The Best - and Worst - Vegetable Garden Mulches

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Mulching squashes

Mulches bring numerous benefits to the vegetable garden that combine to boost productivity while making the garden a more enjoyable place to be. Surface mulches insulate soil and roots from heat and cold, retain soil moisture, smother weeds, reduce disease, and keep vegetables cleaner.

On the flip side, mulches can provide habitat for slugs or other unwanted life forms, delay soil warming in spring, smother germinating seedlings, or bring unwanted chemicals into the garden. Today we are looking at both sides of mulch, and ways to avoid mulch problems posed by pests, pesticides, and forever chemicals.

Slug in mulch
Mulching too early can attract slugs

Mulches and Pests

Once upon a time, my friend Betty dug a pretty spring flower garden and planted it with pansies. She spread leaf mulch from the municipal compost program between the plants, gave the garden a good watering, and called it a job well done. The next morning, many plants were missing, and the survivors were riddled with holes eaten by hungry slugs that hitched a ride in the mulch.

Lesson learned! Mulch piles, including my own, can harbour slugs, earwigs, and random grubs, so it pays to be careful. Put a sample of purchased or imported ready-to-spread mulch in glass jar overnight for observation before moving forward.

Sometimes the problem is not the mulch material but the timing. Mulching early in spring, when the soil is cold and wet, is often a mistake. Otherwise excellent mulches like straw or grass clippings can provide cover for slugs and cutworms, delay soil warming, and smother tiny seedlings when applied too soon.

Plastic mulch
Plastic mulch can distribute microplastics into the soil

Plastic Mulches

Which brings us to plastic mulches, which help raise the soil temperature and keep crops clean. It also degrades into microplastics that persist in soil, which can then be taken up by plants. Recent research indicates that vegetables do, indeed, take up forever chemicals, which persist in plant tissues, and then in animals that eat the plants. In an Italian study, microplastics were present in apples, pears, potatoes, carrots, broccoli and lettuce purchased from six different retail stores. Where did it come from? The big baddie is municipal sludge, but plastic film mulches and other geotextile products used on farms are an ongoing source of microplastics in soil.

We can do better. Plain paper, newspaper, cotton cloth or uncoated cardboard can be used in place of plastic film, which can then be covered with an organic mulch like grass clippings or straw. Do not mulch with paper or cardboard that readily sheds water, because water-repellency is usually due to the presence of plastics.

Grass mulch around garlic
A double mulch of newspapers topped with grass clippings controls weeds in a planting of garlic

Herbicide Contamination in Mulches

If you are buying hay or straw, be aware of the risk of contamination with aminopyralid herbicides, which are used to control pasture weeds and persist in hay, straw, and manure. Tomatoes and beans are highly susceptible to herbicide damage from tainted mulch. Within days after contaminated straw is applied as mulch, the new leaves become curled and deformed. Promptly removing the offending mulch often can undo the damage.

Cotton cloth mulch
Strips cut from an old cotton tablecloth make an excellent soil-protecting mulch between Barbara's lettuce and spinach-mustard plants!

Using Cotton Cloth Mulch to Reduce Erosion

Ready for some positive pointers on mulch? How about using cotton cloth mulch to protect the soil from erosion? When raindrops hit exposed soil, soil particles can be splashed up to 5 feet (1.5 m) high, with wind moving particles horizontally more than 15 feet (5 m). Splash erosion throws soil on lettuce and spinach leaves, and later causes a crust to form on the soil’s surface that can block water to roots or frustrate germinating seeds. To soften the blow of heavy rain long before it’s time to mulch with organic matter, I’ve started rolling out strips of cotton cloth mulch between my spring greens and onions. The strips are fast to apply, sometimes snare a few slugs, and can be taken up quickly when they are needed elsewhere.

Pine straw mulch
Pine straw mulch filters out fungi that cause leafspot diseases while controlling splash erosion

Mulching to Prevent Disease

Erosion also can spread diseases, which may be set back when straw-type mulches are used. Fungal spores that wash down into the mulch are filtered out rather than being splashed about, and the surface of the mulch dries quickly when the sun returns. When tucked in around strawberries, pine straw mulch helps acidify the soil and provides a clean bed for ripening fruits. With tomatoes, a deep mulch of straw or hay reduces threats from early blight and other leafspot diseases.

Wood chip mulch
Wood chip mulch is ideal for berries, fruit trees, and other long-lived plants

Wood Chip Mulch

Finally, wood chip mulch is often the answer for covering the ground under berries, fruit trees, and other long-lived woody plants. Weed free, long lasting, and foot friendly, wood chip mulch is often accessible through tree maintenance companies or municipal composting programs.

When aged wood chips are used atop the soil as mulch, and replenished every year or so, they lead to a steady increase in soil organic matter, with little change to the soil pH. Wood chips retain soil moisture while discouraging weeds, but as with any mulch, they are not immune to weedy invaders. If you are having trouble with a particular perennial weed, please see How to Beat Perennial Weeds Using Mulch, and the lively comments thread that follows it.

Bugs, Beneficial Insects and Plant Diseases

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