All About Garden Mulches
Mulch’s purpose is pretty basic: It acts as a barrier, keeping sunlight and some air away from the soil surface. Sounds simple enough, but mulch’s smothering effect brings with it both good news and bad. Consider these positive and negative effects of tucking in your soil beneath a blanket of mulch:
Without the summer sun’s rays striking it, soil stays cooler and plant roots don’t stress from the heat. The bad news is that slugs, earwigs, cutworms, and other eat-and-run types love cool, moist, dark places. To minimize bugs, use only a thin layer of mulch, keeping it several inches away from plant bases.
Water in the soil doesn’t thaw on sunny winter days, then refreeze at night. That’s good news. The melting-and-freezing cycle makes water shrink and expand, possibly popping shallow-rooted plants right out of the ground — a phenomenon called heaving. Heaving spells the end for plants.
The ground warms more slowly in the spring. This is good because perennials aren’t fooled into breaking dormancy too early. You want the ground to stay cold until it really is spring. The drawback is that perennials may bloom late or soil may not be ready for spring planting. If so, rake back mulch until the soil warms up. Or, if you don’t mulch over winter, wait until plants green up before mulching.
Water evaporates more slowly from cool soil protected from the wind. If you mulch, you don’t have to water as much, saving time, money, and a precious resource. However, heavy rains can make the ground soggy and puddly for days. If beds become bogs, rake off mulch and let soil dry.
Without sunlight, some seeds can’t germinate, and sprouts may not have the oomph to push through the mulch. This prevents weeds, but it thwarts some good seeds, too. Mulch after seedlings are up and have some girth and vigor.
Raindrops don’t hit the soil surface, so soil is less likely to wash away or splash onto plants. This keeps plants cleaner and free of some soil-dwelling diseases.
Everyone asks how much mulch to apply and when to apply it. There are no right answers. It depends on several factors, including your soil, rainfall, the type of mulch, and how weedy the ground is. Here are some guidelines:
- For most mulches and soils, start with a layer 2 to 3 inches deep. For quick-drying sandy soil, go for the higher end of the range. For clay that drains poorly, go with the shallower depth. For a dense mulch like newspaper, use a much thinner layer. See the chart on page 5, “Mulling Mulch,” for details.
- If the soil is dry, water it before applying mulch. Pull any weeds.
- Apply mulch just about anytime, remembering that if you mulch early in the spring, the ground may be slow to warm. If you mulch only in the winter to prevent heaving, wait until the ground freezes. Mulch could delay freezing of the ground, causing roots to go dormant later than normal and possibly damaging them.
- Appearance. In a showy flower garden, you want a mulch that looks good without stealing the limelight: Try bark chips, shredded bark, or cocoa hulls. For a woodland garden, leaves or pine needles are right at home. For a large or no-frills cutting garden, grass clippings or layers of newspaper are budget-smart. You can always top them with something prettier and pricier.
- Longevity. How long do you want the mulch to last? You may want to dig up a bed at the end of the season, in which case compost or another quick decomposer is a smart choice. Around permanent plantings, such as roses or flowering shrubs, a sheet of landscape fabric covered with bark nuggets or river stones will last for years. For perennial beds, consider shredded bark, which lasts a long time. As a rule, the bigger the mulch chunks, the longer they last. Soft or green materials, such as leaves or grass clippings, break down faster than dry woody elements, such as straw, pine needles, or bark. Stone or gravel last an eternity.
- Cost. Homemade mulches, such as compost, grass clippings, and newspaper, are just about free. Bark chips run $2 to $3 a bag. Cocoa mulch can cost more than twice that. If you need to cover a large area, try to buy in bulk, or put something cheap under something expensive. When evaluating cost, remember to factor in how long the material will last.
Dark-color mulches will absorb and retain more heat from the sun than lighter ones. That’s an advantage in cooler regions but a disadvantage in hotter climates.
Light and heat reflected from a light-color mulch (particularly decorative landscaping types, such as white stones) can dangerously overheat surrounding plants.
Some mulches won’t stay put. Gravel and stones creep onto lawns (and make tempting throwables for kids). Cocoa hulls blow away. Small bark chips can wash downstream in a heavy rain. In general, mulches with heavy or large pieces are more likely to stay put. Those that form a mat, such as leaves and pine needles, are usually stable, too.
Organic mulches — grass clippings, leaves, manure, and compost — improve the soil. Stones and plastic don’t. Black plastic, unless it’s porous or perforated, grows a smelly, slimy coating. It also turns brittle and breaks into little pieces that escape the garden. Cheap landscape fabric is not worth it — weeds and roots will tangle in it.
Shredded, chipped, chunks, or nuggets. Usually pine, cypress, or hardwood. Attractive. Long-lasting, especially the large nuggets, but may look too chunky around dainty flowers.
Add nutrients. Fresh hulls carry chocolaty aroma. May compact and mold.
Turn under at end of season to improve soil. Texture is too fine to suppress aggressive weeds.
Turn under at end of season. Can heat up or mold if too thick. Use 1-2 inches if fresh, 2-4 inches if dry.
Loose layer can be about 6 inches deep, will settle down. May contain weed seed.
Use at base of flowering shrubs. Cover with thin layer of attractive mulch. Get good-quality fabric, or weeds and roots will tangle in it. Best type is bonded, not woven.
Leaves composted two to three years. Turn under at end of season to improve soil.
Shred before using if you want them to break down faster.
Use layer 5 to 10 sheets thick. Disguise with thin layer of attractive mulch.
Turn under at end of season. Adds nutrients. Store-bought stuff looks and smells less like the real thing.
Compost Used manure left after mushroom harvest. Can turn over at end of season to improve soil. May contain pesticide residues. Texture is too fine to suppress aggressive weeds.
Use 1-2 inch layer near acid-loving plants. Soak in warm water before using. Never let it dry out completely or it will shed water. Use Canadian peat; Louisiana peat may be dangerously acidic.
Use at base of flowering shrubs. Get kind that lets water pass through. Top with more attractive mulch.
Breaks down quickly. Depletes soil nitrogen, so sprinkle soil with blood meal or other nitrogen source.
Loose layer about 6 inches deep, will settle down. Lasts one to two seasons. May deplete soil nitrogen.
Byproduct of timber industry. Quality varies. Recycled woods from pallets and construction may contain toxins that kill plants and contaminate soil. Don’t use chips if they smell sour; this indicates the presence of harmful acids. Rid fresh chips of acids by letting them decompose in a compost pile or pit before using.