Avocado trees (Persea americana, USDA plant hardiness zones 10 through 12) produce a shallow, aggressive root system, making them susceptible to rot and infection from flooding and damage from surface activities. While many children have stuck toothpicks into avocado pits and set them in a jar of water to watch the roots emerge and grow, most commercial and landscape avocado trees are grown from grafts onto rootstock, not from seed.
Avocado Tree Fundamentals
Avocado trees produce the rich green fruit prevalent in Mexican cooking and prized for its oils. Native to warm, arid areas of North and South America, hardy varieties now grow in subtropical areas including California, Hawaii, Florida, the West Indies and the Mediterranean area. Dwarf varieties have been developed for container gardening. Find containers for growing avocado trees at Home Depot, Amazon, Wayfair and other retailers.
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Avocado Tree Root Growth
Most of the avocado tree's feeder roots grow in the top 6 inches of soil. The roots grow outward from the tree trunk in all directions, reaching beyond the tree canopy.
Avocado tree roots prefer loose, well-drained soil and can tolerate acid or alkaline conditions. Avocado trees have highly competitive root systems that can choke out nearby plants that vie for water, air and nutrients. The strong, aggressive roots can buckle and break pavement as they grow.
The far-reaching dominant root system means avocado trees should be planted at least 30 feet away from buildings and other trees in landscape environments. Planting too close can lead to poor growth of the avocado tree or nearby trees and plants.
Root System Considerations
The shallow nature of the root system makes the avocado tree susceptible to damage from flooding and overwatering. If the avocado tree roots stay wet, nutrients are lost and growth stunted. In prolonged flooding or saturated soil, the tree dies. Death generally occurs due to fungal pathogens brought on by perpetually wet conditions. Constantly wet soil also can lead to a fungal root or crown infection known as Phytophthora rot, which reduces growth and fruit production. The condition can eventually kill the tree.
Avocado Tree Propagation
Fruit quality and yield vary widely among trees grown from seed and are more consistent from grafted trees. That's why commercial avocado trees are started by grafting buds of mature trees onto seedlings or rootstock.
Trees grown from seed normally produce fruit five to eight years after planting, while grafted trees start bearing fruit in three to five years.
Growing Avocado Trees
Avocado trees produce fruit most abundantly when grown in full sun. While avocado trees can't withstand constantly wet soil, they do require moisture so that the plant's shallow roots don't dry out. If you live in an area with abundant rainfall, avocado trees may rarely need watering. However, if you live in a dry, arid climate where rainfall can be scarce, such as Southern California, you will need to water the avocado tree on a regular basis.
Avocado tree seedlings or grafted trees from nurseries should be planted in a hole that is slightly wider than the root ball and covered with loose, loamy soil. Because the root system is sensitive to bumping, breaking and damage, the root ball should be gently lowered into the hole without disturbing the roots.
When planting avocado trees, compost mixed into the soil will promote drainage and aeration. Mulch helps the soil stay cool and moist, but the mulch should be kept at least 6 inches away from the trunk.
Avocado trees don't need much pruning. Prune out dead and crossing branches and otherwise let the tree grow. Overpruning can lead to less fruit production.