Issue: April 27, 1998

Plastic mulch to warm soil

I have always heard that black plastic mulch in the garden warms the soil faster, but now someone told me that clear plastic causes the garden to warm more rapidly. That doesn't really make sense to me, but that person is very knowledgeable so I wanted to check.

Clear plastic does indeed speed the warming of soil, if the garden is in a sunny location, because it allows the sunlight to pass through to the soil, warming the soil directly. It then holds the warmth next to the soil. Black plastic absorbs the light directly and is itself heated. It then must warm the air under it and the air warms t he soil.

So why do people use black plastic rather than clear plastic? Because weeds can grow very well under the clear plastic in the early spring. The black plastic, by excluding light from the soil, inhibits weed growth.

As the season advances and the weather warms, it is important to remember that the plastic is still functioning. Clear plastic continues to warm the soil into the summer, perhaps making it too hot for the plants you are growing. In fact, a process called solarization uses clear plastic in the summer to heat the soil to a temperature which will kill disease organisms and weed seeds in the upper layers of the soil. Such heat is not desirable at the roots of our garden plants. Clear plastic may be used early in the spring to warm the soil for early planting of warm season vegetables but, especially here in New Mexico, it is important to remove the plastic or cover it with another mulch or reflective material as the weather warms. Black plastic has the potential to burn whatever touches it. The shading provided by the plants and the rapid heat loss from the thin film, the low heat holding capacity of the plastic film, allow us to use the black plastic as a mulching material without the excess soil heating that clear plastic provides.

Mulch is an important tool for limiting weed problems in the garden and in conserving moisture as well as cooling the soil in mid-summer. However, it can be used to speed the heating of soil in the spring to allow earlier planting of plants which need warm soil. Proper choice of mulch material and then its proper use allows us to maximize the benefits.

Spring flowering bulbs for New Mexico

As I saw the tulips and daffodils blooming, I wondered why the spring flowering bulbs aren't used more in New Mexico. Do the spring flowering bulbs grow well here?

Yours is a good question. There are many spring flowering bulb plants which should be used here. Many of these plants are native to similar climates in the Middle East, Turkey, Iraq, Iran, Turkestan, and Afghanistan.

Some tulips will serve as annuals or short-lived perennials because they don't tolerate our heat. Other tulips, especially the species types rather than hybrids, should naturalize and persist as perennials. Daffodils naturalize well over most of the state, and there are many other bulb plants which will also do well. Siberian Squill, Galanthus, some of the ornamental alliums, grape hyacinth, crocus, and others should be used more over much of our state.

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!