Mulches around a plant help maintain moisture and soil temperature as well as inhibit weed growth. (Photo by Rick Bogren)

Pine straw is a preferred mulch for many applications. (Photo by Allen Owings)

A variety of landscape mulches are evaluated at the LSU AgCenter Hammond Research Station. (Photo by Allen Owings)

For Release On Or After 04/03/15

By Dan Gill
LSU AgCenter Horticulturist

Mulching is an easy-to-do labor-saving gardening technique that all gardeners should take advantage of. A mulch is a material, usually organic but sometimes inorganic, we use to cover the soil surface around plants. Mulching beds is an important part of sustainable landscaping.

Organic mulches, such as leaves, chopped leaves, pine straw, ground pine bark, dry grass clippings and newspaper, are all derived from once-living materials. They are popular for their ease of use, attractive appearance (except for newspaper) and because, as they decompose, they add beneficial organic matter to the soil. They are the most popular mulches.

Inorganic mulches are derived from nonliving sources and include such materials as plastic sheeting, landscape fabric or weed barriers, stone chips or gravel. Rubber mulch made from recycled tires and synthetic pine straw are inorganic mulches that have the look of organic mulches but last longer.

Some of these mulches, like black plastic, are not very attractive and are only suitable in more utilitarian situations such as a vegetable garden. In more decorative areas, unattractive inorganic mulches, such as landscape fabric, may be covered with a layer of organic mulch for appearance’s sake.

The first and foremost reason to use mulches is to control weeds. Whoever said, “A job well done doesn’t have to be done again” never weeded a garden. Every time weeds are removed, new weed seeds germinate, creating the problem all over again. Mulches work to stop this by blocking light from reaching the soil surface. Most weed seeds need light to germinate because this tells them they are close enough to the soil surface to sprout and grow. When covered over with mulch, they think they’re still deep in the soil and will not germinate.

To create this barrier to weed growth, organic mulches have to be applied thick enough to do the job. Too often, gardeners spread mulch as thin as possible just to cover the soil. This will be ineffective in preventing weeds. Apply organic mulches at least 2 inches thick for best weed control.

Organic mulches are not as effective in controlling persistent perennial weeds such as nutsedge, oxalis, Bermuda grass, torpedo grass and others that grow from below-ground bulbs or rhizomes or run into beds from surrounding areas, but they can help. Woven weed barriers or landscape fabric often does a better, though not perfect, job controlling these difficult weeds.

Another important function of mulches is conserving moisture in the soil. By slowing evaporation from the soil surface, mulches keep beds from drying out as fast. This is especially important in hot, dry weather. Your plants receive a more even supply of moisture, and you save money on your water bill. Shallow-rooted plants with limited root systems, such as bedding plants and vegetables in sunny areas, particularly benefit.

Organic mulches also insulate the soil and moderate soil temperatures – keeping the soil warmer in winter and cooler in summer – which helps the roots. They can even reduce freeze injury to whatever part of the plant they cover in winter.

Black plastic used in the vegetable garden during winter and early spring helps warm the soil by absorbing the heat of the sun. This keeps winter vegetables growing vigorously and allows for earlier planting of spring vegetables. As the weather warms in April, black plastic mulch should be covered with an organic mulch to shade it and prevent excessive heat buildup. Black plastic should not be used to mulch permanent plantings, such as shrubs or around trees.

Have you ever worked hard to turn the soil in a bed until it is nice and loose only to watch rain and watering beat it down again until it’s just as hard and compacted as it was before? You will find that if you mulch as soon as you finish bed preparation and planting, the mulch will substantially prevent soil compaction. A looser soil absorbs water faster and is easier for roots to grow through.

No one mulch is best. Which one you choose depends on a variety of factors, including the gardening situation, your preference based on appearance, what’s available, cost and durability. I like to recycle yard waste, such as leaves and dry grass clippings, and use them as-is or allow them to partially compost and then use them. It’s cheap (free), effective and attractive. If you’re lucky enough to have access to free pine straw, it makes ideal mulch. You can also purchase bales of pine straw rather economically or chopped pine straw by the bag.

If you aren’t currently using mulches in you gardening efforts, I strongly recommend you give them a try. You’ll be amazed at how much work they save you weeding and how nice they can make a garden look. If you are mulching, remember that their primary function is not just decorative and apply mulches thick enough throughout your landscape beds and gardens.

Rick Bogren