The second benefit of a plastic mulch is that it can rise the soil temperature by several degrees, a situation highly desirable during a cold northern spring. Clear plastic leads the way in heating the soil by as much as 20 degrees F, but also causes weeds to grow because it allows sunlight to pass thru. Black plastic also heats the soil, usually by 8 to 10 degrees F, but doesnt allow weeds to propagate the garden. Plastic mulches really cannot lower the soil temperature, but some gardeners like to use black plastic in the spring and fall to heat the soil, and apply an organic mulch over the plastic to cool down soil temperatures during a hot summer spell. This is a nice idea but the benefit depends somewhat upon how tightly spaced your plants happen to be. In our garden, our plants are very close together (intensive gardening), and by July, when it first begins to become really hot, the leaves and flowers are so large that not much sunlight reaches the black plastic underneath anyway. In fact, you actually have to look hard to even see the plastic when things really start to grow.
So adding an organic mulch might not do that much to moderate soil temperatures in small, closely planted gardens. Because soil under a mulch stays wetter and warmer, earthworms love this environment and multiply like crazy. Earthworm burrows help aerate the soil and their nitrogen rich castings add to the nutrient base when they die. Earthworms exposed to hard, dry non-mulched soil, or soil that is too cold or too wet, do not do well, and a garden without a healthy earthworm population is not a healthy garden.
The third benefit of a mulch is to reduce weeds. Weeding is the dread of all gardeners, but with a black plastic mulch it is minimized to perhaps five minutes per day. Weeds that do manage to peek thru are easily spotted (even by Bill!) and can be quickly pulled. Weeds steal water and soil nutrients from other plants and compete for sunlight, thus reducing crop yields, and a row of young vegetable seedlings can be quickly overtaken by aggressive weeds. We have seen gardens in our area where weeding is neglected, and they look just terrible. This especially being true with large gardens, where the weeding job over-powers the owners and the garden gets away from them. It is also true of first time gardeners, who usually dig a bigger garden than they can handle and dont truly appreciate the aggressive nature of weeds. In our opinion, keeping a garden physically small, employing intensive gardening techniques to improve crop yields, and utilizing black plastic as the primary mulch is the best way to go.
Aluminum foil cools the soil because it reflects the suns rays, and also acts as a deterrent to moths and beetles. For instance, the squash vine borer moth has trouble locating the stem of the plant, where she lays her eggs, if the plant has aluminum foil near its base, as this reflects light and confuses her. The same is true with the potato aphids and bean beetles. A secondary benefit is that the foil reflects the rays and gives the plants leaves more light, which basically means more plant growth. We use a product called reflective aluminum foil insulation that comes in 25 foot rolls and is 24 in width. It contains aluminum foil on both sides and air bubbles in between. This is great when the soil becomes too hot for cool soil loving plants, and it confuses the insects while adding extra sunlight to the garden. It is rather cheap as well and lasts a long time, so you might want to investigate this product.
Organic mulches, also called natural mulches, consist of bark, straw, hay, grass clippings, leaves, wood shavings, sawdust, pine needles, compost, peanut or cocoa shells, husks, newspaper, cardboard and similar materials made from natural materials and products. Since these products are made by nature, they decompose readily and return their nutrients to the earth in a relatively short period of time (one to several years). When applied in sufficient depth, natural mulches cool the soil by 8 to 12 degrees F, so they can be used during hot summer months to help plants that prefer cooler soil temperatures. This is important for such plants as peas, which like a cool soil of around 68 degrees F. By keeping the soil cooler longer, you can extend your pea harvest by a week or two - and if you know anything about peas, that can mean a lot of extra peas! However, dont ever apply a mulch to warm-loving plants, such as tomatoes, peppers, melons, cucumbers and the like, as you will only slow their development. Here in cold Wisconsin, these plants need as much sun and heat as possible, and an organic mulch will cool, not warm, the soil. The thing to remember here is that an organic mulch is not always good!
Newspaper is a good mulch and is now printed with soy ink, a natural ingredient. When used in ten over-lapping sheets, they block out weeds and moderate soil temperatures. Newspaper blocks weed growth, but the glossy newsprint can contain higher levels of lead, so avoid these and stick to the black and white papers only, excluding the color comics section. The problem is that it can blow away to the neighbors lawn and looks really bad for visitors unless covered with another type of mulch. We dont use it, but it might make a good mulch for rural gardens and survivalists.
Leaves are free, but they must first be chopped-up and allowed to sit for at least one year, otherwise you could have bad chemical reactions (allopathic effects) in your soil. This is especially true with maple leaves, because they contain a chemical called phenol that reduces the growth of vegetables (especially the cole crops) by inhibiting root formation. After one year the effects of the phenol will have dissipated, so dont ever use fresh maple leaves as a mulch.
Grass clippings are okay because they add nitrogen rather than take it, and, since the clippings are small, fit nicely around the roots of plants. They can moderate soil temperatures, but must be at least 4 inches think (thats alot!). But clippings may contain herbicides, lead from automobile exhaust and thistle seeds, all of which are the last things you want in your vegetable garden. They also have the bad habit of matting over time, preventing moisture penetration, and can develop an offensive smell which your neighbors will just love. Clippings that have started to go to seed can introduce a new crop of unwanted plants, and have the bad habit of providing a perfect bed for flies and gnats. Be advised that grass clippings can bring along a plague of problems, so use them with caution.
Hay is readily available but you must be wary because the product contains weed seeds. If you mulch with hay you will probably add many more weeds to your garden than you thought. When the first cutting of the hay crop takes place, many of the grasses contained in the hay, such as orchard grass, June grass, timothy grass and others, have gone to seed, which become bailed with the hay. These are perennial grasses that are difficult to remove once entrenched, so be careful here. Second and third cuttings of hay, later in the season, will contain fewer seeds, so, if you must use hay, try to acquire a later cutting rather than an earlier one. Straw is much like hay and can contain some of the same grasses and seeds, so use this with caution as well. Salt marsh hay is said to be the finest and most preferred, because the seeds dont tend to germinate outside of the saline marsh in which it is acquired. It is both expensive and hard to find, so you might not be able to locate it. Here in Wisconsin, it is not available as far as we know.
One of the important things to understand about natural mulches is that they will decompose over time, and the decomposition process will normally require nitrogen. Items like wood chips, sawdust, newspaper and straw are high in carbon but low in nitrogen, so the microorganisms that break them down will draw the nitrogen from the soil, thus causing a low nitrogen situation for your plants. This is just another reason why we dont use organic mulches on our garden, because unless you know what you are doing, you could be doing more harm than good. The small amount of nutrients natural mulches return to the soil usually isnt worth the effort to purchase or apply them, and they make great beds for rodents and insects, especially slugs and snails. Natural mulches do look the best however, and are preferred by organic gardeners and purists.
Geotextile mulch fabrics, also known as landscape fabrics, are woven synthetic strands and contain small holes that allow water to pass thru them and, supposedly, act as a weed barrier. They dont raise soil temperatures as clear/black plastic does, and are rather thick and very difficult to work with. Cutting and making holes and slits in them is a nightmare and the geotextiles will eventually allow weeds and roots to become intertwined with the woven fabric, and then you will learn why we dont use them. DONT EVER USE THESE MATERIALS. You will regret the day you ever put them in your garden, and they are terrible to remove once roots get intertwined in the fabric as it breaks down. Forget these!