Avocado trees grown from a seed from a grocery store avocado may produce a tree if you plant it, but its fruit may be different or even inedible, due to the fact that the tree grown from seed will be genetically different than the original tree that produced the fruit.
The new tree will, however, be a good base for grafting branches from an existing avocado tree that's producing reliable fruit. Once you've grown your seedling to a height of about three feet, it's time to start grafting fruit-bearing branches onto it in a process called topworking. Begin grafting in the spring when bark slips easily from the inner wood of the tree.
Select budwood from a healthy, productive avocado tree. The best buds are located near the ends of branches that are 1/4 to 1 inch in diameter.
Cut 6-inch lengths of healthy branch tips that each contain several buds, using a sharp knife. Take six to eight cuttings, wrap them in damp paper towels, and lay them in a bowl of ice to keep them cold and moist.
Make a T-shaped cut on a branch of the rootstock tree, about 12 inches from the trunk. The long part of the T should be about 1 inch long. Make a shorter, crossing cut that goes 1/3 of the way through the branch. Twist the knife slightly to pry the bark away from where the two cuts meet.
Examine the bud sticks you cut and placed in the bowl. Choose a healthy bud, and cut it from the stick, beginning 1/2 inch below the bud and ending 3/4 inch beyond it.
Bring the selected bud back to the rootstock. Slide the long end of the budwood into the long part of the T-shaped cut, matching the bud to the horizontal cut in the T.
Wrap the budded graft with a rubber band, securing it above and below, but not actually on the bud. Repeat steps 3 to 5 in different areas of the tree until you've used buds from all the bud sticks.
Remove the rubber bands when the bud unions have healed and buds begin to open, which should be within three to four weeks. As these new branches grow and mature, avocado fruit will be produced on them.