male opening on the same plant.
  • Whether outcrossed or selfed, hand pollination illustrates that this is not usually the key stumbling block to good  fruit yield.
  • Cross pollination declines markedly with separation distance between pollinizer and receptive flowers, and is also  influenced by the ratio of flower type in neighbouring trees – the less of one type then the less it contributes to pollination.
  • Later and shorter flowering periods are associated with lower yields.
  • Some B type cvs are better at pollinating Hass than others eg Fuerte is better than Bacon.
  • There are fewer determinate inflorescences (ie where there is no functional terminal vegetative buds and growth)  than indeterminate (vegetative growth at the terminus of the inflorescence). Indeterminate shoots can become  functionally determinate by abortion of the vegetative buds. Tip pruning indeterminate terminal stems to reduce  competition between reproductive and vegetative organs can be counter-productive because of branch autonomy  and the need to have sufficient vegetative growth to build up reserves for the following year.
  • A commonly held view is that a tree has to have sufficient photosynthate reserves at flowering, fertilisation and early  growth of fruitlets to avoid abscission, but isotope studies have shown that it is only one factor along with larger  effects from plant hormones; limitations of photosynthate to reproductive organs are more due to resource allocation  than resource availability.
  • Achieving massive flowering is not a goal in itself for increased fruit yield as it has been observed that some cvs set small  numbers of fruit but have good retention, while others (Hass and Fuerte) have larger fruit set with greater  abscission.
  • Hass has a strong tendency to develop alternate bearing, usually triggered initially by adverse climatic events, and  once started it’s very difficult to regain regular cropping.
  • Avocados have a poor root system reflecting the environmental conditions with copious water supply where they  evolved. Moisture stress is a major factor that has to be managed regarding abscission, with timely and sufficient  irrigation in our unfavourable edapho-climatic conditions, particularly through summer. Avocado is extremely  sensitive to saline water (particularly chloride) so check your supply. Mulching is strongly beneficial to conserve soil  moisture levels and also because they’re ‘litter feeders’. Yield will decrease markedly if moisture supply does not keep  up with transpiration demands.
  • Honey bees are not the natural pollinators of avocados and are easily distracted by other flowers more attractive to
Hass avocado yield: expectations?

RFC members are very keen on growing avocados as they’re a delicious, healthy fruit so different from most with their delightful buttery taste, plus they can do so well in our climate and they’re expensive to buy through retail outlets. However some express disappointment when they have massive flowering only to set a few fruit, and then start wondering whether they’re not doing something right, or there’s a pollination problem or whatever. The first consideration is to be sure a plant is sufficiently through its juvenility stage to be able to interpret crop load, as often fruit species can go through a number of seasons with only flowering or low fruit set before finally progressing through to mature stage fruit production. Next, the species has a notorious tendency to become alternate bearing (especially Hass) and so you need to know whether you’re in an ‘on’ or ‘off’ year.

This habit of non-regular bearing is primarily triggered by adverse weather conditions, and unless managed with appropriate thinning of flowers and fruit in the on year, can become extreme. In the past the phenomenon was more commonly known as biennial bearing, but nowadays alternate bearing is the preferred term as depending on conditions the off year may extend beyond a single year. Once this pattern begins, thinning can usually only minimise the problem without eliminating it. When considering the yield of your tree/s, you have to look at multi-year productivity to avoid coming up with misleading diagnostic conclusions.

It’s also important to realise what levels of fruit load avocados will carry, as they’re a relatively inefficient bearer making use of only about a third of photosynthetic capacity at best. Recently, the first large scale analysis of yield in the Californian avocado industry was conducted by a UC Riverside group (Lovatt et al, VIII Congreso Mundial de la Palta [2015], 336). They collected data from about 3000 mono-cultured Hass avocado trees in good health, without visible signs of nutrient deficiencies or pest problems, over a 20 year period from 15 commercial orchards in coastal and inland valley areas with a warm dry Mediterranean climate similar to ours.

The median yield under the best agronomic conditions growers could manage was about 28kg/tree. This meant about 50% of the trees produced less than this; about 20% of them produced <10kg and <2.5% yielded >160kg. With such a highly skewed yield distribution, it’s really meaningless to think of average or mean yield as it’s so influenced by extremely high values, similar to ‘average’ income in a society that has some billionaires skewing the average upwards. Up to 190kg/tree, the yield of both large and small fruit (178-325 and 99-177g resp) continued to increase with yield, but the proportion of large fruit remained a consistently greater proportion (73%) of the total; there was no negative effect on commercially valuable large fruit with increasing yield to this level. Extreme climatic events (excessively high or low temperatures) were the main factors controlling yield and fruit size in any given year. Irrigation and/or precipitation during floral development and the second period of fruit maturation, and the proportion of sand in soils were positively related to yield.

These results were obtained under the usual generous commercial orchard spacing conditions allowing machinery access, so competition between trees for nutrients, moisture etc was probably less than we often have in our own collections. The results are of relevance to us as Hass is the dominant cultivar in the US and Australia, unlike more tropical areas where West Indian varieties can perform better. The research team quantified the variation in yield between on and off years by defining an alternate bearing index (ABI, with a range of 0 = no alternate bearing, to 1 = complete alternate bearing) that showed 47% of trees had an ABI of 0.75 – 1, indicating 75% – 100% differences in yield from one year to the next. No tree amongst the 3000 studied produced >35kg/tree in off years, including the few super performers that produced 300kg in their on years, and only 17% of trees had an ABI less than or equal to 0.25. An example of what these results mean is that a good tree with an ABI = 0.82 that produced 100kg in the on year would have produced only 10kg/tree in the off year.

An important finding in the study was that the very best yielders were not a series of different trees each year but consistently the same ones, and this outperformance was not readily explained by variations in micro-climatic or edaphic zones throughout or across orchards. How can this be when all trees were nominally Hass? One known mechanism is that rootstocks do affect scion behaviour. The best known effect is a vegetative one on tree size (dwarfing), but it also extends in subtle ways to fruit characteristics eg through plant hormone levels/variations, depending on how the many hundreds of genes from the rootstock and scion genomes interact across the graft union when they’re activated and repressed throughout flowering, fruit set and maturation. It’s because of such variable outcomes, including spread of harvest times, that most commercial orchardists nowadays have moved on from use of seedling rootstocks to a preference for clonal material. Another possible mechanism for consistent outperformance is where there may be different heritable epigenetic effects across pollinated trees (and scions used in grafting) depending on their individual past history.

Given the present state of knowledge, the most promising way to increase your long term yield (assuming good management and no disease) is to minimise alternate bearing, as those off years can really cripple multi-year performance. For the 100/10kg/yr example given above, just improving the off year from 10kg to even half its on capacity at 50kg would represent a big increase, and after all, most of us prefer a more regular yearly supply than a feast and famine seesaw. One management aspect revealed in the study that should be given particular attention is to always ensure watering is adequate and timely; having enough sand in our soils is not a problem for most of us who live in coastal areas of WA – quite the reverse in that we usually have far too much. We’d all like to be able to pre-select superb bearers and be riding a winner over the years, but this will have to wait further research on possible factors such as tree age, rootstock cultivars, aspects of cultural management, irrigation water quality, climate, soil characteristics, tree nutrient status etc. In the meantime, it would also help if Nurseries more often gave information on rootstocks used in grafting so we know what we’re dealing with.

Avocados – way to go

Fruits mainly store energy as carbohydrates but avocados, like olives, predominantly use lipids. Lipids are the norm for seeds and nuts, but again there are exceptions such as carbohydrate storage in chestnuts. The edible portion of avos consists of 73% water, 15% lipids, 8.5% carbohydrates and 6.7% fibre, and while having a slightly higher calorie content (160kcal/100g) than typical fruits, they are a nutrient-rich food containing significant amounts of minerals and other important phytochemicals. The 15% total lipids (fats) is made up of approximately 2.1% saturated (SFA), 9.8% monounsaturated (MUFA) and 1.8% polyunsaturated (PUFA) fats, so fat content is largely of the healthy non-saturated types. Furthermore, the ratio of omega-6 to omega-3 polyunsaturates is favourably less than one. On a population level they’re not as frequently eaten as more common fruits such as apples or oranges for a variety of reasons, some of which are – increased cost, availability, buttery texture, lack of sweetness and concern that higher calories would lead to unwanted weight gain. The following US study (Nutrition Journal 2013, 12:1) demonstrates that this last concern is unwarranted, and in fact avo consumers had lower body mass index and better levels of various markers of well-being compared to non-consumers.

Avocados contain monounsaturated fatty acids, dietary fibre, essential nutrients and phytochemicals. However, no epidemiologic data exist on their effects on diet quality, weight management and other metabolic disease risk factors. The objective of this research was to investigate the relationships between avocado consumption and overall diet quality, energy and nutrient intakes, physiological indicators of health, and risk of metabolic syndrome. Avocado consumption and nutrition data were based on 24-hour dietary recalls collected by trained National Health and Nutrition Examination Survey (NHANES) interviewers. Physiological data were collected from physical examinations conducted in NHANES Mobile Examination Centres. Diet quality was calculated using the USDA’s Healthy Eating Index-2005. Subjects included 17,567 US adults 19 years of age or more (49% female), including 347 avocado consumers (50% female), examined in NHANES 2001–2008. Least square means and standard errors were determined using appropriate sample weights, with adjustments for age, gender, ethnicity, and other covariates depending on the dependent variable of interest. Avocado consumers had significantly higher intakes of vegetables, fruit, diet quality, total fat, monounsaturated and polyunsaturated fats, dietary fibre, vitamins E, K, magnesium, potassium and vitamin K, and lower intakes of added sugars. No significant differences were seen in calorie or sodium intakes. Body weight, body mass index and waist circumference were significantly lower and HDLcholesterol (the good one) was higher in avocado consumers. The odds ratio for metabolic syndrome was 50% lower in avocado consumers vs. non-consumers. We conclude that avocado consumption is associated with improved overall diet quality, nutrient intake, and reduced risk of metabolic syndrome. Dietitians should be aware of the beneficial associations between avocado intake, diet and health when making dietary recommendations.

Metabolic syndrome is a significant risk factor for cardiovascular disease and diabetes. It increases markedly with age and is present in more than 42% of adults over the age of 60 in the US. Consequently a 50% reduction in this incidence through consumption of avos is very important for individuals and for societies in terms of lost productivity, quality of life and overwhelming medical costs. Like any epidemiologic study there can be many confounding variables that might have contributed to the reported results. But the 50% reduction was found after allowing for contributions due to age differences, gender, ethnicity, poverty-income ratio, physical activity, smoking and alcohol intake. Only 347 out of 17,567 people (ie 2%) stated they regularly consumed avos (70g or about half a fruit/d). This proportion seems absurdly low but it is in a society where no more than 3% of US men and 6% of women meet recommended daily fruit and vegetable intake. This is such a long way from being ideal, is it any wonder that metabolic syndrome is at epidemic proportions there?

Other studies addressing the unfounded concern about possible weight gain or negative effects with avos have been reported. A US 2013 clinical trial found that addition of half an avo to a standard lunch was associated with improved meal satisfaction and decreased desire to eat over the next 5 hrs, and substitution for an equivalent number of calories in the lunch led to a significant reduction in blood insulin level over the next 3 hrs. An Australian crossover trial in men studied the effects of two iso-caloric diets – one high SFA (milk, butter, cream, cheese and fatty meat)/low MUFA and the other high MUFA (avos, nuts and olive oil)/low SFA for 4 wks on each diet.

Even though calories were the same in each diet, fats provided by MUFAs led to lower bodyweight and fat mass. And another Sth African study found that consumption of 200g/d avos within an energy-restricted diet in overweight and obese adults did not compromise weight loss when substituted for an equivalent number of calories in the daily diet. One underlying mechanism that helps understand these bodyweight outcomes is that MUFAs, the principal component of avo lipids, are more likely to be oxidised in the body (increased thermogenesis) whereas SFAs are more prone to fat storage. At a broader level than all the above, an increasing number of clinical and pre-clinical studies in the last 20yrs have now reported positive effects on cardiovascular health, DNA damage protection, osteoarthritis, eye and skin health and various cancers.

Some other features of avos worth considering are:

  • They have the highest fat-soluble antioxidant capacity of the 24 and 22 most commonly eaten fruits and veges (resp)  in the US.
  • The two ways that most people eat them fresh are by cutting them in half, followed by seed removal and then either peeling off the skin or spooning out the flesh. This second approach can often mean that some of the greener-coloured flesh just inside the skin is left. However like many fruits, phytochemical levels are higher near the skin and decrease towards the seed.
  • Another aspect parallels changes in ripening with more conventional fruits. In these, energy is stored as carbohydrate and ripening leads to substantial conversion of acids to sugars. In avos with predominant lipid storage, SFA content decreases and healthy MUFA increases as they mature and ripen.
  • All up, avos are worth inclusion in your regular diet. They’re an expensive fruit, but if you grow your own you’ll have a plentiful supply on tap as a well-managed tree in our climate can produce more than sufficient for a family. And you can store them on the tree for a little while till needed.