Growing Avocados: Persea americana

Garden Oracle / Drought and Heat Tolerant Gardening / Tucson - Phoenix - Arizona - California

Growing Avocados:
Persea americana

Botanical Overview

A member of the Cinnamon and Bay Laurel family (Lauraceae), the Persea genus contains about 150 species, of which Avocado is the most prominent. Avocado plants fall into three major categories: Mexican, West Indian, and Guatemalan. Some Mexican cultivars are the most cold hardy while some West Indian are the most salt tolerant. Guatemalan cultivars are intermediate between the two.
Over 900 cultivars and hybrids have been created, with only a few grown commercially. Mexico leads the world in avocado production. In the United States, major production occurs only in California, Florida and Hawaii.

Description

Form: Tree.

Lifespan: 400 years is possible. 30-40 years is typical.

Leaf retention: Evergreen. Some leaves start dropping during flowering, and on into warmer months, and are quickly replaced.

Growth rate: Usually rapid. A few cultivars are slow.

Mature Size: 30-40' (9-12.2m) high and 25-35' (7.6-10.7m) wide is typical. Some can reach 80' (24m) in a suitable climate. Dwarf cultivars are available.

Flowers: Yellow-green, small, clustered. While flowers have both male and female parts, avocado trees fall into two flowering types called A and B, depending on the cultivar. In the morning, type A are open as females, and type B are open as males. In the afternoon, type A are open as males and type B are open as females. This unusual flowering behavior increases the chance of cross pollination and genetic diversity.
There is an overlap on the same tree between male and female openings, however, especially when previous night temperatures were below 70°F (21°C), increasing the chance of bees pollinating flowers within the same tree. Usually no fruit set occurs below 60°F (15.6°C).

Bloom: Late winter or spring. Avocado trees can display "alternate bearing" with a heavy crop in one year followed by a light crop the following year. Some cultivars are more prone to this than others.

Self-fruitful: Yes. Most trees get better at producing fruit with age and residential gardeners do not need a second tree. For commercial growers, two trees, one of flowering type A, the other B, can provide a 20-95% increase in fruit production depending on the distance between the trees and the cultivars used.

Years before fruiting: Grafted rootstock take 3-4 years, trees grown from seed take 8-20 years.

Fruit: Egg-shaped, pear-shaped, or spherical. The yellow-green to purple-black skin can be smooth or pebbly, and range from thin to thick. The flesh is light green to yellow. The single seed varies from small to large. Since the tree has an extended flowering period, it will have fruits of varying degrees of maturity. The weight of the fruit ranges between 3-40 ounces (85g to 1.13kg) depending on the cultivar.

Months for fruit to ripen: 6-18 months to mature, depending on cultivar. Avocado fruit attain maturity on the tree but can only ripen after reaching maturity and having dropped or being picked off the tree. For many cultivars, the fruit reach full size before they mature, so the only reliable indicator of maturity is when the fruit falls from the tree, making a tarp under the tree worthwhile. One harvesting problem is picking the fruit too soon, before maturity, and then the fruit never ripens.

Storage after harvest: Mature avocados can take 4-5 days to ripen at room temperature. To speed ripening, place the avocado fruit in a paper bag or bowl with apples or bananas. Once ripe, the only storage is to cut them open, mash the ripe flesh in a bowl, and add lemon juice. This will keep for a day refrigerated or longer frozen.

Leaves: Dark green, glossy, oval, remaining on the tree for 2-3 years. Some leaf drop when flowering is normal and may continue into warmer weather. Mexican cultivars have an anise aroma and can be used in cooking. Guatemalan and West Indian leaves, without an anise aroma, are mildly toxic.

Stems: No thorns. The thin green skin on the trunk is prone to sunburn, especially during the first three years. This tree should not be pruned, particularly the lower branches, so that its leaves can shield its trunk from direct sun. After three years, the trunk skin gradually hardens into a thicker, bark-like covering.

Roots: This tree has a tap root plus feeder roots that extend 2-5' (0.6-1.5m) deep into the soil. Most trees are grafted onto a hardy rootstock.
The 'Lula' rootstock, which originated in Florida, is salt tolerant, does well in alkaline soil, is cold hardy to at least 28°F (-2.2°C), and is vigorous and heavy bearing as a tree. This is a preferred rootstock in Texas and Arizona.
The 'Dusa' rootstock is used in California because of its disease resistance, high yield and salt tolerance. It is also known to be hardy in Arizona.
In climates with winter freezes, the graft union should be placed low on the rootstock to make it easy to protect with soil or mulch.

Cultivars of Note:

'Bacon' - Hardy to 25°F (-3.9°C), grows 20' (6m) tall, with an average yield. The medium sized fruit has a large seed, green skin, a low-oil watery flavor, and tends to spoil quickly when ripe.

'Hass' - This is the standard grocery store avocado everywhere but Florida. It is hardy to 33°F (0.56°C), performs poorly above 85°F (29.4°C), and grows 15-20' (4.5-6m) tall. The fruit are medium sized, with a small to medium seed, pebbly thick skin, with an excellent flavor.

'Lamb' / 'Lamb Hass' - A cross between 'Gwen' and 'Hass' with fruit similar to 'Hass'. It is hardy to 33°F (0.56°C), handles high temperatures very well, has a tall, slender shape, growing 20-25' (6-7.6m) tall, can be planted closer together, and has a higher salt tolerance. The fruit are well shaded by leaves but take 12-18 months to mature. The skin must have turned black before the fruit can be safely harvested. This tree is prone to fruit drop during flowering when young, but improves with age.

'Lila' / 'Opal' - Hardy to 18°F (-7.8°C), possibly lower, medium sized fruit, green skin, average flavor, average and inconsistent production. Slowly grows about 10" (25.4cm) a year, reaching 20-30' (6-9m) tall. Easy to grow in a container for those wanting a small outdoor tree. Will need 50% all day shade protection in the summer for up to 10 years. Sold by Texas nurseries.

'Mexicola' - Hardy to 18°F (-7.8°C), heat tolerant, grows to 25' (7.6m) tall and 20' (6m) wide, with a high yield. It produces small fruit with a large seed, thin edible skin, and very good flavor.

'Wurtz' / 'Little Cado' - Hardy to 25°F (-3.9°C), slow growing dwarf tree to 8-15' (2.4-4.5m) tall, weeping growth habit, small to medium fruit with small seeds, medium-thick green skin, good flavor, consistent moderate yield. Best avocado to grow in a container.

'Zutano' - Hardy to 25°F (-3.9°C), this tree grows 30-40' (9.12.2m) tall and wide, and is grown primarily as a pollenizer for 'Hass' in California. Medium sized, glossy green skin, large seeds, low-oil watery flavor, high yield, poor disease resistance leading to short life span.

Wildlife: The flowers attract bees. The fruit attract mammals.

Toxic / Danger: All parts are mildly poisonous to animals. Unripe fruit are mildly poisonous. The leaves of Mexican varieties which have an anise aroma, and the ripe fruit of all varieties, are not dangerous to humans except in the case of rare allergies.

Origin: Mexico.

Persea americana: Avocado 'Wilma' is prolific and produces very large fruit.

Persea americana: Avocado 'Wilma' on 'Lula' rootstock. The tree is four years old and has grown 15' (4.5m) tall. Tree photos by Ken Stockton.

Cultivation and Uses

USDA hardiness zones: 8b-11 for a few cultivars, 9b-11 otherwise.

Chill hours: None.

Heat tolerant: Most cultivars listed on this page tolerate temperatures over 100°F (38°C), but fruit drop is more likely without protective measures. Fruit that is not shaded from the sun will be baked and ruined in high temperatures.

Drought tolerant: Yes, after three years in the ground, but only at moderate temperatures.

Sun: Provide all day part shade in the first three years. Allow full sun only when the tree is over 8' (2.4m) tall and in the ground three years. In its native habitat, this tree begins as an understory plant.

Planting: Avocados are successor plants. That is, they grow in forests where other plants have already been growing. The best location is one where another tree, such as a fig, has successfully grown ten years or more and has been chopped down, or is next to large trees whose roots have colonized the planting area over many years, and is partly shaded most of the day, especially in the afternoon. The rootlets of the previous tree(s) provide organic material, have aerated the soil, and have supported an extensive mycorrhizal fungi network that the avocado tree needs for nourishment. This need is especially great in hot, dry regions that are marginal to the environmental needs of avocados. An avocado will fail in soil where other large plants or trees with deep roots have not grown.

Time of Year to Plant: Transplant the sapling from a pot into the ground in mid to late fall (in regions without freezes), or mid to late winter, avoiding both freezing temperatures and summer heat, never when temperatures are high. The plant needs time to grow enough roots before summer's heat so that it can supply sufficient water to the leaves. Avocado leaves need to transpire large amounts of water in high temperatures to stay healthy.

Soil: Very well draining, high in organic content, low salinity. The pH tolerance varies by rootstock.

Fertilize: Use an organic fertilizer monthly from mid-winter to end of summer. Spread it inside the drip line and 8" (21cm) away from the trunk. Do not fertilize the first four months after the tree has been planted.

Water after becoming established: Basin irrigate weekly when fruiting, monthly at other times, especially during drought. The soil should be dry 9" (23cm) deep before watering again. Root rot and other diseases occur in moist soil. Spraying water on the leaves, especially the underside of the leaves, greatly helps the tree flourish when temperatures are over 105°F (40.6°C).

Mulch: Use up to 1' (0.3m) of mulch. Keep mulch 8" (21cm) away from trunk in moderate to high rainfall regions. Avocado leaves that have been dropped by the tree make the best mulch because their decomposition provides the best nutrients. Use leaves, pine needles or shredded bark when avocado leaves are not available.

First three years care: During the hottest months of the summer, provide afternoon shade until the plant is over 8' (2.4m) tall. For some cultivars, all day 50% shade cloth may be necessary in the hottest months of the year.
When nights are below freezing, and until the plant is 8' tall (2.4m), a cloth (not plastic) sheet must be draped over the entire tree and fastened on the ground.
Water the tree every two to three days, unless freezing temperatures are near, using basin irrigation, to establish a strong root system.

Prune: Avoid pruning branches, especially lower branches, except to keep them barely off the ground. The leaves must protect the trunk from direct sun top to bottom and also protect the fruit. Fruit develop on new growth.
Pruning can be used to dampen alternate bearing. In the fall after a light bearing year, remove 50-75% of the new growth that would result in new flower clusters. This will lower fruit set in the following year.

Litter: Fruit if not harvested. Fallen avocado leaves should not be discarded, but used as mulch under the tree, directly on the soil.

Propagation: Cuttings grafted onto rootstock. Seeds are only viable for one month after being removed from their fruit and often do not grow true.

Uses: Ornamental, edible fruit.

Comments

Most avocados found in supermarkets, such as Hass, can only survive a 33-80°F (0.56-27°C) temperature range and do poorly in hot, dry climates. A tree grown from a grocery store seed, if not grafted, will take 8-20 years to fruit and will not be true to its parents.

Avocado trees purchased in nurseries are better suited to local climates and are grafted on rootstock specially selected for local soils. They will also fruit in just a few years. In spite of that, avocados are one of the most difficult fruit trees to grow in hot, dry climates. A Walled Tropical Fruit Garden can help provide the proper microclimate.

Avocados do best when planted in soil where other fruit trees have successfully grown ten years or more. Avocados must be "babied" for the first three years, protected from freezes, and protected from sunlight in high temperatures.

Do you have additional information or a different experience for these plants that you would like to share? Email info@GardenOracle.com. All contributions are welcome and appreciated.