It was nice while it lasted, but now the autumn`s spirited coloration lies sprawled across our lawns like an unrepentant dog across a couch.
With rake in hand, we go out to do the season`s battle with the leaves, knowing that to ignore them will make for a very dead lawn next spring. But what do you do with those leaves? The smell that used to blanket communities on fall weekends when leaves were burned in the middle of countless driveways has gone, probably for the better. These days, plastic bags line the curb or alley on garbage pickup day, each storing a few pounds of the trees` seasonal production.
The neighbors have gotten used to seeing a few of those bags disappear overnight. Their contents, in turn, disappear down the throat of an old shredder to become the basis of that season`s compost pile, which will eventually be worked into my vegetable garden. But in this world where everything seems to have its price, could anything that`s free be that beneficial? Are all those leaves good for a garden?
Leaves ”add a lot of bulk to your compost and are a great addition to garden soil, making it more friable, porous,” says Michelle Eggerss, a horticulturist for the University of Illinois Cooperative Extension Service.
”As far as fertilizer, you don`t really add compost for fertilizer, more for soil structure. We have such highly clay soils that you want to add compost so water drains better, roots don`t get waterlogged and plants therefore grow better.”
”In small amounts, leaves are very good for the soil,” says Jim Schuster, an Extension senior horticultural adviser. In the same breath, he warns that in large amounts, the nutrients supplied by the decomposing leaves may make the soil unbalanced, with some elements in such great supply that they interfere with the plant`s ability to take up other nutrients, stunting growth.
”I`ve seen it where within six years, nutrient levels are so high, meters can`t even read them anymore; they are off the scale,” Schuster says. Such cases are the exception, occurring where year after year several feet of leaves have been piled on the garden and tilled under. In breaking down the leaves to their final state, microorganisms draw nitrogen from the soil to convert the carbon in the leaves, robbing any plants growing in the area of that essential element. As the chemical reaction proceeds and the leaves are broken down, the nitrogen is released back into the soil, feeding the growing plants.
The end product of this reaction is a highly alkaline residue, similar to the ashes produced if the leaves are burned. This resideue can actually raise the soil pH, which in the Chicago area is already on the alkaline side of the scale, Schuster says.
While he doesn`t disagree with the point of Schuster`s message, Dr. Gary Watson, a plant physiologist at the Morton Arboretum in Lisle, says soil pH is not usually affected by the incorporation of leaves into the soil. ”I have mulched soils with leaf mixtures, and over the long run the soil pH has actually been lowered,” he says. ”Well-drained soils tend to be more acidic than poorly drained soils. So over the long haul, I had a much better-drained soil, and the leaves were the primary reason pH was lowered.”
Watson says pH is the last thing to worry about when working with types of soils that really need organic matter. ”The benefit of composting leaves and adding to those soils far overshadows the dangers of pH changing. The potential for good far outweighs the potential for bad.”
Watson lists three types of soils found in most of the Chicago area. One is the soil around a new home, where what passes for topsoil may be the lowest clay dug from the home`s foundation. This clay, which may look like the darkest topsoil, needs large amounts of organic matter, of which leaves are just one source, to make it grow plants to their optimum state.
A second soil type is found in areas that were once forested. Watson says those soils have very thin topsoil layers with most nutrients in the top several inches. As the gardener works the soil, more of the lower clays are brought up, requiring addition of organic matter to increase soil porosity.
The third type, which predominates in most of Illinois, is prairie, where organic matter was added each year as the flowers and grasses died back. Topsoil in a prairie area can be several feet thick and contain sufficient organic matter. Although each year`s gardening burns up organic matter, an overzealous attempt to turn more back in could lead to Schuster`s scenario of a soil that`s too rich in certain nutrients.
The Extension service recommends a soil test every three years to determine the levels of nutrients (other than nitrogen, which is not stable enough to measure accurately). If the soil proves to be deficient in some areas and the gardener finds that water drains slowly, digging in organic matter would be one of the first prescriptions. The tests also show the soil`s percentage of organic matter, which Schuster says should be in the 4-to-7-percent range. ”If it is running 7 to 8 percent, you don`t want to add a lot more,” he says.
Composting, away from the garden, is the best way to treat the leaves. When a nitrogen source is added to a pile of leaves, the microorganisms can work their magic within a year and provide a product that won`t tie up nutrients for plants trying to grow there. A well-made compost heap has layers of various materials in specific ratios, but for a pile composed mostly of leaves, nitrogen can be added in the form of manure, grass clippings, fertilizer or commercial products that add the bacteria to start the decomposition.
”If you`re collecting right now, the ideal situation is to compost them through next summer and add them (to the garden) after that growing season,” Watson says. His method is to dump the compost on the garden in the fall to make room for that season`s leaf collection, whether the compost is turned under then or the next spring.
There are other potential problems with raking leaves directly into flower and vegetable gardens:
– Do not use diseased leaves unless you compost them to kill the pathogens that could survive the winter. The other alternative is to destroy those leaves, says the Extension`s Eggerss. She says there have been many reports of leaf diseases in the area this year.
– Never use whole leaves to mulch perennials, as they tend to mat down and suffocate the plant material they are supposed to protect. They also tend to blow off their intended home. Use something fluffier, Eggerss recommends, such as straw, and apply after the ground freezes solid.
To allow leaves to break down faster, shred them, either with a shredder or a rotary lawn mower. Watson says he used to rake leaves into a line along his shrubs and run the mower over them, throwing the shredded product under the bushes as mulch.
Even shredded leaves may suffocate lawns. Schuster recommends mowing north-south and east-west to break them up as much as possible; Watson recommends raking as many as possible and using a catcher on the lawn mower during the last mowing of the season to remove them. ”I remove the oak leaves, which are bigger and tougher than the ash leaves that fall earlier,” Schuster says. Too many leaves, even if shredded, can help cause thatch problems in lawns.
There isn`t much argument that leaves are helpful in the garden, just in how many neighbors should be tapped to help in the endeavor. As one neighbor says, ”I used to go collect them but I had a fight with one neighbor so I don`t do it anymore.” There`s that potential problem as well.
