Staff from our horticulture department were asked to write this article for Home Grown Magazine.
Avocados come from from tropical America, but some varieties are surprisingly easy to grow in temperate regions; so long as they are protected from frost
The fruits are unusual in that they have oily flesh and unlike most fruits are not sweet - instead they have a very unique flavour.
Although the avocado has the common names of avocado pear and alligator pear, the fruit tastes nothing like pear. The flesh is soft and oily rather than crisp and crunchy. Avocado fruits grow on trees which can reach about 10m tall. But there are also many smaller-growing cultivars which are ideal for growing in small garden spaces and which make cropping fruits easier. Avocado (Persea americana) trees generally take 7 years or more to produce their first fruits, but many grafted cultivars will bear fruits after 3-4 years. Once they have begun to bear fruit productivity usually increases as the trees mature.
Tip: If you don't have the patience to wait several years for your first fruits, buy three to four year old grafted cultivars which should be ready to bear fruit.
Avocado trees flower in winter or spring and bear fruit some 5 14 months later. This is during the wet season in subtropical to tropical regions, and summer months in temperate regions. Avocados can only partly self-pollinate. The flowers only open twice. They open on the first day as female, but open the next day as male. They are classified as type A or type B cultivars depending on the timing of the flowering - opening in the morning or afternoon. Although a single tree will bear fruit, you can increase your harvest by growing a type A cultivar with a type B cultivar so they cross-pollinate.
All avocados grow well in tropical and subtropical regions. In cooler climates, Mexican varieties will withstand lower temperatures. Avocado ‘Bacon’ withstands night temperatures to minus 5°C, and avocado ‘Hass’ withstands down to minus 1°C but it needs very deep soil. In frost-prone areas it is necessary to provide protection, and overhead watering has proven most successful in reducing frost damage. In areas prone to heavy frosts, trees are unlikely to survive.
Avocados require plenty of water during the spring and summer. In areas of low rainfall you'll need to give them a deep watering every couple of weeks otherwise fruit quality will suffer. Probably their most important requirement is good drainage, and this may be a problem in areas of high rainfall and dense soils. Soil fertility is not critical but if soil is overly wet the roots will suffocate and trees will die. It is advisable to plant avocados on mounds or ridges to improve drainage and reduce the risk of root rot. Trees can also be grown in containers which can help you to control root moisture.
You also need to avoid exposed windy sites. If wind is a common problem you might need to install windbreak plants and grow cover crops between trees. In small gardens, position trees near sheltered walls where they will be in full sun.
Trees should be fertilised regularly. Add well-rotted compost to the soil in spring and autumn each year and apply a layer of animal manure mulch over the surface each spring. Add it to the soil as far from the trunk as the tree canopy reaches. A slow-release fertiliser may be used every couple of months once growth has resumed in spring until the end of summer. Otherwise, use a liquid feed every two weeks. Don't fertilise too heavily since overfeeding makes trees more susceptible to root diseases. Likewise, not enough fertiliser will weaken trees. The right amount of nitrogen is critical too much and flowering is reduced, too little and leaf growth is reduced. As well as nitrogen, nutrient deficiencies sometimes occur with zinc, boron and iron but if you follow a good fertiliser regime these will be avoided.
Avocado trees should be pruned to develop and maintain a bushy canopy and keep trees from growing into each other. Proper pruning leads to increased productivity, better quality fruit and easier access to the tree when spraying and harvesting. Trees which grow too high may be pruned to encourage more lateral shoots.
Handle trees gently when planting. Tease the roots if they are pot-bound otherwise do not disturb them at all. Apply fertiliser and mulch around the base of newly planted trees. Keep the soil moist, but not saturated for the first few months. It may be necessary to water twice weekly. In some areas, plant protection (e.g. tying hessian around the tree) may be valuable to reduce the effects of wind and sun.
For orchards, trees are often planted in a grid pattern with 5 metres between them. The weaker trees are thinned out to leave 10 metre spacing. Smaller-growing varieties may be spaced closer together.
Pest and Disease Problems
The main problems include:
Phytopthera cinnamomi (Cinnamon fungus) this causes deterioration in the whole plant, and ultimately death. The roots rot, and the leaves turn pale and wilt. It is more likely in poorly drained soils where it can become a very serious problem. Control it by removing and burning damaged plants. Ensure good drainage, use organic manures, mulch trees, and apply lime or dolomite frequently.
Anthracnose this causes small light brown circular spots on fruit which enlarge rapidly and turn black. Control it by spraying with recommended fungicides. Remove the fruit from heat promptly after harvest and store in cool, well-ventilated places. Handle fruit carefully avoiding damage to the skin.
Verticillium wilt this causes leaves to wilt and turn brown, but they remain attached to the tree. It is a common problem. Control it by removing the affected branches.
Insects - these are not normally a serious problem. A community of biological predators which control pests usual develops around trees, and so spraying can often do more damage to these beneficial colonies than to the pests.
Fruit is handpicked upon maturity over several pickings. When ready, the stalk often starts to change colour. Larger fruits are removed first giving smaller fruits time to increase size before picking. The fruits will then ripen after picking and this takes about 10 days. The fruit will keep for about 1 month in cool storage between 4 and 7ºC, depending on the variety.
Tip: Fruits don't ripen on the tree but ripen after picking. It can be tricky to tell when fruits have reached the ripening stage. Pick them when they have reached full size.
'Pinkerton' - (type A) suited to a wide range of climates
'Walden'- (type A) ideal for subtropical and tropical regions
'Hazzard'- (type A) great for cooler regions but will also grow well in the subtropics
'Bacon' - (type B) perfect for cooler regions
'Hass' - (type A) great for cooler regions but needs very deep well-drained soil
'Reed' - (type A) a smaller-growing cultivar at 4 metres
'Sharwil' - (type B) great for warmer regions
'Wurtz' - (type A) perfect for the small garden reaching just 2-3 metres
'Shepard'- (type B) suited to warmer climates
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