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Mulch provides garden heat defense

By Urban Harvest

Mulch provides garden heat defense

We have been blessed with a cool spring. On these beautiful days it's easy to forget that the heat of summer is right around the corner. The next few weeks are your best chance to get your garden ready to beat the heat.

Plants that are in top condition deal with the stresses of Houston's heat and humidity better than weaker specimens. This is one reason we fertilize and do major pruning in late winter and early spring. The next best thing we can do for them is to apply and maintain a layer of organic mulch.

Mulch works for us in several ways. The more you know about mulch, the better decisions you can make about what kind of mulch is best for you, and how to apply it affectively.

Weed suppression

Small weed seeds contain photoreceptors that detect the light that triggers germination. Mulch prevents this light from reaching the seeds below it. A few weed seeds will still germinate and grow but the numbers will be reduced to numbers that can be handled with a few minutes of attention each week.

Mulch will not stop all weeds. Persistent perennial weeds must be removed mechanically or by using herbicides before the mulch is applied. To be an effective weed suppressant, mulch must be applied at a sufficient depth, and must be maintained regularly.

As tempting as the ads may be, do not combine the use of woven weed mat under a cover of mulch. These products were originally intended to suppress weeds under brick or flagstone walkways, and they work quite well for this purpose. However, when used under organic mulch, a larger problem is created.

New weed seeds are introduced into your garden constantly by visiting birds and on the wind. When these weeds seeds germinate in the mulch layer they will root into the weed mat and are difficult to remove.

Plastic sheeting has no place in the garden and it should not be used under mulch. If you have an existing bed that has known weed problems, and you wish to use something under the mulch to get a foothold on the weeds, use a layer of newspaper 12 to16 sheets thick. Newspaper does not harm the soil food web and works very well in combination with mulch.

Preserving soil moisture

Mulches work to preserve soil moisture by slowing evaporation. This means that you can water less often, conserving this precious resource. This effect contributes to plant health by lessening the danger of drought stress.

Moderating soil temperature

The temperature of bare soil can vary greatly over the course of a day in peak summer months. A cover of mulch shades the soil and keeps it cooler. The same is true in peak winter months, where mulch acts as a blanket keeping warmth in the soil. In both seasons mulch moderates the extremes, keeping soil at a more consistent temperature.

The combination of preserved moisture and moderated temperatures is important for soil health. The inhabitants of the soil food web, those beneficial microbes that really do all the heavy-lifting and macro-organisms such as earthworms, will find the environment under the protection of a layer of organic mulch quite hospitable.

Types of mulch

There are many types of mulch available. All of them will function to suppress weeds and moderate soil moisture and temperature, but not all mulch is created equal. All organic mulches will break down over time and contribute to the make-up of your soil, but some will provide additional nutrients from the start and others will not. Read the bag or ask your bulk supplier to tell you what their mulch comprises. Some mulch is made up mostly of chipped construction waste that is dyed to have a natural appearance.

  • Aged bark mulch is chipped bark that has been partially decomposed. It is a much better product than recycled construction waste and it is readily available at bulk suppliers at a reasonable price.
  • Native mulch is a better choice still. It is composed of chipped whole trees and contains nutrients not available in the products above. The nutrients stored in this material are released into the soil as they decompose.
  • Pine straw is a superior mulch product. Pine straw has a beautiful finished appearance in a bed. It is easy to put down, comfortable to walk on, and smells wonderful on a summer day.

Pine straw is a more effective weed suppressant than some mulch materials because new weed seeds that fall onto it seldom reach the soil surface. They just fall into a pocket in the straw layer. Without soil contact they cannot absorb sufficient moisture to germinate and grow.

Most mulches need to be held several inches away from the stems and trunks since they hold moisture and do not allow air circulation. Since air can circulate through pine straw it can be pulled up very close to the stems and trunks of your plants, leaving less soil exposed for weed growth.

Contrary to rumor, pine straw does not acidify the soil. The needles break down to a soft, fluffy layer and a new layer can be placed right on top each year.

Pine straw is available locally at independent nurseries such as Buchanan's, The Arbor Gate, and Joshua's. Ask your local nursery or landscape professional to order pine straw for you if they do not stock it already.

This column is produced by Urban Harvest. Learn about gardening classes, community gardens and orchards, farmers markets and more at Urban Harvest’s website at

The article is written by Angela Chandler and Heidi Sheesley. Chandler is an instructor at Urban Harvest and several local nurseries around Houston and is knowledgeable in vegetable, fruit gardening, as well as composting and low volume irrigation. Sheesley is the owner of TreeSearch Farms Inc., a wholesale grower of perennials, natives and unique plants.

Angela may be reached at The Garden Academy. Heidi may be reached at Tree Search Farms website.