Ask the gardeners

I have several beautiful oxalis plants. At times they are breathtaking, but they seem prone to white fly infestation. lSpraying helps but only for a short time and changing the soil does no good at all. What do you suggest I do?

White flies have five stages in their life cycle, thus making them difficult to control as new adults are perpetually emerging.

Yellow cards, coated with sticky stuff and hung near the plants, work well in catching the adults. These (or the ingredients) are available in garden centers, or you can make your own by using SAE 90 oil over yellow paint.

Also, 1 pint of rubbing alcohol in 1 quart of water, to which has been added one-half teaspoon of liquid dishwashing detergent, will destroy the adults and some immature stages. Spray each week under and above the leaves until the infestation is eliminated.

If too badly infested, after spraying cut off the stems just above the pot (after carefully slipping a plastic bag over them), and seal the bag and discard.

Keep new growth sprayed for a week or two and you should like the problem. White flies do not live in the soil but on the backs of the leaves.

Is it a good idea to move houseplants outdoors for the summer? I have lots of ferns and African violets.

Ferns love it outdoors during the summer, but put them in a shady spot. Our African violets have never fared well outdoors, however, so we keep them inside in a north window.

I have a Stayman's Winesap apple tree that's 20 years old. Each year I remove a few of the branches and wonder if that is the right thing to do. The fruit doesn't color up well.

Perhaps you are not removing enough growth. It's important to prune so that plenty of sunlight gets to the inside crown of the tree. Also, branches that form narrow-angle crotches should be removed as they form weak limbs.

When branches criss-cross one another, remove the one that's least desirable.

I planted a row of sweet corn in my garden last year, and the ears had only a scattering of kernels on them. What happened?

Corn must be planted in ''blocks'' of at least four rows of any variety, side by side, in order to get good cross-pollination from the wind.

Pollen from the tassels (male elements) must fall on the silks (which are the female parts attached to the kernels), in order to produce mature, edible kernels. A single row does not allow for good pollination.

Is it possible to start plants, such as philodendrons, from seed?

Summer is a good time to start philodendron seeds since they like a germinating temperature of 70 to 85 degrees F. Most large-leaved types take from 16 to 22 weeks to reach a size that would look presentable in a 4-inch pot.

If you have a question about your garden, inside or out, send it to the Gardening page, The Christian Science Monitor, One Norway Street, Boston, Mass. 02115.