Q&A: Bringing avocado trees to bear; lily flowering

QUESTION: It is easy to grow avocado trees " most of us have grown a few in coffee cans over the years. Some of mine have reached a rather large size, but none has ever flowered or formed fruit. Is there a way to grow them in a greenhouse in containers and get them to produce avocados?

ANSWER: 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).

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.

QUESTION: I have lilies of the valley that I brought from my former garden in Massachusetts. I've planted them in three locations, one in total shade, one that is slightly brighter and one in part sun. The clump in the darkest site grows leaves and produces a few blossoms but nothing like the flowering in New England. The other clumps have leaves but no flowers at all. Why is this happening and what can be done about it?

ANSWER: Lily of the valley does not perform as well in the Washington area's hotter climate as it does farther north. It also does best if it is growing in porous organic soil that never completely dries out. An early season drought when the shoots first emerge may cause the flower buds inside the shoots to dry up. My experience has been that lily of the valley takes several years to settle down and cover the soil with its network of rhizomes before it begins to flower very heavily. You can speed this process by providing ample moisture and mulching the area with chopped leaves. A small amount of fertilizer in spring after flowering and attention to slug control can also help keep the leaves in good shape, which is important to production of next year's flower buds.

- Scott Aker is a horticulturist at the U.S. National Arboretum.