Ask the Gardeners

For the past two years the blooms on my peony plants have been only 3 inches across, whereas they used to be about 6 inches across. In the same area, my lilies used to be 5 feet tall with lots of blooms, but lately they've been only about 18 inches tall. What's wrong?

It's probably overcrowding. Use a spade in early fall to divide peonies. You don't have to remove the whole clump, but rather push the spade shovel through the clump, then by working around one of the sections remove it from the ''mother'' plant.

This will leave you one part undisturbed, to bloom well next year. The separated portion may take a year or two to bloom profusely. The lilies can be dug up in late August or early September, separated, and replanted.

Work some rotted compost into the soil if it is hard and claylike. Both peonies and lilies like a feeding of bone meal or superphosphate for good bloom.

If all the plants remain stunted after you've separated and fed them, you may have nematodes in the soil. These are microscopic worms that feed on plant roots. If you suspect this problem, consult with your cooperative extension office. Nearby tree roots will also cause dwarfing of plants as they rob nutrients and moisture.

Is there any way we can save our white birch trees? The leaves at the top of the trees are turning brown and branches are unusually droopy.

Sounds as if the bronze birch borer (larvae of a small wasp) is boring tunnels in the trunk. Look for small square holes. They work down from the top, and you can sometimes stop their devastation by cutting the top out just below the dead part and burning it.

Arborists recommend systemics (which work into the tree's sap stream), but we have had no success with these. We've decided not to plant birches where they do not occur naturally. In native habitat they appear healthier.

We are in middle California, which was the object of a huge spraying of malathion for the medfly. Our insect problem has changed radically, causing new problems.

Foremost is the huge increase in whitefly, which surfaced early in the spraying. Our zucchini plants were so heavily infested that we finally just dug them up because of the deteriorating fruit. I read about a parasite that feeds on whitefly. Where can I get it?

There's a tiny wasp called encarsia which feeds on whitefly eggs. We've had great success with them. You can find advertisements for them in publications such as Organic Gardening magazine, Rodale Press, 33 East Miner Street, Emmaus, Pa., 18049.

There are other natural controls such as ladybugs, crickets, and preying mantis that help reduce whitefly. Unfortunately, the massive spray programs eradicate the beneficial insects. Chemical sprays cannot completely eradicate whitefly, since there are five immature stages, some of which are immune to sprays.

Yellow cards covered with a sticky material and suspended horizontally over plants catch whiteflies by the hundreds, as they are attracted to yellow. These are available at garden stores or you can make your own using molasses, honey, or old motor oil for stickum.