2008-07-26 / Columnists

Opinions Vary on Wide Range of Available Garden Mulches

Nature Notes
By Patricia Martin

Last week, I began to write about the wonderfully controversial topic of mulch, and I discovered that every gardener seems to have a personal opinion about mulching, its advantages and disadvantages, what type to use, when to apply it, and to what plants, etc. This week I would like to continue the discussion, focusing on types of mulch and how to choose which one to use.

There is no perfect mulch, but there are factors that make one mulch preferable in any situation. Some things to consider include cost, availability, ease of application, appearance, water retention and penetration, air exchange, lasting qualities, staying power, and odor.

Let's look at each of these factors.

Mulch should not be too expensive. There are so many mulches available that you should find one at reasonable cost that will meet your needs. The mulch you choose should be relatively easy to apply. After all, who wants to be a slave to mulching? You want to pick a mulch that will not offend your neighbors, and mulches that may be good for a vegetable garden may not be the right thing for your flower beds. You want to select a mulch that will let water soak down to the plant roots, and also allow gas exchange. If you're mulching a path, these two factors may not be so important. In certain cases, such as in a vegetable garden, you want a mulch that can turn into the soil at the end of the year so that you would want one that decomposes quickly. Chopped or ground mulches decompose faster than coarser mulches. Inorganic mulches don't break down at all. If you live in a windy area, you probably don't want lightweight mulches like straw or buckwheat hull, as they will blow away. The same is true with paper or plastic mulches, which have to be anchored. If your garden is on even just a small incline, small, fine bark chips can wash away in a downpour. Certain mulches have strong odors, such as poultry litter and manure, and even grass clippings can smell. And there are those people who don't like the smell of chocolate, given off by cocoa hulls.

As I mentioned last week, there are inorganic and organic mulches. Inorganic mulches are those that contain no plant material in their origin. Here are some of the inorganic mulches which are popular:

Aluminum foil can be fairly expensive, and some say so artificial that it's unattractive. It can be used between rows of vegetables living an opening of a couple of inches. It's an okay insulator, and it does reflect the sun up onto the plants and so increases photosynthesis. It does not, of course, biodegrade, and there may be some danger of aluminum toxicity.

Fiberglass is another mulch which is completely fireproof, though it is quite costly, and many think not very attractive. It may cause a skin reaction when being handled. Fiberglass readily absorbs moisture, like a sponge, and then compacts. It's a great insulator, however.

Geotextiles are sort of a compromise between black plastic and organic mulch. They aren't as feeding as true organic mulches, but they do allow air and water exchange, and keep down weeds. They're usually made from petroleum byproducts. There are a few polyester products available which last longer. These fabrics should be laid down and then covered with another mulch to keep them in place, to prevent the sun from breaking them down and to make them look better. X's can be made where you want to put in plants. Weeds, by the way, will still grow in the upper layer of mulch.

Plastic can be clear, green aluminized, or black (black being the most popular, as it prevents light getting to the soil). No weeds can grow beneath plastic, particularly the black, and it will help soil retain moisture by preventing evaporation. Most people cover it with bark or wood chips, etc. Black plastic does cause the soil temperature to increase, as one study found, between 3 to 7 degrees Fahrenheit on a sunny day. Black plastic does cause water from penetrating and the soil doesn't breathe. It's usually more appropriate for vegetable gardens than flower gardens. The edges much be weighted down, and holes, X's, or T-shaped should be cut so the plants can come through and some water can get down. Plastic doesn't break down, though new photo-degradable plastics are being developed.

Stones, including gravel, shale, crushed marble or limestone, etc. can be used as mulch, and it's one of the most decorative, but it's not much use in the vegetable garden or anywhere you have to till. It's fairly costly, and is a permanent mulch. Weeds do find their way through crushed stone fairly easily, unless it's underplayed with geotextiles or plastic. Stone does retain some of the heat from the sun to warm the soil. Some elements will leach out of the stone over time, and remember, limestone will raise the pH of the soil and should not be used around acid-loving plants. Stone mulches won't blow away with the wind, nor mat down in rain, nor tie up soil nitrogen, and they do allow water penetration and air exchange.

There are a wide variety of organic mulches, those made of plant products, and these seem to be the most popular. Here are a few that are commonly available around here.

Bark is probably the most commonly used, and most versatile of the landscape mulch. It comes in all kinds of colors, shapes, sizes, and texture. Bark is different than wood chip as it doesn't contain the heart wood. Around here, the most commonly used barks are the shredded cedar and Cyprus barks which, until recently, I thought were fine mulches. According to Jeff Young, the arborist who works with lilac trees up here, the problem with these barks is that they shed water, allowing little of it to reach the soil. They biodegrade very slowly and so add no nutrients to the soil. They may prevent weeds from appearing, or at least very few, but they also hold insects, fungi, and over time may damage perennial plants. He recommends ground hardwood bark for mulch. If you look at the pod that he worked on in Marquette Park this spring, you will see that he replaced the redwood bark chips with ground hardwood mulch and kept it well back from the trunks of the trees, so that mice, etc. will not live right next to the trees and eat the bark.

Cocoa hulls are dark in color, and absorb heat and warm the earth. They decompose slowly, add lots of nitrogen phosphorus potassium to the soil, and they smell of chocolate. A couple to three inches are plenty around most plants. They retain moisture for long periods of time, and may get slimy after about six months. They may also compact, and may develop molds in humid conditions but this is usually harmless. These things can be lessened if cocoa hulls are mixed with sawdust or pine needles at a ratio of 2:1. The biggest drawback to cocoa hulls is the cost, so they're usually only used for rose and flower beds. They are fairly high in potassium, and may be toxic to some plants.

Coffee grounds are a good homemade mulch, though it may take a while to build up enough, unless you drink a ton of coffee. They will cake, and so they need to be spread no more than an inch or so. They're great in a vegetable garden, or added to the compost pile.

Compost, or at least partiallydecomposed compost, is a great feeding mulch. It biodegrades quickly, and becomes humus. In the process, it adds many nutrients to the garden. We're fortunate on Mackinac, in that the city runs a wonderful composting center, which mixes horse manure, yard waste, and other biodegradable materials, so that it's not just lawn clippings, as in many municipal centers. If you're using the compost from the Department of Public Works, make sure it's not too hot when you spread it. If you're composting your own, I have some suggestions, but I'll save that for another column.

I've used evergreen boughs to cover plants in the winter, particularly my roses, and it does give them good winter protection, but you do have to remove them in the spring.

Roofing paper or felt is fairly good along vegetable rows, as it absorbs heat and warms the soil around the roots. It's fairly expensive, but if you have some left over, you can use it in the vegetable garden. Be a little cautious, however, because some of the chemicals used in producing it may be toxic to some plants.

Grass clippings can be used in vegetable gardens, but they should be spread sparingly when new seeding are popping up. It's best if they're dried before they're applied to the garden, and spread lightly, as otherwise they can make a slimy mess. Lawn clipping may also have with them weed seeds that you would rather not have in the garden.

Hay and straw have often been used as mulches, but they're more appropriate for vegetable gardens than flower gardens, as they can be a bit unsightly. Chopped hay and straw generally look better, and allow more water penetration and break down faster. First cutting of hay usually has more grass and weed seeds to your garden, though second cut has fewer. These are popular for mulching strawberries, grapes, and fruit trees. It periodically needs to be fluffed periodically. Partially-rotted hay is better than fresh, as it doesn't take so much nitrogen from the soil.

Leaves are nature's favorite mulching material and contain many of the nutrients needed by plants. Some leaves like maple, birch, and elm leaves tend to mat and become soggy. If they're chopped, they and most leaves make a better mulch as the water can pass through easily. Running them over with a lawn mower usually chops them up sufficiently, and then they can be raked into the garden.

Well rotted manure can be used as mulch which, of course, adds all sorts of nutrients to the soil, but it has to be well rotted, otherwise you will burn your plants. Manure will also encourage the growth of weeds, as well as your garden favorites.

Peat moss is often associated with mulch, however, it's not really a very good mulch, though it's a good soil conditioner. It is extremely slow to decompose, is slightly acidic, and has no value as an organic fertilizer. It also ties up a ton of water. It can tie up six to 12 times its weight in water. It takes a heavy rain to soak through the peat moss.

Sawdust is often used under blueberries and other acid-loving plants. It decomposes slowly, and seems to discourage earthworms, so it's not the best choice for vegetable gardens. It may pack down fairly quickly, and does tip up a good bit of nitrogen. Sawdust can be combined with other mulching material to increase water penetration, and deal with some of its other drawbacks.

Wood chips, from a brush chipper, make a good mulch, realizing that like other wood products, will tie up some of the nitrogen, at least initially. Some people mulch too deeply with wood chips, five to six inches instead of two or three, which causes problems. If you're buying wood chips, be careful that they've not soured. Souring occurs when mulches are stockpiled and begin to break down anaerobically, which produces toxic substances like methanol acetic acid, and ammonia (they stink sort of like rotten eggs, vinegar, or ammonia). Different chips are better for different types of uses. Cedar chips, like their bark, break down slowly and so are great for paths, etc. Like the bark, they should not be mounded up against trees or shrubs.

There are a number of other mulches that I haven't talked about, including paper, seaweed, salt hay, etc., but they're not commonly used around here. This whole thing is probably more than you ever wanted to know about mulch, and now you know why you were afraid to ask.

Trish Martin is a yeararound resident of Mackinac Island, has earned a master's degree in botany from Central Michigan University, and owns Bogan Lane Inn.

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