Reviewing the use of landscape mulch

Richard Bogren, Young, John, Gill, Daniel J., Owings, Allen D.  |  1/4/2011 1:07:59 AM

Sustainable Landscape News Distributed 01/15/10

By LSU AgCenter Horticulturists Dan Gill, Allen Owings and John Young

Mulching is a great sustainable landscape practice when done correctly. The new year is a good time to review the use of mulch in the landscape and how to apply it properly to achieve the maximum benefit.

A mulch layer around trees, shrubs and planted beds and covering bare ground provides many benefits. In areas that are difficult to mow, irrigate or otherwise maintain, mulch can replace turf or groundcovers. It’s also appropriate in shady areas where plants don’t grow well.

A few reasons to use mulch include:

– Organic mulch materials improve soil fertility as they decompose.

– Mulch buffers soil temperature, keeping soils warmer in winter and cooler in summer.

– Mulch helps maintain soil moisture by reducing evaporation. A layer of mulch also minimizes water needs for established plants.

– Fresh mulch inhibits weed germination and growth.

– Over time, many types of mulch improve soil aeration, structure and drainage.

– A mulch layer can inhibit certain plant diseases.

– Mulch around trees and shrubs – but not against the trunk – eases maintenance and reduces the likelihood of damage from string trimmers.

– Mulch gives planting beds a neat and uniform appearance, adding a contrast of color and texture that complements plantings.

Follow these tips when adding mulch to your landscape:

– For well-drained sites, apply a 2-inch to 3-inch layer (after settling) of mulch around trees, shrubs and bedding plants. With drainage problems, use a thinner layer. Coarse materials, such as pine nuggets, may be applied to a depth of 4 inches, but don’t allow mulch to accumulate deeper. If mulch is already present, check the depth. Do not add mulch if 2 to 3 inches already are in place.

– “Volcano mulching,” or mulch applied too deeply, hinders oxygen exchange to roots, which stresses the plant and causes root rot. Do not place mulch on top of a tree’s root ball or against the trunk. More than about 1 inch of mulch on the root ball of newly planted trees and shrubs can stress plants because mulch can intercept water meant for the plant.

– If mulch is piled against a tree trunk, pull it back several inches to uncover the base of the trunk and the root flare. Mulch piled against tree trunks holds moisture against the trunk and stems, and trunks that remain constantly wet are prone to root rot. Mulch piled high against the trunks of young trees also may create habitats for rodents that chew the bark and can girdle the trees.

– Bring the mulch layer out to a tree’s drip line – the outer edge of the canopy – or beyond to at least an 8-foot diameter around the tree. Remember that in a forest environment, a tree’s entire root system (which usually extends well beyond the drip line) would be mulched naturally.

– Thick blankets of fine mulch can become matted and may prevent water and air from seeping through or become like potting soil and support weed growth. Rake old mulch to break up any matted layers and to refresh the appearance.

– Organic mulches may require weeding and replenishment once or twice a year to maintain a total depth of 2-3 inches.

– Shell, crushed stone or pebbles can be used as mulch, but they won’t contribute to the soil’s nutrient and organic content or water-holding capacity. Limestone and shell both raise soil pH. They also reflect heat, increasing the water needs of plants.

How much mulch do you need? Bulk mulch is sold by the cubic yard. To calculate the amount of mulch you need, measure the area to be mulched in square feet and multiply this by the depth you need in feet. For example, 3 inches equals 0.25 feet. Then convert cubic feet to cubic yards by dividing by 27. To cover a 100-square-foot area to a depth of 3 inches, for example, you will need .926 cubic yards.

Visit LaHouse in Baton Rouge to see sustainable landscape practices in action. The home and landscape resource center is near the intersection of Burbank Drive and Nicholson Drive (Louisiana Highway 30) in Baton Rouge, across the street from the LSU baseball stadium. For more information, go to www.louisianahouse.org and www.lsuagcenter.com/lyn.

Rick Bogren

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