CORVALLIS, Ore. – Good mulch performs many functions in the garden. Mulches can reduce soil compaction and erosion, suppress weeds, keep the soil moist, regulate soil temperature and improve soil quality, fertility and texture.
The Oregon State University Extension Service Master Gardener program strongly recommends mulching in autumn.
Weed-suppressing mulches save a lot of work. An occasional weed may poke through the mulch, but you can easily pull it out. Mulches prevent loss of moisture from the soil by evaporation during warmer, drier months. Moisture moves by capillary action to the surface and evaporates if mulch does not cover the soil. Sun and wind hasten this loss of moisture.
Mulches also help regulate soil temperature, warming soil sooner in the spring or keeping it cooler in the summer, depending on the type of mulch. Plastic mulches will warm the soil; organic mulches will prevent soils from warming up as fast as bare ground, but also will stop them from getting as cold. Many plants, including those in vegetable and flower gardens, need a cool surface soil. Others, such as tomatoes and vine crops, may benefit from warmer soils.
Organic mulches condition the soil and furnish food for earthworms, which are valuable in aerating the soil. The organic matter helps to keep the soil crumbly and easy to work. Energy from falling raindrops is dissipated on a mulched soil, with less soil erosion and less soil compaction.
The end of the growing season is the optimal time to work mulch into the soil; this practice will help supply organic matter for the following growing season.
On your perennial beds and beds that are now empty of summer crops, mulch the soil surface with an organic material. Sawdust, straw and compost make excellent mulches and are easy to apply. Simply spread a 2- to 4-inch layer of one of these organic materials on the soil surface around your plants, making certain you do not cover the plants.
Adding mulch is not a one-time task. Eventually, you will need to add more mulching material over the old layers. If you are adding more mulch to an existing layer, use a garden fork to rough up and loosen existing, compacted mulch before adding a fresh layer.
Many organic materials, such as straw and autumn leaves, are low in nitrogen. Weed-free straw is excellent, but loose straw can be a fire hazard, and some people consider it unsightly. Usually, it is beneficial to add nitrogen fertilizer to straw or leaves before applying it as mulch. One cup of fertilizer high in nitrogen for each cubic foot of organic material, five pounds per cubic yard of mulch or compost, is about right. To avoid burning the plants, do not let the fertilizer touch them or use composted manure.
Be aware that some mulches may contain weed seeds. To alleviate the weed problem in manure, compost it for one year, and then use it as mulch. Avoid mulching with hay, especially ryegrass straw, unless it is thoroughly composted. Otherwise, it has too many seeds that will eventually sprout and will create a serious weed problem.
Mulching with grass clippings is a good way to dispose of the clippings, but don't use too thick a layer at one time. If you apply them too deep, they will pack and/or get slimy and prevent water from entering the soil. And if you use weed and feed type fertilizer, don’t use your cut grass as mulch, as it may contain herbicide residues.
Sawdust makes a better mulch if it is well rotted, or if you add one cup of ammonium sulfate or calcium nitrate to each cubic foot of fresh sawdust before applying the mulch. (If you mulch around camellias or other plants that like an acid soil, use ammonium sulfate.)
In areas where you don’t want anything to grow, such as in between beds, use a weed barrier cloth or quarter inch stacks of newspapers, then pile on at least eight inches of a mulch that decomposes slowly, such as bark chips, river gravel or hazelnut shells.
Larger sheets of plastic can be used on top of seed-ready raised beds to protect soil from winter’s wrath. Or drape clear plastic over hoops made from PVC pipe to protect your garden from the cold and rain. Your covered bed will have very few winter weeds, will remain dry and heat up when the sun comes out, making it ready for early spring planting, way ahead of unprotected beds. Weight the plastic down with rocks or other heavy objects.
Mulch heavily around perennials such as rhubarb or globe artichokes in the winter as protection from freezing, remove it in the spring to expose plants to light and let the soil warm.
Shrubs such as roses, azaleas, rhododendrons and hydrangeas can really benefit from low-nutrient mulch. Flowers such as lilies and dahlias, and spring bulbs like tulips and daffodils will do better with this type of mulching also.
Blueberry bushes and lingonberries send out roots along the soil surface, and they appreciate a thick layer of well-rotted sawdust. Caneberries will benefit from organic mulches such as composted manure. Rhubarb and asparagus beds do best covered with seed-free strawy manure. Dormant vegetable beds can benefit from a 6-inch blanket of manure and leaves.
When mulching around tree trunks, do not place the mulch up against the bark. Mulching against tree bark encourages crown rot and allows easy access for mice, which might girdle the bark around the base of the tree.
For more information on mulching, download these OSU Extension publications:
Improving Soils With Organic Matter (EC 1561) [PDF download]
Or call 1-800-561-6719 to purchase printed copies.