A mulch is any substance, usually an organic material, that is spread on the ground around plants to conserve moisture, prevent erosion and control weeds. A mulch may also be used to cover low-growing plants and provide winter protection. See Protecting Plants.
Mulching, as practiced in gardens, is an adaptation of a natural process. Nature mulches the roots of plants with fallen leaves and with the tops of herbaceous plants and ground covers. There is no repeated cultivation of the surface soil (clean cultivation) where plants grow naturally.
The popularity of mulching as a garden practice has increased greatly in recent years, and its usefulness is generally limited only by the availability of suitable materials at an economical cost. Fruit growers, especially growers of Apples and small fruits, use mulches extensively. Mulching is especially useful to the small commercial growers who do not have the expensive tillage machinery needed for clean cultivation. An advantage to them is that the mulch may be applied during slack periods, whereas cultivation must be done at busy times in spring and summer. Home gardeners more and more are learning to appreciate the value of mulches for flowers, vegetables and shrubbery.
Advantages of Mulching
Mulching has several substantial advantages over clean cultivation as a method of managing the soil in the garden and orchard. These advantages are discussed here for the gardener who may be considering whether to adopt this practice or not.
The greatest advantage of a mulch is the conservation of soil moisture that results from its application. Evaporation from the surface of the soil is greatly reduced by protecting the soil from the direct rays of the sun and moving air. Rain which falls on the mulch is all absorbed and does not pack the soil surface. Erosion, a great destroyer of soil, is eliminated.
A mulch of wood chips around a newly planted tree.
The moisture content of a mulched soil varies much less than that of a clean-cultivated soil in dry seasons, and this uniformity favors root activity and brings other advantages. Uniform growth of Tomatoes, for example, eliminates cracking and blossom end rot, a trouble caused by a fluctuating moisture supply. The size of tree fruits ripening in late summer or fall in a dry year is substantially larger where a mulch is used.
The second important advantage is the control of weeds, particularly annual weeds. Strong-growing perennial weeds are not controlled by any mulch that it is practicable to maintain in a garden, except an overlapping mulch of special paper or aluminum foil. A mulch greatly reduces the amount of weeding required in the garden. I£ used in conjunction with a weed killer that kills germinating weed seeds, the benefits in weed control will be greater.
Lower and more uniform soil temperatures in summer follow mulching, and these favor bacterial activity in the soil. High summer temperatures may injure beneficial micro-organisms as well as roots close to the surface and speed the destruction of humus.
Frost penetration under a mulch is much less than where the ground is bare. Where winters are severe and a good snow cover is lacking, the insulating effect of a mulch may prevent serious injury to tender rootstocks and to evergreens that of necessity must absorb moisture from the soil in winter and cannot do so if the soil is frozen to a great depth.
Most mulches improve soil structure and tilth because the decaying organic matter works down into the topsoil. The increased friability (crumbly condition) of the soil favors water penetration, and the better aeration that follows stimulates both biological activity in the soil and growth of the roots. Materials which decay rapidly bring these benefits about more quickly than slowly decomposable materials. Chopped Alfalfa is, for example, much superior to peat moss in achieving a desirable crumbly condition of the soil.
Decaying mulch materials add plant nutrients to the soil. The amount and variety of these vary greatly with the type of mulch used. Some mulches, such as peat moss, Buckwheat hulls and shredded Redwood bark, provide little in the way of actual nutrients; a mulch of partially rotted manure is a rich source of nitrogen as well as other nutrients, and Tobacco stems afford a good supply of potash in addition to some nitrogen and phosphorus. Hay, often favored as an orchard mulch, is an excellent source of plant nutrients and so is good compost, being formed of a variety of vegetable wastes such as leaves, weeds and grass.
A mulch of hay, straw or other organic material has a very beneficial effect on the potash content of the soil and its availability for plant growth. In a clean-cultivated soil, the potash is fixed in an unavailable form in the surface soil and moves downward very slowly because of alternate wetting and drying. In a mulched soil, fixation is very slow under the continuous moisture under the mulch material, and the potash moves downward to the root zone, where the plants can get it. If potash is the limiting nutrient in fruit tree growth and production, or in the growth of other plants, it may be supplied quickly and effectively in conjunction with a mulch.
A mulch prevents the packing of heavy soils either by vehicles or by their being walked upon soon after a rain. This is especially beneficial in the case of clayey soils, which may easily be permanently damaged by such traffic.
Root injury from surface cultivation, which can be much more extensive than is generally thought, is eliminated when a mulch is used. Feeding roots develop in the topsoil just under the mulch. This is normally the best-quality soil, and a mulch makes it possible for the plants to make full use of it. This is especially important with surface-rooting plants such as Rhododendrons, Mountain Laurel, Azaleas and Blueberries.
Some mulches, especially peat moss, Buckwheat hulls, Pine needles, shredded Redwood bark and some others, improve the appearance of the garden, and are especially valuable for covering beds near the house or in other areas where neatness is an important factor.
A mulch prevents the bruising of fruits that drop and land on it rather than on stones, hard ground or stubble. A mulch keeps soil from splashing on Raspberries, Gooseberries, Strawberries, Daffodils and other fruits and flowers that are near the ground.
Mulched orchards can be sprayed earlier in wet weather in the spring, as the spray rigs can get around when they cannot on cultivated ground. The mulch prevents muddy shoes when one is walking among the flowers in the spring or after a heavy rain.
Preventing Winter Injury. Mulches are used to protect low-growing, evergreen plants and somewhat tender plants, including many bulbs, from winter injury from low temperatures when there is no snow cover. Placed around plants newly set out in the fall, a suitable mulch will prevent them from being heaved out of the ground by alternate freezing and thawing during the winter.
Disadvantages of Mulching
The cost and lack of availability of some mulches are a drawback to large-scale mulching, especially on farms. In small gardens, the costs are not generally excessive for the benefits and the area involved. With straw, hay and some other mulches, however, there is a fire hazard. A lighted cigarette carelessly dropped among pine needles, salt hay or dry leaves used as a mulch about shrubs can easily result in the loss of valuable plantings.
Mice are often more serious among mulched plants than others, but they may be controlled by poisoning and trapping. Nitrogen starvation sometimes occurs when sawdust and straw are used as mulches, but this is easily corrected by using additional nitrogen as a fertilizer. If mulches are applied too early in the year to crops that need a warm soil, such as Tomatoes and Corn, growth may be delayed.
These disadvantages, however, are minor compared to the benefits derived from mulching.
A suitable mulch for extensive use, to be worth while economically, must be available in quantity from, nearby sources. Mulches are grown or purchased, or sometimes may be had for the hauling. Many materials are suitable, but only a few are generally available at reasonable cost. Some of those listed are available only from processors in certain areas. The user of mulches should search out cheap, local sources. Something that may be had free from a nearby source at the user’s convenience is probably the most economical material to use if it is suitable for the plant and area to which it is to be applied.
Hays of various types are available wherever grasses are grown. They are suitable for orchards and for use around newly planted trees. Because of their appearance, they are less suitable for many garden purposes than other neater-appear- ing mulches. Rain-spoiled hay from a dairy farm and roadside hay from the highway department are often available. Orchardists often grow hay between the trees in a young orchard, fertilize it heavily, mow it and rake it around the trees. Land not suitable for fruit because of poor drainage or frost hazard is sometimes used to grow hay for mulching.
Orchard grass will grow on a wide range of soils and, if heavily fertilized with nitrogen, will furnish a nearly complete supply of nutrients for fruit trees, in addition to its mulching effect. Timothy is a high-yielding grass for good soils, and Reed Canary Grass will produce heavily on wet soils not suitable for other crops.
Millets, Sudan Grass and wild grasses from marshlands are all good mulches. Siberian and German Millets sown about June 1st, cut September 1st, and followed by Rye which is cut in the spring, will produce a lot of mulch if fertilized heavily.
An acre of land is needed to produce the mulch for an acre of fruit. Hay is excellent for fruit and the larger vegetables, but not for the flowrer garden.
Legume hays, Alfalfa, Clovers and Soybeans are high in nitrogen and are valuable sources of plant nutrients, besides being good mulches. Pea vines are high in nitrogen, and are suitable orchard mulches. With hay, particularly well-fertilized hay or legume hay, the mulch can provide most, if not all, of the nutrients needed in addition to those available to the plant from the soil. Heavy mulching with hay builds up a reserve of available nutrients which lasts for several years. In fact, with fruit plants the money spent on fertilizers and tillage might even be expended to better advantage for mulching materials.
In one experiment, mulched Apple trees substantially outyielded cultivated trees, even when the mulched trees were not fertilized and the cultivated trees were fertilized. The profitableness of this practice will depend on local circumstances, the availability of mulching material at a reasonable cost, the labor situation, moisture relations and soil fertility in the fields to be mulched. Straw is lower in nutrients than hay, but furnishes considerable potassium. The other mulches are mostly low in nutrients.
Wheat, Rye, Oat and Buckwheat straws are good mulches but furnish less nutrients than hay. They constitute a fire hazard and may bring in weed seeds, especially Bindweed. The insulating value of straw is good, which makes it excellent for preventing deep freezing of the soil and providing protection for low-growing plants. The application of supplementary nitrogen in the form of fertilizer may be needed during the first three or four years of a straw mulch. Shredded cornstalks are good when available.
Tree leaves are available almost everywhere and are excellent for fruit trees, shrubbery and evergreens, but they mat down too much for perennials and bulbs unless used thinly. Leaves that have rotted down to leaf mold are excellent wherever a mulch is needed. Leaves rot rapidly, and a covering 8 or 10 in. of fresh leaves will be nearly all gone in twelve months.
Because of their value as mulch material, the burning of leaves is a wasteful practice. They are a valuable source of humus, and gardens are much in need of the organic matter they can supply. Leaves contain as much plant nutrients per pound as does manure of the same moisture content. For these reasons every gardener should collect all leaves possible and either compost them or use them as mulches.
Oak leaves mat down less than the others and are especially recommended for Rhododendrons and Azaleas. Pine needles are often collected in Pine forests for mulching Strawberries, and they are equally good for the flower garden. They do not mat down and are fairly durable. In the southern states, Spanish Moss is sometimes used as a mulch, and it may be had for the gathering.
Peat moss is generally available from garden supply stores, and is a good mulch, but is generally too expensive, except for the flower garden and ornamental plantings. It is attractive to look at and durable; however, when dried, it intercepts most of the rain from light showers. When wet and frozen, it loses its insulating value, but it is a good insulator when dry.
Sawdust is being used in increasing amounts as a mulch material as it becomes known that it is not toxic to plants and does not make the soil acid. It does cause nitrogen starvation, because the bacteria which rot the sawdust draw heavily on the soil nitrogen for their life processes. Supplementary nitrogen should be added to the soil at the rate of 110 lb. of ammonium nitrate, or its equivalent in some other nitrogen fertilizer, to a ton of dry sawdust. The need for supplementary nitrogen is indicated by the development of a yellowish color in foliage which should be dark-green.
Sawdust is very low in plant nutrients, decomposes slowly, and tends to pack down, so that light rains penetrate it with difficulty. For mulching purposes it matters little whether the sawdust is fresh, or well-rotted, or from hardwood or softwood trees.
Sawdust is excellent for Blueberries, Rhododendrons, Azaleas, Heathers, and other members of the botanical family Ericaceae, as well as for bulbs and small plants which push through it more readily in the spring than they do through the coarser mulches.
Wood shavings are similar in most of their properties to sawdust, but mat down less and tend to blow away in exposed situations. Wood chips from chipping machines are a new product available in some places. Other wood materials are shredded Redwood bark, excelsior and Balsam wool.
Spent hops from a brewery are one of the best mulches. They ignite with difficulty and are durable; moreover, the beery odor, which may at first be unpleasant, is soon dissipated. Grape growers use the pomace (Grape refuse) from wineries and grape-juice processing plants in their vineyards.
Ground corn cobs are an excellent mulch. On Roses, they have proved more effective than peat moss in conserving soil moisture and in increasing the organic matter of the soil. They have a pronounced beneficial effect on soil structure.
Peanut shells, cocoa bean shells, coconut fiber refuse, shredded sugar cane fiber, shredded tobacco stems, shredded banana stalks and cotton screenings are other possibilities as mulch materials.
Manure is sometimes used for mulching purposes, but often contains weed seeds and may cause the soil to become too fertile for some plants. It should be used on crops responding to high fertility, such as Rhubarb, and on trees and shrubs that need fertilizing as well as the other benefits that a mulch supplies.
Special mulch paper was tried extensively as a mulch for vegetables in the 1920’s, but it was expensive and did not stand traffic well. It did increase the yield of hot-weather crops such as Peppers and Tomatoes, but soon passed out of fashion. Aluminum foil is also effective, but is not likely to be used widely because of its comparatively high cost. Special plastic films are also available for mulching.
Glass wool is a good winter mulch for small plants which remain green through the winter; light passes through it and it gives good protection without matting down. Vermiculite is also suitable for small-scale mulching. Newspapers and flattened cartons have a smothering effect on weeds and conserve moisture. If used, they should be covered with something that is better- looking than themselves and will keep them from blowing around.
Stone chips or gravel is used around rock-garden plants and is very satisfactory. As a mulching material, flat stones have their advocates. They are certainly durable, and no weeds or moisture can get through them, but obviously their usefulness is very limited. These nonor- ganic materials do not, of course, supply plant nutrients or humus.
Ground-cover plants provide the shading effect of a mulch for bulbs and shrubbery and also some cover for winter. Evergreen boughs laid on the ground provide some protection and hold the snow; this is important because the snow itself is one of the best of all winter mulches, although it unfortunately does not form a dependable, lasting cover in most areas.
Managing the Mulch
A mulch may be applied whenever convenient for fruit trees and shrubbery. If there is a fire hazard, late fall application is safer, as the mulch will soon be wet from rain and snow, and partly rotted by the time dry weather arrives in the spring.
Perennials and annuals should be mulched after they are nicely started in the spring. Fall- planted bulbs should be mulched any time after they are planted, but before deep freezing occurs. Tomatoes and vine crops should be mulched before they spread out, but not until the soil has warmed. No warm-weather crops should be mulched until the soil is warm and the plants are nicely started.
Mature Apple trees need 200 to 300 lb. for the first mulching and 100 lb. a year thereafter. A depth of 6 in. of mulch is necessary to keep down weed growth.
Sawdust, peat moss and similar materials are applied to a depth of one to several inches, depending on the crop or plants to which they are applied and the amount of mulch available. An inch is sufficient for moisture conservation, but two or three inches or more will give better weed control. If the mulch is in short supply, it should be used on dry areas and on slopes subject to erosion.
Fruit trees and other trees and shrubs should be mulched as far outwards from their trunks as the limbs spread. With old trees the bulk of the mulch should be under the outer portions of the more far-reaching limbs, as the shade will keep down weed and grass growth near the trunk and the main feeding roots are out towards the limit of the branch spread.
Nitrogen. With sawdust and straw, supplementary nitrogen may be needed for three or four years to prevent nitrogen starvation. With hay, especially legume hays, the nitrogen content of the soil may increase to the point where Apples color poorly and drop excessively, and tree growth is overvegetative. The mulch should then be reduced and some grass allowed to grow up through it to use the excess nitrogen. Straw and hay, especially near highways and public areas, are likely to be fire hazards. Cultivated strips should be left between the trees to serve as fire breaks.
Mice are often a problem in a mulched orchard and among mulched trees and shrubs. The trees may be protected with mounds of coarse gravel, crushed stone or cinders 5-6 in. high and 3-4 ft. in diameter. Wire guards around the trunk furnish good protection. Poison baits should also be used in mulched orchards. These may be obtained from local farm bureaus in fruit-growing regions.
Special Hints on Mulching. Planting sites for trees may be mulched to good advantage a year before planting the trees. The mulching kills out the grass and weeds and may have other benefits: the trees grow very well when planted on a mulched site.
A mulched vegetable garden on heavy soil should be plowed in the fall or the mulch removed early to permit the ground to dry out for spring plowing. Turning the mulch under adds organic matter to the soil.
Fall-bearing Raspberries respond well to a mulch, as the fall crop ripens during the driest part of the year.
Strawberries are mulched to protect the crowns from low winter temperatures, to keep the berries clean, to conserve moisture and keep down weeds. Wheat, Rye and Oat straws and marsh hay (salt hay) are the best materials, but others may be used. The mulch is spread on the bed to a depth of 3 or 4 in. in late fall after the plants have experienced 2 or 3 hard frosts, but before temperatures have dropped below 20 degrees F.
Everbearing Strawberries are best grown in hills and mulched with an inch of sawdust, which is applied a few weeks after planting. No runners are allowed to set, and weeds are removed by pulling. Yields are much heavier than with the clean-cultivated matted row.