Avocado trees in a greenhouse

A mature avocado tree may grow 50 feet high and 20 feet across -- a challenge for any greenhouse. Even then, you may not get fruit because some varieties are not self-fertile.

However, there are several dwarf varieties that can bear fruit in a greenhouse. A variety named Don Gillogly has received a lot of attention among avocado fans. It is offered by mail order, but it is relatively new and hasn't been widely grown to determine its performance over a wide variety of conditions. Two others, Holiday, and Little Cado, have been more thoroughly tested. Holiday is a small tree that tops out at 12 feet. Little Cado is a bit larger at 15 feet, with smallish fruit that no doubt give the variety its name. Holiday is said to have a very good flavor and ripens earlier than others, about midwinter. Don Gillogly is the only dwarf cultivar widely available by mail or Internet. Little Cado and Holiday are available from Growquest in California, www.growquest.com; (866) 986-7673. Avocado trees from GrowQuest.

Another challenge is to tell when the fruit is ripe. As a rule, avocados don't change color to signal ripeness, and remain dark green and hang on the tree for months after they are fully developed before they deteriorate at all. The only way to tell whether your avocados have ripened is to cut one open and check the color of the seed. If it has turned brown, the flesh is fully developed, meaning you can pick it and allow the fruit to soften as you would an avocado you find in any produce aisle. Don Gillogly is said to turn from green to black as it ripens, bypassing this uncertainty.