Boab tree, also known as baobab, bottle tree, upside down tree, bottle tree, tree of life

Adansonia gregorii

Origin:

There are 9 species in the genus, with 2 endemic to Africa, 6 to Madagascar, and A gregorii to the Kimberly region of WA; all have similar appearance and common names as above. Continental Africa appears to be the original centre of distribution, with spread elsewhere by ocean currents rather than the tectonic breakup of the continent of Gondwana. More widespread distribution in recent times has been anthropogenic. The best known species is the African A. digitata which is tetraploid, while all others are diploid.

Climate:

Arid tropical conditions, mainly in low-lying areas but up to elevations of 1000m with hottest/coldest months of 35-39°C/13-18°C. It is not suited to regions with annual precipitation above 1200mm per year, and has low tolerance of frost. Trees in the north-west are often subject to periodical fires which they often survive given their high trunk water content, with subsequent establishment of new canopy from pre-existing meristems.

Plant Description:

Boabs are long-lived (hundreds of years) deciduous trees growing slowly once mature. They usually don’t exceed 10m in height, and occasionally there may be multiple trunks. Large swollen trunks that expand and contract depending on water supply are commonly bottle-shaped with soft wood and smooth grey bark. The irregularly rounded crown is compact with disorganised branches, and there is an extensive broad but shallow root system. In the Kimberly region they grow in open woodlands where individual trees are usually widely spaced; they don’t compete well in more densely forested areas. Foliage can be absent for several months of the year, hence the name ‘upside down tree’ with the bare branches looking like roots. Juvenile plant leaves are usually simple and with maturity they become increasingly palmate elliptic or lanceolate compound, 8-16 X 2-4cm, and 5-9 foliate. Sub-sessile leaflets with entire margins are glabrous above and pubescent below.

Relatives:

It is a member of the Bombacoideae sub-family of the recently expanded Malvaceae Family. Fruit tree relatives include cacao, durian, malabar chestnut, kola nut, cupuassu, South American sapote, and saba nut.

Soils:

Adansonia gregorii grows on a wide range of soils including light-textured soils derived from sandstone and basalt, but also on rocky outcrops on limestone hills and occasionally distributed along creek lines. However, neutral pH and high organic matter are preferred with good drainage essential.

Propagation:

Propagation is mainly by fresh seeds, but springtime cuttings and grafting can also be used. Cuttings should be prepared leaving about three leaves, dried for a few days to reduce the chances of fungal attack, and dipped into a rooting hormone. The cutting is then inserted in a soil mixture of sand and peat. Success rate without hormone treatment is compromised.

For seed germination, crack the woody shell of the fruit and remove as much of the pithy flesh surrounding the seeds as possible. In the Perth area, sow the seeds in late October to December, and use a native plant seed-raising mix in 100 mm-size pots. The best seed pre-treatment is to use acid, as scarification can sometimes allow too much water imbibition leading to frequent kernel rot. Sow seeds 1-2cm under the surface, place pots in full sun, and then keep the mix moist but not soggy at all times. Germination may take 3 or 4 weeks. Culture techniques for mass propagation have also been developed.

Cultivars:

There are no established cultivars, although bark colour is used to indicate different flavours in Africa.

Flowering and Pollination:

Axillary inflorescences borne on short, sturdy, erect peduncles/pedicels are single (rarely 2) with large and spectacular, perfect, hypogynous white flowers 10-15cm long with a faintly sweet smell. There are 5 calyx lobes that partially split and twist at anthesis, and a distinct long staminal tube with numerous free filaments. The densely pubescent ovary has many ovules and a long style that is enclosed in the staminal tube. Flowers usually appear in summer; they open early in the evening, are receptive to pollination through the night and early next morning, and then abscise over the next few days. Pollinators are mainly long-tongued hawkmoths and perhaps honeyeaters, and cross-pollination can occur over several kms. Other animals visiting the flowers are likely pollen and nectar thieves. Flowers seem to be functionally self-incompatible (post-zygotic abortion).

Cultivation:

Water seedlings daily and keep them moist at all times, but not wet. They can grow quite rapidly while juvenile. In early April, the seedlings' leaves will turn yellow and abscise and watering should then be withheld to prevent root rots. They will be dormant until December, and during this time the pots should be kept in a warm and sunny location protected from winter rains. When new buds appear, usually in December, begin watering and apply 3-to-6 month slow-release native fertiliser; appropriate watering has more of an impact than fertiliser for rapid growth during the juvenile stage. Repot in bigger pots as seedlings grow. A soil-wetting agent is recommended plus the addition of some sharp, coarse river sand into the native potting mix for best drainage. Continue this routine of winter protection, potting and fertilising for about 5 to 10 years before planting out in early summer in a hot/sunny location when new buds appear, with protection from any salt-laden wind. Soil should always be free-draining. Watering should only be done when plants are in leaf and growing. During winter months, use of black plastic covering the root zone to shed winter rain away from the base of the trunk and the roots is recommended. The protective sheet should be installed when leaf-fall occurs in May, and removed in early November.

Mature baobab trees in the ground do not need any fertiliser. Fertilise young and potted plants with liquid succulent fertilizer that is high in potassium and low in nitrogen every month.

Wind Tolerance:

Sensitive to strong winds because of their shallow root system.

Pruning:

Pruning should not be needed.

The Fruit:

The fruit is a large dry, ovoid-shaped, many-seeded berry, with a brittle 3-4mm thick pericarp (40% fruit weight) containing numerous kidney-shaped seeds having a tough testa embedded in a chalky or spongy white-creamy pulp. It is functionally dehiscent, tending to crack on the tree and usually breaking open when falling. Vitamin C, Ca and Mg are the main nutritional attributes of the fruit and seeds.

Fruit Production and Harvesting:

Seedlings may take up to 20 years to begin flowering but grafted plants can reduce this period substantially. The fruit shell is greenish when immature and turns brown on maturing, after which it can be picked if preferred (difficult) rather than waiting for fruit to fall and probably split open.

Fruit Uses:

The fruit pulp, seeds and leaves are the main edible components of boabs. Fruit has a dry consistency and can be eaten fresh or made into sauces, porridges and beverages. Seeds can be roasted and eaten like peanuts or ground up as a powder. Historically in developing countries, almost all parts of boabs have been used in some way, whether as sources of food, fibre, medicine or mysterious spiritual welfare, providing an important input into the well-being of small subsistence communities.

Pests and Diseases:

Some common pests that are attracted to baobab trees are mealybugs, spider mites and fungal gnats.

Comments:

Growing a boab is a long-term project with sufficient space needed to accommodate this iconic tree. To produce fruit, you’ll inevitably need more than a single tree. They grow well in pots while young.