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Horticulture Questions and Answers

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What can I use as a mulch?

A mulch is any material that covers the soil surface around and under plants to protect and improve the area. Two major reasons for mulching today are to conserve moisture and to create a better planting bed. Additionally, by recycling yard waste back into the landscape we mulch our planting beds for an environmental, as well as horticultural, benefit.

Organic Mulch

Mulch comes in three types: organic, synthetic, and inorganic. The majority of mulches we use are organic materials that we recycle from our own yards. Garden centers also offer a wide assortment of packaged organic mulches. The most common organic mulches include peat or sphagnum moss, wood chips/shavings, sawdust, shredded/chipped bark, and lawn clippings.

In summer, organic mulches keep the ground cooler in the daytime. In winter, they keep the ground warmer and less subject to heaving. The real value of organic mulch is that it improves soil structure and the quality of the root zone.

In St. Louis, organic mulches should be applied around annual plantings in mid-May, after the soil has warmed up. Generally, a three-inch layer of compost, peat moss, or chipped or shredded bark is sufficient to protect against moisture loss and temperature fluctuations in the summer. If using fresh grass clippings, apply only a very thin layer of less than one inch around the plants. Preferrably, mix fresh grass clippings with leaves, wood chips or other brown plant debris, keep moist and let them decay for atleast 2 to 4 weeks before using as a mulch.

Synthetic mulches

Synthetic mulches include clear or colored polyethylene plastic films and spun or woven polypropylenes, or what is called "landscape fabrics." Unlike organic mulches, synthetic mulches tend to warm the soil in the summer by as much as 10 degrees because they magnify and trap the heat in the soil.

Colored plastic mulches will suppress weeds and may increase yields of heat-loving crops like peppers, tomatoes, and melons. They also retain more moisture than other mulch materials and for many gardeners are much more convenient to acquire and use.

Plastic mulches should be laid down at or before planting time. Plastic can warm a spring soil very quickly, allowing for early season planting if the tops are protected in some other way. Before applying a plastic mulch, the soil should be fertilized and tilled. The plastic mulch ideally should be removed at the end of each growing season.

A newly developed synthetic material made from polypropylene or polyester is permeable to air and water, but blocks light to suppress most weeds. Landscape fabrics can be used around perennials because they allow more air exchange in and out of the soil. They generally last for many years provided they are covered with some other materials, such as, bark chips or rock.

Inorganic mulches

Inorganic mulches include stones, gravel, and other rocks that are used for the same purpose as other mulches. A layer of gravel or pebbles is often applied in areas where the most durable mulch is required. Disadvantages include poor weed control and the inability to add organic matter to the soil once it is in place. Inorganic mulches are relatively inexpensive and can be considered a permanent mulch for woody planting beds.

Mulch Types for Different Crops

Organic mulches tend to keep the soils relatively cool in the spring and these work best for raising cool-season crops. Synthetic mulches tend to warm and retain the warm condition of the soil. In this case, they are best used with warm-season crops.