Grow more than one avocado tree indoors if you want them to bear fruit, and be prepared to roll their containers outdoors from late spring to late summer. The reason for this is twofold, the trees will get air circulation and access to pollinating insects. Purchase avocado seedlings that are at least 2 feet high if you want to see a crop within three years. Growing avocados from seed will add several years to the wait for delicious, edible fruit for salads and dip.
Fill planting containers at least 2 feet in diameter and 2 feet deep with high-quality potting soil. Wet the soil thoroughly and check to see that drainage holes in the bottom of the pot are functioning.
Nestle the seedlings into the soil so that the root crown (where the trunk emerges from the roots) is 1/2 inch above the soil line. Make sure the smaller roots and the central root are hanging straight down, and covered with soil. Tamp the soil lightly down around the seedlings them with your fingers to ensure firm contact of soil with roots.
Place the containers in a sunny, warm room. Allow the potted avocados to dry out slightly between waterings -- they do not tolerate "wet feet." Be on the lookout for yellowing leaves, a sign of too much water.
Move potted avocado trees outdoors in late spring. A sunny, sheltered spot on a patio, deck or in the garden is ideal. Use a hand truck for moving large containers, since they can be quite heavy. When the avocados flower, bees and butterflies will cross-pollinate them and fruit will set.
Fertilize avocados twice during the growing season, once in April and again in July. Use a 7-4-2 fertilizer with added zinc sulfate, heavy in nitrogen and phosphorus that avocados need -- the numbers indicate the percentages of nitrogen, phosphorus and potassium in the fertilizer. Follow instructions on the package for correct proportions for the size and age of the tree. Bring the trees back indoors in September, well before nighttime temperatures are 50 degrees F.
Repot your avocados every two to three years as soil becomes exhausted and trees outgrow their containers. This will give the avocados a steady stream of nutrients and keep the roots from becoming coiled and potbound.