By Hugh Skinner

Conserve moisture. Reduce weeds. Improve winter survival. Add to soil fertility and organic matter. And reduce waste from your yard. Sounds too good to be true? Mulching will do all this and also reduce the amount of work needed to grow a fine garden.

Mulching a garden improves growing conditions for most plants. A 10 to 15 cm. (four to six inch) layer of organic mulch applied in early summer after the soil warms up will cut down on the time you’ll need to spend weeding and watering in the garden. The mulch layer shades the soil and prevents germination of weed seeds, most of which need light to get started. If weed seeds do blow into the mulch the seedlings are usually easy to pull. The mulch layer will moderate the soil temperature, slow drying significantly and let you walk in the garden sooner after rain without compacting the soil.

For most plants these conditions promote better growth. For plants that prefer cool soil (for instance peas, cabbage, and most woodland plants) apply mulch in late June to allow the soil to warm moderately. For warm season crops like vine vegetable, tomatoes and peppers and most sun loving plants, apply the mulch in mid-July to allow the soil to warm more.

Perennial flowers, especially more tender varieties, will benefit from an application of mulch in the fall after the ground begins to freeze. The insulating layer will moderate soil temperature and will prevent freeze-thaw cycles that damage roots and contribute to winterkill. If you leave the mulch in place until weather begins to moderate in the spring, it will delay growth and protect flower buds and tender shoots from severe spring frosts. This strategy can also delay flowering of fruit trees to help flowers avoid harsh frosts.

A mulched area around the base of trees will eliminate the need to trim grass around the trunk and help to avoid damage from lawn mowers and string trimmers. Moisture retention and reduction of competition from grass are added benefits. When applying mulch around trees, be careful not to pile it against the trunk – it may cause rotting by keeping the bark continuously moist.

Organic materials that can be used for mulch are readily available to most homeowners. If you use organic materials that break down quickly (grass or straw for instance), the micro-organisms that cause the decay will take up nitrogen from the soil as they increase in number. To avoid nutrient deficiencies, apply a layer of compost or composted manure rich in nitrogen before applying the mulch.

Grass clippings can be dried and used as garden mulch. Wait for two mowings before using clippings from a lawn where herbicide has been applied. Apply grass clippings to vegetable gardens or flower beds and for a more attractive look apply a thin layer of peat moss on top of the grass. Apply nutrient rich compost or composted manure first. As the grass clippings break down they will release the nutrients to the soil and when incorporated, will improve tilth.

In the fall, cleaning up leaves is a chore for most gardeners. Gain value by saving the leaves (preferably after they have been broken down by a mulching mower) and use them as mulch in perennial beds. If the mulch is applied after the soil begins to freeze, it will eliminate freeze-thaw cycles and protect the crowns and roots of plants from extreme cold. Leave the mulch in place until temperatures begin to moderate in spring. In the spring, move the mulch to the side to allow the soil to warm up. Then incorporate it into the soil to add organic matter and slow release nutrients.

Mulch Materials

Grass clippings and fallen leaves are the most readily available mulches but other organic and inorganic materials are suitable for mulching gardens. Straw or clean hay is effective for winter protection in the garden, for strawberry plantings or for summer mulch in the vegetable garden. Straw has more tendency to blow around than hay and has less nutrient so if you mulch with straw you should apply compost or composted manure before mulching to prevent nutrient deficiencies.

Wood chips will prevent weed growth by shading the soil surface. They are longer lasting than grass clippings or leaves and are better suited to mulching tree and shrub plantings. The wood chips will break down slowly to become incorporated into the soil but because the breakdown is slow, there is less likelihood of nitrogen deficiency. Wood chips can be obtained from tree pruning, fence post peelers, and wood processing plants. Bark chips break down even more slowly than wood chips but behave in a similar way in the landscape. Garden centres sell bark chips of various sizes in bags.

Flax shive is what remains after fibre is extracted from flax straw to make paper. It forms a clean mulch material. The Lily Nook at Neepawa uses flax shive for weed suppression, moisture conservation and winter protection in their bulb fields. They’ve found that by applying about five cm (two inches) in the fall just after the ground begins to freeze they are able to over-winter tender Oriental and trumpet lily varieties that would not survive without this protection. It breaks down slowly and can be left in place for up to five years around trees, but if it is incorporated after a year, it improves the tilth of the soil and prevents it from compacting.

Peat moss is available in compressed bales from garden centres. It has a very high affinity for water and will add to the soil’s water holding capacity when incorporated. Be careful when using it as mulch. A thick layer of peat moss will shed water and make it difficult to get water to plants. It is preferable to use it in conjunction with a more porous material such as straw or grass clippings.

The magic of organic mulches: Take waste materials. Layer them on your garden beds or vegetable plot and with a little effort make your garden more productive and healthier. Save time weeding, save water and improve your soil.

Hugh Skinner operates the Frank Skinner Arboretum near Roblin and for 30 years has been involved in the testing and ithe nursery production of a wide variety of hardy plants.

*Originally published on Manitoba Gardener Fall 2007 issue.