10/16/2005

Daily Herald (Arlington Heights, IL)

By Diana Stoll The Planter's Palette

Mulch is one of a gardener's best friends. It reduces the amount of moisture lost from the soil through evaporation; acts as an insulator against heat and cold; and decreases the amount of weed growth.

Mulch also moderates soil temperatures - important in our indecisive winters where temperatures fluctuate between glacial and springlike.

A layer of mulch also reduces soil erosion; prevents mud from splashing up onto foliage; and protects against soil compaction. It slows runoff of rainwater; protects tree trunks from lawnmower and weed whip attacks; and some mulches add nutrients to the soil as they decompose.

Organic Mulches

Organic mulches come from sources that are, or were once, alive. Organic mulches improve soil structure as earthworms wriggle through incorporating them into the soil. They add micronutrients to the soil as they break down, increasing the soil's fertility. Generally, these materials are readily available, many are inexpensive, and some are free.

Sound too good to be true? There are some drawbacks. Some organic mulches contain weed seeds; some provide a home for uninvited, unwelcome pests such as slugs, mice or voles. Organic mulches also need to be replenished as they break down.

Pine bark and nuggets, cypress, cedar, and shredded hardwood are probably the most popular choices of organic mulch material, and are all excellent weed inhibitors. As microorganisms work to break these mulches down, nitrogen in the soil may be "tied up," depriving plants of this essential mineral.

Adding nitrogen in the form of fertilizers will alleviate this problem. The larger, coarser organic mulches are ideal for use around trees and in shrub borders. Smaller materials are perfect for perennial gardens.

Composted yard wastes are excellent at improving the soil, but decompose more quickly than wood mulches and are not as effective at weed control. If they are used in combination with wood mulches you gain the benefits of both.

Leaves are a delicacy for earthworms and are plentiful this time of year, but shred them before you use them as mulch. Non-shredded leaves can form a soggy mat impenetrable to water and air.

Grass clippings are best left on the lawn where they can provide up to 25 percent of the lawn's total fertilizer needs. If you do bag your clippings, instead of putting them on the curb in landscape bags, use them as mulch in the garden. Wait until they are dry before using them. Apply no more than 1 inch at a time or excessive heat and foul odors may result, and never use grass clippings if you have recently applied herbicides to your lawn.

Pine needles are attractive and fragrant. Over time, they may slightly increase the acidity of the soil. Pine needles can be difficult to find and may mat down if they are used too thickly.

Cocoa bean hulls make another attractive, fragrant mulch. They tend to be more expensive and may float away during a heavy rainstorm. Cocoa been hulls are not a good choice if you have a dog that chews his way around the yard, as they are toxic to canines.

Sawdust and animal manures are also options, but both should be well aged before using. Un-composted sawdust is low in nitrogen and will pilfer it from other plants; un-composted animal manures may burn plants.

Cover crops and green manures are living mulches. Vegetable gardeners plant winter wheat or rye as green manures in early fall. Cover crops are grown on unused garden plots during the regular growing season. Both prevent weeds and are turned into the soil while they are still green.

Inorganic Mulches

The good thing about inorganic types of mulches is they do not break down; the bad thing about them is they do not break down. Because they do not decompose, they do not need to be replaced, but they also offer no benefits to the soil.

Advantages to inorganic mulches are many. They are less likely to blow away or wash off a sloping site; they are fire resistant; and dark-colored mulches can be used to warm the soil - perfect for getting warm-season crops off to a quicker start in the spring.

Plastic completely inhibits weed growth, but it cracks easily from freezing temperatures and allows for no air or water exchange. Black plastic raises the temperature of the soil and some studies suggest that red plastic increases the fruit production of tomato plants.

Landscape fabric controls most weeds, is more durable than plastic, and lets water and air through to plant roots. Another mulch is usually added to cover the fabric. Weeds may germinate on top of the fabric.

Stone, gravel and pebbles are permanent mulches that are typically used over plastic or landscape fabrics. Used alone, they do not prevent weed growth, and it may be difficult to rake out leaves or evergreen needles.

Applying Mulch

Spreading mulch too thickly and too closely to the stems of plants are common mistakes. Leave some air space between the stems of herbaceous plants or the trunks of trees and shrubs. Thick mulch against trunks gives borers and other insects easy access to your woody plants; thick mulch against herbaceous stems may cause them to rot.

The depth at which to spread mulch is based on its porosity and how easily it can be packed down. The less permeable and the more compactable, the thinner the mulch should be applied.

For example, a 2- to 3-inch layer of finely shredded hardwood or mini bark nuggets is plenty, but if larger nuggets were used, 3 to 5 inches would be better.

How Much to Buy

It may seem complicated to calculate the quantity of mulch needed, but it is really simple. If you are buying mulch in bulk, you will probably purchase it by the cubic yard. All you need to know is a cubic yard is 3 feet wide by 3 feet high by 3 feet deep (or 27 cubic feet). At this point, some gardeners panic, but take a breath and read on.

Determine the size of the area you want to cover in square feet. To do this, multiply the length by the width of the space. Next, decide how deeply you will mulch in inches. Multiply the square feet of your garden space by the inches of mulch depth. Divide that number by 324. This is the number of cubic yards of mulch you will need.

If you are buying mulch in prepackaged bags, each bag typically contains 2 cubic feet. Use the same mathematical process and take it one step further. After figuring cubic yards, you have to convert cubic yards into cubic feet. A cubic yard is 27 cubic feet, so multiply by 27.

In the perennial border above needing 4 cubic yards, multiply by 27 and total cubic feet is 108. Remember each bag contains 2 cubic feet so divide by 2 to determine the number of bags needed (54 bags).

Mulching is a labor-saving way to save water, save work and save plants. Choose a mulch and start mulching!

Diana Stoll is a horticulturalist and shade specialist at The Planter's Palette