Mulch
Materials Research Report
By Beth Huxta
If organic gardeners named a most valuable player
after every season, mulch would win over and over again. Mulch prevents weeds
from sprouting up in your garden, keeps soil moist and aerated, protects your
plants from soilborne diseases, replenishes the soil as it decomposes, and
keeps your yard and garden looking well tended. But which mulch is best for
your needs? And how much mulch to use and when to use it? We asked a few
leading experts to answer those very questions for you.
For Vegetable
Gardens:
Mulch comes in a lot of different forms, many of them
at a price you cannot beatabsolutely free. The best for vegetable beds are
organicthat is, parts of once-living plants.
1. Grass clippings:
These are reat for vegetable, annual, and herb
gardens, because you can get them by the bagful (though never use grass that
you suspect has been treated with herbicide). They decompose quickly,
especially in very hot weather. If applied too thick, they tend to mat down and
become impervious to waterstart with a 2-inch layer and reapply often. For best
results, allow grass clippings to dry before spreading them.
2. Leaves:
If weed control is your goal, shredded leaves are your
star. Leaves from just about any deciduous tree work well. Contrary to popular
belief, leaves such as oak will not acidify the soil. Oak leaves are acidic
when they are fresh, but they lose this acidity as they decompose. To keep the
whole leaves from blowing away or forming an impenetrable mat, coarsely shred
or chop them (running them over with a lawn mower is an easy way to do it)
.
Like grass, leaves should be spread 2 inches deep and replenished as needed.
When you dig into soil that's been mulched with leaves, you'll find lots of
plump earthworms, who thrive on turning them into the best fertilizer for your
garden. Climate concern: "Heavy, easily matted materials like wet
tulip-poplar leaves do not work well in cold northern climates," says Mary
Meyer, Ph.D., professor of horticultural science at the University of Minnesota.
Wet leaves can easily freeze into a block of ice, damaging plants instead of
insulating them and often leaving a mess behind when they thaw, Meyer adds.
3. Pine needles:
The foresty scent is reason enough to use pine needles,
but they are also attractive as they decompose (slowly) and they allow water to
seep through easily. Though they come from plants that prefer acidic soil, a 2-
to 3-inch layer of pine-needle mulch will not substantially change the pH of
your soil.
4. Straw:
If you are buying straw for mulch, be sure you get
straw and not hay. Straw has just the stems of plants; hay has the seedheads,
which will sprout into weeds in your garden. Straw breaks down quickly, adding
nutrients to the soil. Because straw does not mat like grass or leaves, you can
pile it 6 to 8 inches deep in your vegetable beds and strawberry patch. Climate
concern: In very rainy climates, avoid straw mulch, because wet, partially
rotting straw makes a perfect hideout for slugs. Cover crop.
Plants such as
hairy vetch or alfalfa that you grow specifically to improve your soilsometimes
called green manureare effective as mulch, too. We planted a cover crop of oats
in our Pennsylvania
test garden in late summer, and when it died back the following spring, we
simply dug holes for tomato transplants right through the oats. "Plant a
winter legume like hairy vetch in the early fall," advises Paul Hepperly,
Ph.D., research and training manager at the Rodale Institute, in Maxatawny, Pennsylvania.
"When the hairy vetch flowers, cut it down and leave the residue as
mulch." Caution: Do not let hairy vetch produce seed, as it will sprout
where you don't want it.
5. Plastic:
We generally prefer mulching with organic matter that
builds soil as it decomposes. But where you need to heat up the soilfor growing
melons and other warm-weather crops in cooler climates, for instanceor for
total weed control, plastic can be very useful. Clear plastic warms the soil
best but allows weed growth. Black plastic inhibits weed-seed germination as it
warms the soil slightly. Newer (more costly) types, such as infrared
transmitting (IRT) plastics, combine the weed-control ability of black plastic
with the soil warming of clear, and have been shown to improve plant growth
and, in some cases, yields.
For Flowerbeds and Shrubs
Where you are growing perennials or in other spots
where you don't turn the soil often, you can use mulch materials that break
down very slowly or not at all. These also tend to be more attractive than the
mulches we've already covered (though a cover crop of red clover is quite
lovely).
1. Bark:
Typically sold as chips, nuggets, or shredded pieces,
bark decomposes slowly but stays in place well (pine bark nuggets may float in
a heavy rainfall). You'll find both hardwood and softwood options. Common
hardwood types include hickory, oak, and elm. Softwood bark, such as pine, fir
and redwood, decomposes more slowly than hardwood. Stay away from cypress
mulch, because overharvesting is depleting vital coastal wetlands, and studies
have proven it to be no more durable than more sustainable choices.
Douglas-fir
bark mulch is ideal for gardeners in the Northwest, says Rita Hummel, Ph.D.,
environmental horticulturalist at Washington
State University.
"It works well in landscapes and is readily available here."
Coarse-textured mulches like bark can be layered up to 4 to 5 inches thick,
because more air circulates between the bigger particles and water passes
through them more easily than it does with finer-textured mulch like grass or
leaves. Watch your wallet: Bark mulches can be expensive to use in large areas.
2. Wood chips:
You can find tree and utility companies,
arborists, and yard-waste facilities willing to give you wood chips for free.
They tend to lose color more quickly than bark mulch. Wood chips are a good
choice for paths and where you have a lot of ground to cover, but don't use
them close to your house, because termites and other destructive insects may be
living in them. A popular low-cost choice for landscaping is a recycled mulch
made from construction wastes and wood pallets. Keep this away from your
vegetable garden, as it may contain unknown industrial contaminants.
Furthermore, a University
of Ohio study showed that
the use of ground pallet mulch offers little benefit to the microbial soil
community. Aerial Attacks: "Wood mulch can harbor the artillery
fungus," warns Hepperly, "which shoots a black, sticky spore into the
air. This can be a nuisance if it lands on your house siding and even your
car's paint job." Current research shows that mushroom compost (available
in garden centers), suppresses these shooting spores. Pennsylvania State
University researcher
Donald Davis, Ph.D., recommends using a mulch mix of 40 percent compost, 60 percent
bark (not wood) if you are concerned about an artillery assault. But, he says,
"even 20 to 25 percent mushroom compost would likely help."
3. Newspaper:
A four-sheet layer of newsprint is very effective at
suppressing weeds. Wet it down and cover it with a layer of bark chips, and
you'll have beautiful and hard-working mulch layer for any ornamental bed. Ink
issues: We do not recommend using colored newspaper in the vegetable garden.
Researchers at West Virginia
University who have done
extensive research using newspapers in vegetable production caution against
using slick paper of any kind in the garden. These materials may still contain
heavy metals, they report.
4. Rocks/stone/gravel:
Durability is both the appeal and the drawback of
stones as mulch. They stay put and don't degrade, which means they don't need
to be replenished but neither do they improve the soil. Use in paths or around
trees and shrubs about 1 inch deep for good weed control and water
permeability. Climate Concern: "In hot regions like the South, rocks can
radiate heat and cause extreme temperatures around plants," says Gary
Wade, professor and extension coordinator at the University of Georgia.
"This encourages water loss and can result in severe plant stress. On a
hot day, rock mulches in full sun can cause the temperature around the plant to
soar into the triple digits."