Perennials in winter

I read in a gardening book that once frost has killed the tops of my perennials I should cut the tops into small pieces and leave them in the garden. In the past, I have left them through the winter and cut them in the spring. The problem is that they are dry and woody in the spring and compost very slowly. Which is better, leaving them until spring or cutting them in the fall? - Sue F.

Either way of dealing with perennials has advantages and disadvantages. Standing perennials may trap more snow if you live in an area that gets snow. In our arid climate, any moisture harvested into the soil is beneficial. However, the chopped perennial debris serves as mulch and helps hold moisture in the soil.

In either case, leaving the perennial debris can be a problem if there are disease organisms in the stems and leaves. The debris also provides over-wintering habitat for insects. If insects or diseases are a problem, removing the debris in the fall may be best.

Finally, chopping and leaving the debris on the garden can help with the composting process. While serving as a protective mulch, the plant material will begin the decomposition process. Being green when chopped, it will decompose faster than if left to dry through the winter. In the spring, the decomposing stems and leaves may be left in place or perhaps removed to the "official" compost pile. If it is removed before the perennials begin to grow, there will be less chance of spreading diseases from the previous season. Removing the mulch also allows the soil to warm more rapidly and may help the perennials develop more rapidly in the spring.

Scale on cacti

I want to rid my prickly pears of cochineal scale. I've heard about blasting them with water, and using an alcohol or soap solution. Is there anything more I can try? The water spraying helped, but can't that go too far with a cactus? - Cynthia D. M., Tucson, AZ

The cochineal scale, source of a natural red dye, can be damaging to the cacti if they build to large numbers. Blasting with water will help keep the population low as will manual removal with swabs treated with rubbing alcohol. Insecticidal soap can also be used. Insecticidal soap, manufactured and labeled for pest control, is better to use than other types of soap. If all else fails, there are insecticides labeled for controlling scale insects.

This scale insect can kill whole prickly pear plants, but that takes a long time. Using water or insecticidal soaps on a frequent basis may be sufficient to prevent significant damage to the prickly pear cacti. In time, natural predators should arrive to help with the control of the scale. In considering your questions and the treatments mentioned, it appears you prefer not to use the more environmentally impacting insecticides. They are an option if all else fails. If you use the commercial insecticides, remember that you may also eliminate these beneficial predators. Regardless of the method you choose, if you use soaps, oils or the other insecticides, use them carefully and according to the label directions.

You also wondered if blasting with water could be damaging to the cacti. Prickly pear cacti are fairly tough, so they may tolerate the extra water if you do not treat too frequently. Treatment once a week for a few weeks should do little damage if the soil is well drained. If you use a low volume, high-pressure spray, you can control the insect without applying a lot of water. The objective is to keep the pest population low enough for the plant to tolerate until beneficial insects arrive to help.

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!