Prevent weeds, improve soil and save water by mulching your garden

This article has been updated. Please check our website for the most recent story.
Last Updated: 
July 2, 2007

CORVALLIS, Ore. – There are many reasons to use mulch in your garden. Mulch keeps weeds down, improves soil structure and makes your garden more attractive.

Another big plus for using mulch is that it prevents water loss and conserves the amount of water you will have to use in the summer.

A mulch is any material you place over garden soil to affect either the soil or the plants growing there. Organic mulches are derived from plant materials that eventually decompose and become part of the soil. Inorganic mulches include both plastic sheeting of various kinds and gravels.

"Different kinds of mulches have very different effects," explained Neil Bell, horticulturalist with the Oregon State University Extension Service. "In some cases a mulch may actually do the opposite of what you intend it to do. So it's a good idea to do a little homework before you make a choice."

Foot traffic, weather and even the impact of raindrops tend to compact the top layer of bare soil, making it more likely that rain will run off rather than soak in. A good mulch reduces the compaction and allows the water to infiltrate the topsoil layer slowly and effectively. Organic mulches, such as wood chips, compost, sawdust or even straw, usually allow soil to be more absorbent to rainwater than bare soil.

Once the soil layer is moist, a layer of mulch helps to keep it from drying. With bare soil, the heat of the sun causes the top layer of soil to heat up, causing water in the soil to evaporate. As the moisture in the top layer of soil evaporates, more moisture wicks up through the soil to replace it, until the soil is essentially dry. A coarse organic mulch layer reduces soil temperatures. In addition, water can't wick well across the interface between the soil and mulch. So the surface of the soil stays moist longer and much less water is lost to evaporation.

A layer of mulch can cut down evaporation by as much as 75 percent, according to Bell.

Gravel can also be an effective mulch to conserve water. It lets rain or irrigation water through readily and reduces evaporation from the underlying soil surface. White gravel, because it reflects the sun's heat, is usually the most effective gravel at keeping soil temperatures down, consequently reducing evaporation.

Woven plastic sheets called geotextiles or landscape cloth allow water and air to penetrate the soil and effectively reduce evaporation. They're also good at suppressing weeds. On the other hand, they usually need to be covered by an additional layer of organic bark for aesthetic reasons, and they don't eventually decompose and improve the underlying soil the way organic mulches do.

"Beware also of weeds that can germinate and grow on top of the cloth in the decomposing mulch," warned Bell. "Plastic sheets, while effective at weed control, are not good at allowing water and air to penetrate the soil."

The best time to apply mulch for conserving water is while the soils are still well charged with winter moisture in the spring. Once they've dried out thoroughly in the summer heat, much of the benefit of the mulch is lost for the season.

Here are some things to do before mulching. If the soil is compacted, roughen it up to aerate it and make it easier for water to penetrate. Add any soil amendments that you're planning to use before placing the mulch.

A two-inch layer of organic mulch is usually enough for weed suppression and moisture retention. If you already have a layer of mulch in place, wait until it's decomposed to add more. Try raking it up to aerate it and freshen its appearance. Do not apply too thick a layer, which can keep oxygen from getting to the roots of shrubs and trees.

While mulches can be very effective in reducing evaporation, they can't stop it altogether. So if you want to conserve water, don't forget to group plants together according to their need or lack of need for water. Choose new plants that are drought tolerant. Plan and plant for shade in your garden.

Author: Davi Richards
Source: Neil Bell