Mulch: a Garden Miracle

Reuse garden waste to enrich the soil and protect from invading weeds, erosion and frost.

Credit: Elizabeth Rowlands

Wood-chip mulch tidies up a blueberry bed.

Sometimes I really miss the obvious. For years, I piled grass clippings and raked-up leaves in one corner of our yard while struggling to fight back weeds overtaking the garden in another.

Finally the light dawned and I began to redistribute the piles of waste onto the beds as mulch. Now the garden is gaining a lovely sense of order with all the yard waste being recycled into a barrier that adds nutrients to the soil and protects it from invading weeds, erosion, frost and summer drought. As simple a strategy as mulching is, it helps to have a few hints.

First and foremost, maximize the benefits of your mulch layer by keeping it to 5 to 10 cm (2 to 4 in.); going deeper can lead to a lack oxygen and surplus of moisture that will sour the soil. To boost the effectiveness of mulch as a weed suppressant, lay down a bottom layer of newspaper (without coloured inks as they can carry dangerous metals), heavy cardboard or a natural fabric like cotton. (Wetting the newspaper and cardboard down first makes them easier to maneuver.) Some gardeners advise sprinkling manure or compost onto the soil first. Leave at least 30 cm (1 ft.) between mulch and tree trunks – mulching too closely can smother the roots and also create a habitat for rodents that chew bark and girdle trees.

If you are concerned that leaf mulch might be carrying scab spore, add lime to encourage decomposition before the next spring. Compost tea accelerates this process. Turn your mulch under in spring to discourage egg-laying by snails and slugs. If neighbourhood cats treat your mulch as their litter box, add a top layer of pinecones – they strongly dislike the texture. Don’t mulch with peat moss – it dries into a hard mat that resists water. There are many choices of mulch. Bark (ground or chipped) is perfect for rhodos, azaleas, woodland plants and other acid-soil lovers.

If you want to use it around alkaline-soil fanciers, dust the area with dolomite lime first to sweeten things up. Wood chips, shavings and sawdust from pruning or the workshop (no treated lumber or cedar, though!) work well around trees. Pulled-up weeds can be chopped and used as a nitrogen-rich mulch for annual and vegetable beds.

Avoid those with seeds or persistent roots. Pesticide-free grass clippings are great. Straw is effective for suppressing thistles. Before using it, though, horticulturist Carolyn Jones suggests laying the straw on a tarp, watering it and letting any seeds germinate and die; otherwise straw can sprout in your garden. The organisms that thrive on wood and straw mulches derive nitrogen from the soil, so balance things out by adding in some nitrogen-rich fertilizer. Leaf mulch is full of nutrients.

Always chop leaves to make them more porous and allow enough oxygen and water through to your soil. Simply run your lawn mover over them, use a leaf shredder/wood chipper or, as our Organic Gardener Sheena Adams suggests, throw an armful of leaves into a large plastic garbage bin and run your weed whacker through them.