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Organic mulches: how to use them

October 30, 2001|By KEITH MAYBERRY

University of California-Imperial

County Cooperative Extension

Top gardeners have long realized the value of a mulch layer in both vegetable gardens and flowerbeds. Some expert gardeners have become mulching experts, using them for every flowerbed, around all shrubs and

down each row of vegetables. Some gardeners use specific mulches for each site and never throw anything organic in the trash can.

Mulches conserve moisture by reducing the amount of soil water lost through evaporation. In addition, mulches help maintain a uniform soil temperature.


They act as insulators, keeping the soil warmer during cool weather and cooler during the hot months of the year. Mulches aid in improving water penetration. Mulches help with weed problems. If the mulch material is weed-free to begin with, and if it is applied correctly, weed seeds in the soil won't germinate. Mulches often give a neater and more finished appearance to a flowerbed or the vegetable garden.

Mulches increase availability of certain nutrients to plants. In addition, in the vegetable garden, mulches keep vining plants off the soil and vegetables such as lettuce and cabbage remain much cleaner.

Mulching materials are readily available if you want to gather your own. Each type of material has its merits and its disadvantages. Here are some possibilities:

Grass clippings are readily available when the lawn is actively growing. Using grass clippings has several disadvantages, however. If the clippings are applied too deeply and packed too tightly they will release heat and a foul odor as they decay. Avoid clippings from lawns that have been treated with weed killers. These herbicide residues can injure tender plants. If the clippings are weed free and allowed to dry such as on a driveway for a few hours before moving them to the flowerbed, the risk of the grass taking root or becoming a problem is greatly reduced. One expert gardener prefers grass clippings to all other types of mulches as she claims the clippings are easy to place and stay put in the wind.

Leaves are falling now and are easy to collect. However, leaves may be difficult to keep in place in windy locations. Very dry leaves can be a fire hazard, and packed, wet leaves interfere with air and water movement into the soil. By composting leaves in the fall, you can make leafmold. The material should be partially decomposed by the following spring. It is a good mulch but difficult to apply evenly and may not be very attractive. We save all leaves for mulch around shrubs.

Broken or old alfalfa bales may be obtained from your farming friends. Alfalfa makes good mulch, especially as it releases nitrogen when it starts to decay.

Peat is often called "peat moss" but this is a misnomer. Moss peat comes from mosses, while other types of peat originate from cattails, reeds, sedges and other similar water plants. Sphagnum peat moss is very resistant to decomposition and has many fibers into which roots often grow profusely. However, sphagnum moss is relatively expensive to buy.

Wheat straw is a product available locally. It is highly flammable, so do not use it where cigarettes or matches could be carelessly flipped into the material. Additional nitrogen must be applied to prevent a nitrogen deficiency when this mulch is used. Weed seeds may be introduced with this mulch. Unless the straw is chopped into short lengths, it will be difficult to apply between plants growing close together.

Strawy manure is a good mulching material and available locally. The most readily available is horse or steer manure. Nearly anyone boarding horses will be happy to give you some if you clean the pens. The only drawback to manure is initially there is a strong smell if the manure is fresh and not aged. Manure from sheep, rabbits and goats make good fertilizer but they have a high available nitrogen content and must be applied sparingly to avoid burning the plants.

Biosolids are also marketed as mulch or planting mix. This material is the product recovered from processed sewage. We prefer to use these products for ornamentals but not in vegetable gardens.

Even the Imperial Valley Press newspaper can be used as mulch. If you use newspaper, apply several layers directly on the soil and then cover it with some natural organic mulch. Some gardeners prefer to keep the newspaper a few inches away from stems or trunks of plants as newspaper is somewhat acidic. The newspaper will allow you to stretch your natural mulch if it is in short supply. Cardboard boxes may be used in the same fashion.

Organic mulches need to be applied at the proper depth to be effective. Decomposed leaf mulches and manure need to be 3-4 inches deep, grass clippings should be 4-6 inches deep, and loose material like straw, loose leaves, or hay should be 8 inches deep.

Lastly, we should mention that not all mulches are organic, there is also living mulches (plants such as vetch or clover) and synthetic mulches (plastic). These will be covered in future columns.

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