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 (see Figure). 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. 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.

Barry Madsen