Issue: May 19, 1997

Apply compost to established lawn?

I read that it is a bad idea to place compost on an established lawn. Is this true?

In New Mexico, with the grasses commonly grown as lawn grasses and with the cultural methods employed here, it is advisable to topdress established lawns with compost. The compost may be applied over the lawn in a thin layer or applied after aeration to fill the holes left by aeration. Compost applied following aeration should increase the permeability of the soil to air and water, and increase the water-holding capacity of the soil.

Perhaps the author of the article you read was concerned that too thick a layer of compost would shade the grass and weaken it. If the grass is in poor health from winter stress, insect pressure, or disease, then heavy applications of compost over the top of the grass could be detrimental. In this case, application following aeration would be the preferred method of application. Take care not to leave a thick layer of compost over the grass.

John White, Bernalillo County Extension Horticulture Agent, recommends that not more than one-fourth inch of compost be applied to a healthy lawn to minimize shading effects. He warns that if there is an impermeable layer below the soil surface, a black layer may form if heavy irrigation follows application of compost, but this is probably more a function of the over-irrigation and the impermeable layer.

Another concern which would be greater in more humid climates is that the compost, if improperly cured, could contain fungi and bacteria which could infect the lawn. This should not be a problem with properly prepared and cured compost and in the arid Southwest U.S.

Herbicide and ground water contamination?

My wife and I just bought a house with a lovely lawn and an 80-foot well for our home water supply. With the lawn, we inherited a fairly sumptuous crop of dandelions. How bad are the standard "weed and feed" products when it come to contamination of ground water?

If the product is used properly, there should be no problem. That means you apply the weed-and-feed material as directed on the label, no more per square foot that recommended, and not irrigating following application. If you want to be even more careful, you may choose to use manual weed control near the well casing and be sure that no water runs toward the well casing when you irrigate. Unless you apply the product at an excessive rate and irrigate heavily, water contamination potential through the soil is not a concern. However, water which dissolves the product and runs down the well casing has direct access to your ground water supply.

In addition to concerns regarding water contamination, there are other problems when weed-and-feed products are used. They come in two types, weed-and-feeds with pre- and post-emergence herbicides. They are often use improperly. The post-emergence herbicides can cause damage to trees and shrubs when used improperly. If watered immediately after application or over applied, the post-emergence herbicide can be absorbed by tree and shrub roots damaging the tree.

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!