Beginner’s Guide to Mulching
Originally Published at: http://www.organicgardening.com
The best time-saving measure a gardener can take is applying mulch. This goes for every garden site, from vegetable garden to flower bed. Mulched gardens are healthier, more weed free, and more drought-resistant then unmulched gardens, so you’ll spend less time watering, weeding, and fighting pest problems.
There are two basic kinds of mulch: organic and inorganic. Organic mulches include formerly living material such as chopped leaves, straw, grass clippings, compost, wood chips, shredded bark, sawdust, pine needles, and even paper. Inorganic mulches include gravel, stones, black plastic, and geotextiles (landscape fabrics).
Both types discourage weeds, but organic mulches also improve the soil as they decompose. Inorganic mulches don’t break down and enrich the soil, but under certain circumstances they’re the mulch of choice. For example, black plastic warms the soil and radiates heat during the night, keeping heat-loving vegetables such as eggplant and tomatoes cozy and vigorous.
Using Organic Mulches
There are two cardinal rules for using organic mulches to combat weeds. First, be sure to lay the mulch down on soil that is already weeded, and second, lay down a thick enough layer to discourage new weeds from coming up through it. It can take a 4- to 6-inch layer of mulch to completely discourage weeds, although a 2- to 3-inch layer is usually enough in shady spots where weeds aren’t as troublesome as they are in full sun.
Wood chips and bark mulch: You can purchase bags of decorative wood chips or shredded bark from a local garden center to mulch your flower garden and shrub borders. A more inexpensive source of wood chips might be your tree-care company or the utility company. They may be willing to sell you a trunkload of chips at a nominal price. Many community yard waste collection sites offer chipped yard debris or composted grass clippings and fall leaves to residents for free (or for a small fee).
Shredded leaves: If you have trees on your property, shredding the fallen leaves creates a nutrient-rich mulch for free. You can use a leaf-shredding machine, but you don’t really need a special machine to shred leaves—a lawn mower with a bagger will collect leaves and cut them into the perfect size for mulching.
You can spread a wood chip or shredded leaf mulch anywhere on your property, but it looks especially attractive in flower beds and shrub borders. Of course, it’s right at home in a woodland or shade garden. Wood chips aren’t a great idea for vegetable and annual flower beds, though, since you’ll be digging these beds every year and the chips will get in the way. They do serve well as a mulch for garden pathways, though.
Grass clippings: Grass clippings are another readily available mulch, although it’s a good idea to return at least some of your grass clippings directly to the lawn as a natural fertilizer (see the Lawns entry). It’s fine to collect grass clippings occasionally to use as mulch, and the nitrogen-rich clippings are an especially good choice for mulching vegetable gardens. Your vegetables will thank you for the nitrogen boost!
Compost: If you have enough compost, it’s fine to use it as a mulch. It will definitely enrich your soil and make your plants happy, but keep in mind that when any kind of mulch is dry, it’s not a hospitable place for plant roots. So you may want to reserve your compost to spread as a thin layer around plants and top it with another mulch, such as chopped leaves. That way the compost will stay moist and biologically active, which will provide maximum benefit for your plants.
Pine needles: Pine needles are a trim-looking mulch for garden beds. They allow water to pass through easily and they break down slowly. Despite what you may have heard, using pine-needle mulch will not make your soil significantly more acid.
Straw and hay: Another great mulch for the vegetable garden is straw, salt hay, or weed-free hay. It looks good and has most of the benefits of the other mulches: retaining soil moisture, keeping down weeds, and adding organic matter to the soil when it breaks down. But be sure the hay you use is weed and seed free, or you’ll just be making trouble for your garden. And don’t pull hay or straw up to the stems of vegetables or the trunks of fruit trees or you’ll be inviting slug and rodent damage.
Organic Mulching Mechanics
Spreading organic mulch saves labor and nurtures plants by:
- Preventing most weed seeds from germinating; the few weeds that do pop through the mulch will be easy to pull.
- Keeping the soil cool and moist in summer, reducing the need to water.
- Decomposing slowly, releasing nutrients into the soil.
- Encouraging earthworm activity, improving soil tilth and nutrient content.
- Keeping dirt from splashing on flowers and vegetables.
- Preventing alternate freezing and thawing of the soil in winter, which can heave plants from the soil.
Nothing, unfortunately, is perfect. When using organic mulches, keep in mind the following facts:
- As low-nitrogen organic mulches such as wood chips and sawdust decay, nitrogen is temporarily depleted from the soil. Fertilize first with a high-nitrogen product such as blood meal or fish meal to boost soil nitrogen levels.
- An organic mulch retains moisture, which can slow soil warming; in spring, pull mulch away from perennials and bulbs for faster growth.
- A wet mulch piled against the stems of flowers and vegetables can cause them to rot; keep mulch about 1 inch away from crowns and stems.
- Mulch piled up against woody stems of shrubs and trees can cause them to rot and encourages rodents, such as voles and mice, to nest in the mulch. Keep deep mulch pulled back about 6 to 12 inches from trunks.
- In damp climates, organic mulches can harbor slugs and snails, which will munch on nearby plants; don’t spread mulch near slug-susceptible plants.
- Organic mulches are usually more or less acidic, depending on their content; mix some lime with the mulch beneath plants that prefer neutral or slightly alkaline soil.
Double mulch stops weeds. If you know that a garden bed is filled with weed seeds or bits of perennial weed roots, use a double-mulching technique to prevent a weed explosion. Set plants in place, water them well, then spread newspaper and top it with organic mulch.
Using Plastic Mulch
Mulching a vegetable garden with sheets of black plastic film can do wonders. When it’s spread tightly over a smooth soil surface, black plastic will transmit the sun’s heat to the soil beneath, effectively creating a microclimate about 3°F warmer than an unmulched garden. Because the plastic film remains warm and dry, it protects the fruits of vining crops such as strawberries, melons, and cucumbers from rotting and keeps them clean. And of course, the mulch prevents weed growth and retains soil moisture.
Infrared transmitting (IRT) plastics cost more than standard black plastic, but they can result in even higher yields. These plastics warm the soil as well as clear plastic does, but also control weeds as well as black plastic does.
In raised bed gardens, lay down a sheet of plastic over the entire bed. Bury it at the edges or weigh the plastic down with rocks. Then 384punch holes in it for the plants. A bulb planter makes quick work of hole cutting. Sow seeds or plant transplants in the holes.
Because water can’t permeate plastic, the mulch retains soil moisture but it also keeps rainwater from soaking the planting bed. Thus, the ideal watering system for a plastic-covered bed is soaker hoses or drip hoses laid on the soil surface before you put down the plastic.
Don’t use plastic as a mulch under shrubs. Although it keeps out weeds and can be camouflaged with decorative mulch, black plastic destroys the shrubs’ long-term health. Because water and air cannot penetrate the plastic, roots grow very close to the soil surface—sometimes right beneath the plastic—seeking moisture and oxygen. The shallow roots suffer from lack of oxygen and moisture and from extremes of heat and cold. Eventually the plants decline and die. Stick to organic mulches such as shredded leaves, bark, wood chips, or compost under your trees and shrubs.
Although black plastic mulch seems like a great boon to organic gardeners, its use is not problem free. One issue of concern with black plastic is its manufacture (it’s a petroleum product) and its disposal—there are very few places it can be recycled. If you carefully lift black plastic at the end of the growing season and store it in a dry place over winter, you should be able to reuse it for several years, but eventually if will become torn and you’ll have to throw it away.
An alternative is a biodegradable plastic mulch (cornstarch based). These materials are designed to break down in place by the end of the growing season, and you can dig any remaining bits into the soil. However, one of the breakdown products of biodegradable plastic mulch is carbon dioxide. Black paper mulch made from recycled paper is also available, but these products are usually treated with a synthetic antimicrobial substance to prevent them from breaking down too quickly.
Unlike black plastic, landscape fabrics let air and water through to the soil beneath while keeping weeds from coming up. But landscape fabrics (geotextiles) have some of the same drawbacks as black plastic. To begin with, they are petroleum products. When exposed to light, they degrade over time, so to make them last longer, you have to cover them with a second mulch (they’re ugly, so you’d want to, anyway). However, many gardeners have discovered that shrub roots grow up into the landscape fabric, creating real problems when you eventually want to remove it. And weeds that germinate in the surface mulch send roots down into the fabric, too, tearing it when you pull them out.