Avocado fruits come from evergreen trees (Persea spp.) that reach 40 to 80 feet tall. There are of avocado, though several hybrid varieties are available within each of these three species:
- Guatemalan avocado (Persea nubigena var. guatamalensis L. Wms)
- Mexican avocado (Persea americana var. drymifolia Blake)
- West Indian avocado (Persea americana Mill. var. americana)
Regardless of type, avocados have similar characteristics and care requirements. Avocados all prefer full to partial sun, and grow best in sandy loam soil with good drainage.
All three avocado varieties are hardy in U.S. Department of Agriculture plant hardiness zones 10 through 11, but individual hardiness still varies. West Indian types will not survive freezing conditions, Guatemalan types tolerate temperatures that occasionally dip down to 26 to 30 degrees Fahrenheit, and Mexican types can survive occasional temperatures down to 19 or 20 degrees F. If you're expecting cold weather, take steps to protect avocado trees from frost damage. This is particularly important for young trees.
When freeze or frost is expected, mound soil up several inches deep around the base of the tree to provide insulation. Water the soil thoroughly two to three days before cold weather arrives. Don't forget to move the soil back after the frost has passed.
Protect young trees from frost by draping a blanket, quilt or tarp over the canopy and anchoring the corners to the ground on the nights that frost is expected. Remove cover in the morning. Older trees are usually too large to cover.
Set a heat source like an incandescent light or camp lantern under the covering to provide more protection from cold damage.
Food and Water
Like citrus trees, avocados benefit from trace nutrients. Zinc is particularly important for growing healthy avocados. You can supply zinc and other needed nutrients by using an avocado or citrus plant food when fertilizing. For an avocado food with an N-P-K ratio of 6-4-6, begin fertilizer applications when new growth starts and then repeat every four to six weeks until growth slows in late summer.
Apply granular fertilizer evenly to the soil starting 1 foot away from the trunk and extending 1 to 2 feet beyond the leaf drip line. Scratch the fertilizer into the top 1 to 3 inches of soil, then water thoroughly. Application rates vary based on the size of the tree:
- For trees under 4 years old, apply 1/4 cup of fertilizer per 9 square feet, or 2/3 cup per 16 square feet.
- For older trees, apply 3/4 cup of fertilizer per 16 square feet or 1 cup per 25 square feet.
Avocados require about 50 inches of rainfall distributed throughout the year to survive. For the best fruit production, water avocados enough to soak the soil thoroughly one to three times a week. This requires about 2 gallons of water at each watering for a newly planted tree, and up to 20 gallons for a mature tree. To check if the trees require water, pick up a handful of soil from under the tree's canopy and squeeze it. If the soil sticks together and holds the impression of your hand, then the tree does not need water yet.
Avocado trees do not require pruning to keep them healthy or to encourage fruiting. The only reasons to prune are to remove dead, diseased or damaged branches, or to shape young trees if desired. When branches have been damaged, wait until new leaves and twigs start to grow and then cut the damaged branches back to new growth.
If you do decide to prune avocado trees to improve the shape or keep them small, they can be pruned any time of the year. Pruning in late winter means the trees will grow back slower than when pruned in warmer weather. When pruning, try to keep as much of the leaf cover as possible, since it shades the trunk and protects it against sun scald.
Anthracnose (Colletotrichum gloeosporioides) is a fungus that typically affects avocado fruits, though it can show up on other parts of the plant as well. It causes tiny brown or black spots that are round and sunken. The spots can reach over 1/2 inch in diameter, and may cause fruit to split. Anthracnose is often present in avocados, but rarely causes damage if you keep the trees clean of dead twigs and leaves. To control the problem on fruits, keep them dry and cool after harvesting.
Dothiorella canker (Botryosphaeria ribis) starts out infecting the trunk and causing dead patches. Infected spots excrete a reddish sap that dries to a white and brown powder, and if the bark peals off the wood beneath looks brown or red instead of a healthy pale color. This canker can spread to the fruits, causing them to rot. Mexican avocados do not get trunk canker, but the fungus can still cause fruit rot. To control and prevent the problem, keep the tree clean from dead leaves, twigs and fallen fruit. Use low-salinity water when irrigating, and keep with a regular watering schedule.
Root rot (Phytophthora cinnamomi) is a fungus that typically targets avocado trees growing in locations with poor drainage. It causes the roots to die, and results in stunted leaf growth. To avoid this problem, plant healthy trees in locations that drain quickly enough that there is never standing water around the trees for more than a few hours. Irrigating with low-salinity water and mulching can help control the development and spread of root rot. Use a coarse-textured organic mulch in a layer 4 to 6 inches deep beneath the canopy, keeping the mulch several inches away from the trunk. With proper care, infected trees may continue to produce edible fruit. Trees can be removed if they continue to deteriorate. If you remove infected trees, do not plant avocados or other plants susceptible to Phytophthora rot in that location for several years.