Mulching to Control Weeds and Save Water

Mulching is the practice of covering the soil surface with a material that smothers weeds and delays the evaporation of moisture from the soil. Although most people think of mulches as being dead plant material such as pine needles or bark chips, anything that keeps weeds down and moisture in can be considered a mulch (including aluminum foil, old carpeting, and low-growing non-competitive plants).

Good Reasons for Mulching

The right mulch can just about eliminate the need to pull weeds. Only the most tenacious perennials poke through, such as yellow nutsedge or johnsongrass. Mulches also prevent weeds seeds from reaching the soil. Instead, they fall on an inhospitable surface, where they languish.

By covering the soil surface, mulches reduce the amount of sunlight and wind that reaches the soil, so water evaporates from the soil more slowly. And the soil stays cooler, so the roots stay cooler (a big help in the heat of the summer).

A mulch derived from plants (such as grass clippings, cocoa hulls, or shredded pine bark) can also add organic matter to the soil, improving its structure and fertility and encouraging a healthy population of microorganisms.

Finally, mulches can make a garden more attractive, especially a new one where the plants have yet to fill in. And when it rains, a mulch keeps soil from splashing onto the leaves or nearby paved surfaces or buildings.

A Few Cautions

Mulches do have some drawbacks. For one thing, insects such as slugs and earwigs love the moist shady hiding place a mulch provides. If the insects are abundant enough to cause serious damage, you need to rake off the mulch for a while.

Another drawback is that organic mulches cause a temporary boom in the population of soil microorganisms, which can tie up some soil nutrients. This boom is especially a problem with mulches high in carbon, such as wood chips. To compensate, fertilize first with a high-nitrogen fertilizer such as rotted manure or bloodmeal. Or put a high-carbon mulch on top of a layer of balanced mulch, such as grass clippings or compost. You can add nitrogen, phosphorus, and (in alkaline soils) sulfur to make sure there's enough left for the plants.

Choosing and Using Mulches

When choosing a mulch, consider how long you want it to last, how attractive you want it to be, what's available, and how much you want to spend.

In general, the bigger the pieces in the mulch, the longer it lasts. Soil organisms can digest a blade of clipped grass pretty quickly, but might need a year or two to chomp through a pine bark nugget. And juicy materials, like compost, deteriorate faster than woody ones, like newspaper. So if you want a long-lasting mulch, as you might around trees and shrubs, choose bark nuggets or another big, woody material. If you want a mulch that deteriorates in a season, as you might to improve the soil in a vegetable garden, go with something small and juicy like grass clippings or apple pomace.

Whatever material you choose, you want to apply it deep enough to control weeds, but not so deep that it smothers the roots of your garden plants. How deep that is depends on the plant, how dense the mulch is, and how well aerated the soil is. Start by putting a 1-2 inch mulch around flowers and vegetables and a 3-4 inch mulch around trees and shrubs. If weeds pop through, add another inch. As the mulch breaks down, add more.

The final considerations are how easy the mulch is to find and how much it costs. Some mulches, such as newspaper and grass clippings, are easy to come by and inexpensive. Others, like bark mulch, are also easy to come by but cost a few bucks a bag. And some mulches, such as mushroom compost, cocoa shells, apple pomace, and pecan shells, are often limited to areas where there are mushroom farms, candy factories, cider presses, and pecan groves.

Some Mulching Materials

Bark chunks, sometimes called nuggets, are attractive pieces of bark about three inches in diameter. Because they're so large and woody, they last a long time (years in some climates). Of the various sizes of bark mulches, they are the best for weed control. Bark chunks are so coarse textured that they don't encourage a big growth spurt in the soil microorganism population that can tie up soil nutrients.

Their slow decomposition means you'll have to put a finer organic material under them if you want to improve the soil; try manure or compost. Because the bulky pieces decay slowly, don't turn them under the soil, where they can create air pockets near plant roots.

How to Use: Bark chunks are a good ornamental mulch for trees and shrubs, or for covering the spaces between plants in a new planting. Spread to a depth of about 3-4 inches.

Wood chips are small pieces of wood about an inch in diameter. The chips make a decorative mulch that breaks down faster than bark chunks, adding more organic matter to the soil. Wood chips are high in carbon, making soil organisms tie up nitrogen and other soil nutrients as they work to digest the chips. Bags labeled "wood chips" can contain wood other than bark, including leftovers from the timber trade that might have been treated with chemicals that harm plants. Freshly chipped wood needs to sit for a few months before you use it, since the tree might have contained natural substances toxic to young plants. Small chips may bounce onto sidewalks during a downpour.

How to Use: Spread to a depth of about 3-4 inches around trees and shrubs, or in the spaces between plants in a new planting.

Shredded Bark is narrow strips of bark a few inches long. They break down faster that chunks or chips, so you can turn them into the soil after a year or two as a source of organic matter. Shredded bark costs less than bark chunks. Shredded bark is high in carbon, so soil microorganisms draw nitrogen and other nutrients from the soil supply as they break the bark down. Shredded bark can float out of plant beds in a downpour. Fresh shredded cypress mulch may be toxic to seedlings. Shredded bark is far less effective for smothering weeds than are bark chunks.

How to Use: Spread a 4-inch layer around trees, shrubs, and flowers.

Compost is a fertile mulch you can make. It's an ideal soil-building mulch for vegetable gardens or flower beds that you plan to turn under. However, fully decomposed mulch is too fine to smother aggressive weeds. Partially decomposed compost is bulkier and better at controlling weeds, but can trigger a burst of activity in soil organisms that temporarily which ties up nitrogen; compensate by adding a little bloodmeal or other source of nitrogen.

How to Use: Apply at 2-4 inch layer around vegetables and annual and perennial flowers, keeping it about an inch from the stems to prevent rot.

Hay and Straw are loose mulches, so they don't block air flow to the roots of plants. Both are good winter mulches for roses, strawberries, and other perennials. Hay is higher in nitrogen than many mulches, so its decomposition doesn't tie up soil nutrients. Hay can contain weed seed. Straw is usually free of weed seed and is often used to keep newly seeded lawns moist between waterings. It attracts spiders that control insect pests, repels some aphids, and impedes the movement of Colorado potato beetles. Straw lasts longer than hay, but is woody enough to make soil organisms tie up nutrients, so add a little bloodmeal or other nitrogen source. Because neither is a dense mulch, some weeds can pop through it.

How to Use: Apply in a 4-6 inch layer.

Shredded leaves make a lightweight, insulating mulch well suited to protecting roses and other woody perennials during the winter. Although you can buy shredders, your lawn mower is probably the best shredding tool around. Rake the leaves into along, low pile and run the mower over it, catching the leaves in a bag. Or make a tall round pile and tilt the mower back, then onto the pile.

How to Use: Apply is a 4-6 inch layer.

Grass clippings add organic matter to the soil. Because grass clipping have a good balance of carbon and nitrogen, they don't make soil organisms tie up soil nitrogen. Fresh clippings sometimes get moldy. Because the clippings break down quickly, you have to replenish them often. Grass that's gone to seed or contains weed seeds can create a weed problem in the garden.

How to Use: If you're applying fresh clippings, add a thin layer (1-2 inches). Use 2-4 inches of dried clippings. Replenish as necessary.

Cocoa bean shells are clean, weed-free, and smell of bitter chocolate when first applied. They add nitrogen, phosphorus, and potassium to the soil. The long-lasting shells are attractive, lightweight, and easy to handle, but can blow around. They also mold easily.

How to Use: Apply a 1-2 inch layer.

Leaf mold is leaves that have been composted with no other material. Leaf mold makes a humus rich in micronutrients. Because leaf mold has had time to decompose, its ratio of carbon to nitrogen is more favorable than fresh leaves. When added the soil surface as a mulch, it adds organic matter to the soil, whether or not you turn it under. It can take several years for leaves to decompose into leaf mold.

How to Use: Shred the leaves first by raking them into piles and running the lawn mower over them. Then dump the shredded leaves in a pile; you may want to build a wire bin to contain them. To make the leaves break down quicker, keep them moist and fluff them with a pitch fork every month or so. The leaf mold is ready when its soft and crumbly. Add up to four inches to the soil surface as a mulch.

Sawdust adds organic matter, helps lower the pH of alkaline soils, and repels some insects. Sawdust is so high in carbon that you'll need to add extra nitrogen, unless you compost it.

How to Use: It's a good idea to compost sawdust before adding it to the soil surface, both to lower the carbon-to-nitrogen ratio and to give any toxic substances in the wood a chance to leach out. Then spread 1-2 inches on the soil surface. You can turn it under at the end of the season.

Newspaper sheets provide a barrier that weeds have a tough time breaking but water can easily pass through. Newspaper breaks down over time, adding organic matter to the soil. It's inexpensive and readily available. Deciding how thick to make the layer can be tricky. If it's too thin, it won't last as long as you want and weeds will pop up. Too thick, and you'll have to pull it up at the end of the season if you want to turn the soil or add fertilizer. How rainy the season is affects how long the paper lasts. Newspaper is high in carbon, so if you work it into the soil after it disintegrates you'll need to add extra nitrogen.

How to Use: Lay sheets of paper on the soil surface and weight them with grass clippings, stones, or some other material to keep them from blowing away. Try making the layer 10 sections thick to start with; reapply if the layer breaks down before the season ends. Don't use colored sections of the newspaper, which may contain toxic dyes.


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