Mulches for Weed Control
A mulch is any kind of material applied to the soil surface for protection or improvement of the area covered. The value of any mulch material is measured in how well it improves crop quality. The most common reason for using a mulch is to eliminate weeds or at least retard their growth. Where a mulch layer is sufficiently deep, few weeds will grow. In addition to controlling weeds, mulches also aid the optimum development of the plants that grow in the mulched areas. Other advantages include the following:
- By retarding the amount of soil-water evaporation, mulches conserve moisture, which is particularly important during droughty periods of the growing season.
- Mulches help maintain a uniform soil temperature. They act as insulation to keep the soil warmer during cool spells and cooler during the warm months of the year. By maintaining uniform soil temperatures, they retard freeze-thaw cycles during winter and reduce heaving of perennial plants. Strawberries, for example, should always be mulched in the winter to help them survive the extreme cold temperatures. In the spring, the straw used to cover the plant can be pulled back between the rows to serve as an additional mulch to facilitate harvesting.
- As mulch materials gradually become mixed with the soil, they increase the water-holding capacity of light sandy soils and increase the aeration of heavy clay soils.
- Organic mulches serve as "food" for many microorganisms in the soil. During decomposition of the organic material, soil microorganisms secrete a sticky material that promotes the granulation of the soil. The mulch also maintains more stable temperatures, so the activity of the microorganisms can prevail at an even rate.
Although the advantages of properly used mulches far outweigh the few disadvantages, several inherent problems might occur when mulch materials are used as a form of weed control around plants. Once some of the following limitations are understood, mulches can be adapted to most situations.
- Mulch materials such as hay, straw, and strawy manure might introduce additional weed seeds into the planting area. Hulls or corn cobs might contain grain or seed that could germinate in the mulch layer.
- Several materials used for mulching, including wood chips, fresh sawdust, crushed corncobs, straw, and shredded bark, require the addition of fertilizer to reduce the chance of nitrogen deficiency in the growing plants. Yellowing of the plant foliage during the growing season can indicate the need for additional fertilizer.
- Some organic mulches, especially the fresh types that have not been composted or decomposed, can heat up quickly when damp. This heat, created by the decomposition process within the mulch layer, can "cook" the bark or stem of a plant if it comes into direct contact with the mulch.
- In the late part of the growing season, mulch can also cause the problem of improper hardening-off of the trunk at ground level. To harden off, fruit tree stems must be subjected to gradual temperature changes as the days become colder in the fall. When mulch material covers the base of the stem or trunk of a tree, the covered portion is not able to respond to the seasonal temperature change. The covered part remains in the same condition as it was during the summer growing season, and when the air temperature drops to the freezing point, this plant tissue is not ready for winter and can be injured or killed.
- Plastic films used as mulches are quite effective, but also have some limitations. Problems can occur with plastics when very large areas are mulched. Under these conditions, the amount of water that actually enters the soil as rainfall or irrigation might be reduced. It is important to punch small holes every few yards in large areas of a plastic mulch layer to ensure that moisture penetrates into the soil. Weeds generally will not grow through these openings.
- It also is important to properly grade areas to be mulched with plastic films, in order to prevent excess surface water from collecting in one location and flooding or saturating the soil in that area.
- Mulches around fruit trees provide an ideal haven for voles, which can eat the bark at the base of a tree. If mulch is being used around apple and pear trees, it should be pulled away from the base of the trees in late October.
One final possible problem with mulches involves the area of soil rather than the material itself. Mulching is not recommended around plants in soils that tend to remain too wet. A mulch will only aggravate the waterlogged soil by preventing evaporation of the excess water.