Issue: August 9, 2003

Lilac yellowing

I have a lilac bush that has (until the last week) looked "healthy." It now has yellowing leaves and looks like it is dying. I have a dog that sometimes urinates on it. Could this be the problem?

It is possible that the dog is causing some of the problem, but that would occur only on leaves in direct contact with the urine. Dogs are usually a greater problem with junipers and other small conifers, which are even more sensitive.

Another consideration is moisture and potential leaf diseases. How much do you irrigate? If often, the humidity around the shrub can allow leaf fungus to grow at night. There are several leaf fungi that can cause minor problems (early defoliation) and usually don't kill the plant. The other possibility is that the shrub is not getting enough water, but lilacs don't require a great amount of water. Water them deeply once or twice a month. Clay soils hold more water so they can be watered less often. Sandy soils are easier to water, but they hold less water and must be watered more often. Water slowly at the dripline and beyond the edge of the shrub so that water doesn't run-off over the soil surface but soaks into the soil. Continue this until the soil is moistened to a depth of two feet. You will need to probe the soil with a soil probe or dig a shallow hole a few hours after watering to determine how deeply a given irrigation moistens the soil. If the soil is moist to one-half foot, increase the irrigation time to four times what you had done previously.

Irrigate in the manner described above if you have been watering more often than this. If you have other plants (shallow rooted annuals), you may water them less deeply and more often, but avoid daily watering. The greater the time between irrigations, the less potential there is for diseases.

Can I spray pine tip moths now?

I discovered that I have pine tip moths damaging my Ponderosa and Afghan pine trees. Can I spray now? Tomi K. Albuquerque

Do not spray now. You will do little to affect the pine tip moth larvae and pupae, so there is no reason to apply insecticides. Wait until spring. In the Albuquerque area, years of pheromone trapping by Master Gardeners have shown that the first week of May is the time to spray. This is when the new larvae are just hatching from the eggs and are most susceptible to the chemicals.

Readers in other areas of the state should check with their local Cooperative Extension Service to determine timing for their area.

Marisa Y. Thompson, PhD, is the Extension Horticulture Specialist, in the Department of Extension Plant Sciences at the New Mexico State University Los Lunas Agricultural Science Center, email: desertblooms@nmsu.edu, office: 505-865-7340, ext. 113.

For more gardening information, visit the NMSU Extension Horticulture page at Desert Blooms and the NMSU Horticulture Publications page.

Send gardening questions to Southwest Yard and Garden - Attn: Dr. Marisa Thompson at desertblooms@nmsu.edu, or at the Desert Blooms Facebook.

Please copy your County Extension Agent and indicate your county of residence when you submit your question!