Hemlock and cedar mulches don't raise acid levels, and another look at

Q. With the proliferation of hemlock and cedar mulch as convenient and practical ground covers, I'm wondering what effect they have on soil pH. I suspect they may produce a more acid soil that would be beneficial to evergreens and other acid-loving plants, but what do they do to, say tulip bulbs, and other perennials as well as annuals? - L.N.J., Woburn, Mass.

A. It would take an extraordinary amount of cedar or hemlock mulch to acidify the soil, says Dan Struve, a former landscape architect and assistant professor of urban landscape ecology at Ohio State University.

He says all such commercial mulch is heavily coniferous but has such a minimal effect on the soil that plants should not be affected.

Dan Sullivan, assistant professor of soil science at Oregon State University agrees. He adds that when mulching around shrubs, a medium-sized mulch is better, whereas in a garden, a finer mulch is preferred so it can decompose after the growing season.

Readers respond Several readers wrote in response to the March 17 Resident Expert column, which explained how to deal with tomato hornworns. They described their experiences with pests on tomato plants.

I thought I'd add my 2 cents to the discussion of hornworms. When I was gardening some years back, I planted French marigolds next to my tomato plants. Whenever I did that, we had no tomato worms. I believe this is called companion planting. - J.E.C., via e-mail

Gardeners may notice that some of the hornworms on their tomatoes have lots of egg cases - the size and color of white rice grains - mounted vertically on their backs. I learned in a day-long class in Integrated Pest Management that these are the eggs of a beneficial insect, the Trichogramma wasp, which is almost too small to see with the naked eye.

The wasps don't sting humans or cause other damage, but when hatched, the pupa burrow into the hornworm, essentially killing it.

So if you remove hornworms, don't disturb the ones carrying Trichogramma wasp eggs, which may eventually hatch and do the dirty work for you. - M.K., Noblesville, Ind.

Readers: Pose your questions and we'll seek out experts on home repairs, gardens, food, and family legal issues. Send queries to the Homefront Editor, The Christian Science Monitor, One Norway Street, Boston, MA 02115 or e-mail home@csps.com