There are over 500 varieties of avocados grown and the Hass is the most common. Most of the avocados you find in supermarkets in the United States are Hass avocados grown in California. They have a dark green, almost black skin when ripe that is easily peeled to reveal the smooth green fruit. Care for these trees is minimal and the trees start bearing fruit as early as their second year. The root systems are quite extensive and will kill other plants within 20 feet of the tree.
Water the tree, soaking the soil, and wait until the top of the soil is dry before watering again. The frequency will depend on the weather. Hot dry air may cause you to water every day, while you may not need to water at all during a rainy season.
Place a top dressing of compost on the soil over the roots in the spring and summer of the first year. This will create a better draining soil plus place nutrients into the soil while the tree is too young for fertilizer.
Apply a 2-inch deep layer of mulch over the soil in a 2-foot diameter around the trunk of the tree in the spring. Keep the mulch at least 6 inches away from the trunk itself. The mulch will not only help to retain moisture but will also keep the area weed and grass free.
Begin to fertilize the tree in the second spring. Use a balanced fruit-tree fertilizer and apply in spring, early summer, late summer and late winter. Refer to manufacturer's directions as to how much to use for each application.
Spray the tree with a chelated foliar spray of trace elements containing iron if the tree has yellow leaves. This is an iron deficiency that is common among avocado trees and is easily corrected.
Harvest all fruit by the first frost. Hass avocado trees are not as cold hardy as some of the Mexican types and the fruit will turn black and suffer damage from frost.