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By Scott Aker
Special to The Washington Post
Thursday, June 15, 2000; Page H11

Q: Why won't our magnolia tree bloom? This tree is about six feet tall and was started from a seed. It has been transplanted at least three times, the last time in 1987. It is healthy and full, but has never bloomed. It gets the morning and early afternoon sun. We have tried fertilizer spikes and spoken to nurseries. No one seems to have an answer.

A: Magnolias do not bloom profusely until they reach a fairly large size. This is especially true of the Southern magnolia, Magnolia grandiflora, with its large, gorgeous blossoms and lemony scent. It seems odd that your magnolia would be only six feet tall more than 10 years after you planted it. The tree has more than enough light: If you have watered it during periods of drought and other plants are growing in the vicinity, there is no reason why it should grow so slowly.

Do a bit of digging around the roots to see if there is anything impeding growth. Rubble, stones or other obstacles to root growth can have a drastic effect on growth, as can bits of burlap, rope or staking material that should have been removed but were left to choke the trunk. I'd also suggest that you get your soil tested. Magnolias prefer slightly acid soil. If yours is too sweet, use iron sulfate to decrease the pH.

Magnolias require little pruning; only dead branches need to be removed. Water is very beneficial in times of drought, but magnolias are remarkably adapted to dry conditions as well as wetness at the roots.

Magnolias are not sprinters in the tree world, but they can make good growth in the right conditions. Starting small is always a good idea with a magnolia since they typically suffer transplant shock even when a careful job is done in moving them. I planted a three-foot-tall sapling in my garden just four years ago; it is now more than 15 feet tall. A friend of mine planted two large magnolias that were 10 feet tall at about the same time; his are now no larger than the small tree that I planted. The odd, ropelike roots that lack an abundance of root hairs explain the difficulty in transplanting a magnolia. New roots grow primarily in spring, so transplanting is most successful if done in March or April, not the fall planting recommended for most trees.

Q: The lilac is my favorite flower, and I have three bushes--two purple, one white--but for several years in a row, they have produced very few flowers. The shrubs are big and healthy, and continue to grow in all directions. We prune them to keep the size manageable. But we're obviously far better at urging green growth than flower production. Now it's time to admit that we really know nothing about lilacs. The only worthwhile thing we seem to do is cut out dead branches, and we are generous in giving away new growth that pops up beside the bushes. Do you have suggestions for this year to improve next year's flowering?

A: I suspect the pruning you are doing to keep the size manageable explains the lack of flowers. Lilacs complete their annual growth and set flower buds very early, and if you do any pruning after mid-June, you risk lopping off the buds at the tips of branches and the tiny flowers inside them that will bloom next spring.

Heavy pruning also will promote growth from the base of the shrub of suckers that need at least two years to mature into flower-bearing stems.

The other possibility is that your lilacs do not get enough sunlight. They really need full sun to bloom heavily.

Lilacs are not difficult to grow, but they do not shine in Washington as they do in cooler climates. In the heat of our summers, the thickest branches are very prone to lilac borers, and powdery mildew can give entire shrubs a filmy appearance in the autumn months. The best way to control the lilac borers is to cut the infested branches back to the ground.

New suckers will arise from the crown of the shrub to resurrect it from the damage; these new branches will not be suitable for the borers' needs until they reach two inches in diameter. Although the powdery mildew is not attractive, it has no effect on next year's flowers, so it is best to simply tolerate it.

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

Have a question about gardening? Write Digging In, Home Section, The Washington Post, 1150 15th St. NW, Washington, D.C. 20071 or e-mail home@washpost.com.

TIP OF THE WEEK

CRAB APPLE TREES with leaves now turning orange and dropping off suffer from a disease called apple scab. Spraying becomes unfeasible on large trees. Consider replacing old crab apple cultivars severely affected by scab with newer, resistant varieties, says the Maryland Home & Garden Information Center.

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