Appalachia--Science in the Public Interest

MULCHING

Pat Stone

A good mulch can smother weeds, conserve moisture and improve soil.

1. Inorganic Mulches Plastics. Black polyethylene plastic is a synthetic mulch that can be applied to sufficiently moist soil after cultivating, fertilizing and weeding around the plants. It should be weighed down to keep it from blowing away. Black plastic absorbs heat and will give the vegetables a jump start. It can also ward off certain soil-borne diseases including early tomato blight. But it can be excessively hot and burn out plants. Clear plastic warms soil even more through a greenhouse type effect, but it encourages weed growth. Infrared transmitting (IRT) plastic lets infrared light through but blocks photosynthesis-producing light. Brent Loy of the University of New Hampshire, Durham Plant Biology Department, has almost doubled early melon yields and boosted total melon yields 30 percent (over black plastic-mulched crops) using a version of IRT, called IRT-76.

Landscape fabrics, or geotextiles. Geotextiles are fibrous mats that let water and air through but block most weeds. (brand names include: Miracle Mulch, Weed-X, Weed Barrier, Earth Blanket, Weed Block, and Weed Mat.) Frequently, ornamental gardeners will lay a geotextile around their plantings, then cover it with an attractive organic mulch like cedar chips or pine bark. This results in very good weed control and more natural and attractive surface appearance.

Aluminized mulches. Aluminized mulches confuse aphids (which often carry plant viruses). In various field tests, these mulches have doubled yields of squash, melons and cucumbers -- crops frequently plagued by aphid-borne disease. They have also reduced thrip infestation, markedly improving tomato and pepper yields (both crops are susceptible to thrip carried tomato wilt spotted virus). There is even a new shiny gray and black mulch that combines the repellent quality of aluminized mulch with the heat-retaining capability of black mulch. (Home gardeners can simulate such materials fairly well with aluminum foil* and black plastic). * Ed. note: or less costly aluminum paint.

Plastic mulches are ugly, use petrochemical resources and stop an overlay of organic mulch from reaching - and improving - the soil. Plastic mulches encourage shallow root growth, making trees and shrubs more susceptible to drought and wind damage. If the material doesn't "breathe," it can also promote root disease or a lack of oxygen at ground level. It biodegrades and is a disposal problem for both landfill and incinerator (neither is desired). However, Hortopaper and Planters Paper are made from renewable resources and degrade naturally into benign soil- building components.

2. Organic Mulches

Organic mulches are attractive, help build up the soil, are made from recycled yard and kitchen waste and do not pose disposal problems. Their only drawback is they can harbor slugs. However, organic mulches have advantages which off-set this drawback. For example, a Tennessee researcher found a dramatic increase in spiders and between 60 - 70 percent less plant damage when she compared mulched first-year gardens to unmulched ones. Another scientist, at Virginia Polytechnic Institute, observed that a good layer of wheat straw or grass hay around potato plants reduced damage from Colorado potato beetles.

Mulch containing weed seed will have detrimental effects if anaerobically fermented. Using sour mulch that has fermented anaerobically instead of composting properly should be avoided. Sour mulch can have a very low pH (1.8 to 3.6) while normal mulch usually varies from 6.0 to 7.2 (much closer to neutral). Black walnut wood chips can spread jugalone, a growth inhibiter, and should not be used in mulch. Some kinds of fresh, raw leaves contain growth-inhibiting phenols that are particularly harmful to brassicas and fresh grass clippings can "burn" tender seedlings, if the grass is piled up too close to the plants. Mulch will cool spring growth and should not be applied around heat-loving plants until it warms up.

3. Mulching Suggestions

Raspberries -- Cardboard mulch works well for raspberries. Cornell University researchers found that straw-mulched raspberries bore almost twice as many berries as ones where weeds were controlled by black plastic mulch, herbicides, or hand- weeding. They credited the dramatic difference to "improved soil moisture status under the mulch, and the fact that root systems were undisturbed."

Asparagus -- Since asparagus is harvested only in the spring, why weed it all year? After the last harvest, weed the bed thoroughly, then mulch it heavily with clean hay or straw. In the following spring, pull the mulch back so tender young spears can grow unimpeded. Another option would be to sow annual rye grass seed thickly in the bed after the last spring weeding. This will choke out summer weeds.

Strawberries -- Mulch and perennial crops go hand in hand. A four- to -six-inch layer of weed-free mulch will save a lot of work, weeding strawberries repeatedly all summer.

Potatoes - Some people lay potatoes on the ground surface and cover them with about a foot of hay, eliminating the need for digging.

Corn -- Once it is knee-high,corn should be mulched. This will allow the plants to develop undisturbed and avoid compacting the soil during the most important stages of growth.

Beets -- If beets get leafy spot they should be mulched. This disease spreads through soil splash.

Cabbage -- To help prevent cabbage root maggots the base of the young plants can be mulched with an inch or two of sawdust which prevents flies from laying their eggs there. (Most people do not mulch large sections of their gardens with sawdust. It is acidic and uses up nitrogen. However, it can be used in circles around plants).

Tomatoes -- Problems with fruit cracking and blossom-end rot are frequently a reaction to water stress. A good layer of mulch can help keep the soil moisture levels even.

Seedlings -- Some gardeners cover their seedbeds with a very shallow (1/2", roughly) layer of light mulch. This helps maintain the soil moist, a crucial condition for seed germination.

Living mulch -- One can grow lettuce (or clover) under taller crops to provide the benefits of mulch and producing a harvest (or build the soil) at the same time.

Grow our own mulch -- Idle areas of the garden should be put to work. An annual cover crop like rye can be planted during the growing season and winter wheat or winter rye in the fall. Cut these crops down before they go to seed to provide the next supply of mulch.

At the USDA agricultural lab in Beltsville, Maryland, researchers planted a summer crop of hairy vetch, left it over winter, mowed it down the next spring and transplanted tomatoes directly into the site. The cut vetch plants died and formed a weed-blocking mulch around the tomatoes. The result? A yield increase of 138 percent and no Colorado potato beetle problems.

Leaf mulch -- Leaves can make fine mulch, but they tend to mat unless they are chopped. A lawn mower can be used to chop leaves. If the lawn mower used is equipped with a bag, this makes collecting the mulch easy. If not, mowing can be done in a pattern that pushes the leaves together and makes them easy to gather. Another option would be to put the leaves in a large garbage can and use a string trimmer to chop them. This works best with dry leaves.

Newspaper -- Are newspapers safe to use for garden mulch? While this is still a matter of controversy, the general consensus seems to be that black-and-white newspaper is suitable, but colored paper is not. Some black-and-white papers that have been tested contained negligible amounts of heavy metals. But a USDA soil scientist has discovered harmful metals in four-color advertising inserts; barium in red ink, copper in blue and green, and manganese, cobalt, and lead in metallic inks.

Trees and shrubs -- Mulch provides another important service around trees and shrubs, protecting them from mower or trimmer injury. Mower and trimmer injuries can lead to insect or disease infestation and are the single most common cause of tree decline. A good tree-protecting mulch is about three to four inches deep and at least two feet in radius. It should be kept two to four inches from the trunk to reduce rot and mice problems.

Winter protection -- Many ornamental plantings -- including bulbs, perennials, and small shrubs and trees -- are not bothered by the cold of winter. However, the soil heaving caused by repeated freezing and thawing can disturb their roots or send them the signal to grow too early. To prevent this, a thick layer of mulch can be laid over the plants after the top two inches of ground have frozen hard. (If it is put on too early, mice may move under the mulch and nibble the bark of the plants being protected). This can be pulled back in the spring so the soil can warm up for the bulbs and perennials.

Ornamental mulches -- Many ornamental growers mulch all the plantings they can. Probably the most popular mulches among these growers are pine bark, hardwood bark, cedar chips, longleaf pine needles, and shortleaf pine needles. Of these five, pine bark and longleaf pine needles last the longest, according to tests done at North Carolina State University. But all require occasional replenishment.

How thick? -- The denser the mulch, the thinner it should be applied. Thick materials such as buckwheat hulls or sawdust should be applied no more than an inch or two deep. Bark will work fine at a depth of two or three inches. Hay, however, can be safely applied to a depth of six inches. Applying mulch too thickly can block water and oxygen from getting to the soil.

Nitrogen shortage? -- When carbonaceous material breaks down, it temporarily uses up extra nitrogen. If one applies a carbon-heavy mulch like fresh sawdust in the garden scratch some bonemeal or other nitrogenous fertilizer on the site first.

References & Sources Major Source: BackHome, "How Much Do You Mulch?" by Pat Stone, Summer, 1993. P.O. Box 70 Hendersonville, NC 28793.

The Ruth Stout No-Work Gardening Book

GARDENER'S SUPPLY COMPANY 128 Intervale Rd. Burlington, VT 05401 (Miracle Mulch, Hortopaper, IRT-76, Weed Mat) GARDEN'S ALIVE! 5100 Schenley Place, Lawrenceburg, IN 47025 (Weed Barrier) BOUNTIFUL GARDENS, 18001 Shafer Ranch Rd. Willits, CA 95490 (Planter's Paper)