Tree Leaves

Fall 2011
Autumn leaves
Leaves from deciduous trees, once shredded, make excellent mulch for most vegetable crops, and they enrich the soil. English photo.

By Will Bonsall

Trees and their leaves are probably the greatest natural soil builders on earth, greater even than grasses. (I mean “on earth” literally, as I am not including the oceans.) The incredible proliferation of life that occurs in the forest ecosystems of the world is powered by the prodigious plant biomass of the trees themselves, particularly their leaves.

The lushest grassland cannot compare with forests for bioefficiency, which is hardly surprising when you consider forests’ advantages: dense crowns of broad chlorophyll-laden “solar panels” compared with the narrow blades of grass; the towering upreach of old-growth hardwood trees compared with waist-high grass; the constant accruing of new growth on old as opposed to beginning anew from sod every year.

Also, consider the extensive root systems of trees: Their main structural roots are only a few feet deep, but I have encountered fine feeder roots of beech and maple at the bottom of a 12-foot well in the mountains, and I believe they reach down much farther for water and minerals. Remember that those minerals, which are eventually dropped on the surface as leaf litter, would otherwise never become part of the biosphere; they would stay right down where the glacier dropped them.

Indeed, so efficient are forest ecosystems that the only regions on earth where grasslands are the climax ecosystem – think Kenya, Mongolia, Patagonia, Wyoming – are areas of scant rainfall; otherwise those areas too would have succeeded to forests long ago.

Given the overwhelming ecological advantages of forests, wouldn’t it be great if we humans could just eat directly from them? Perhaps, but we can’t, except for a relatively small number of tree fruits and nuts. Unlike beavers and termites, we cannot digest raw cellulose and lignin; our finicky tummies have evolved to eat more-succulent roots, shoots, fruits and the energy-packed grains. I don’t see us changing that anytime soon.

However, if we could eat INdirectly from the forests – that is, by parlaying deciduous tree leaves and other forest debris into those foods that our guts do prefer – mightn’t that be a very intelligent thing to do, regardless of whether you are also producing and using animal fertilizers?

I use several tons of tree leaves every year, in different forms. For one, I use them as a primary component of my veganic (plant-based) compost, combined with grass, crop residues and ramial chips (shredded brush from deciduous trees – another forest resource). This gets incorporated into the soil, of course, but I also use large quantities of shredded leaves on the soil surface as a mulch. Eventually this too becomes part of the soil, but in the meantime it serves other purposes as well.

Mulching with Shredded Leaves

Most gardeners, including organic, like the look of bare weed-free soil between their rows of crop plants. I like the “weed-free” part, but I’m not so excited about the “bare.” Where in nature do we find the earth naked? In the most severe deserts! Is that our model for benevolent land use?

I don’t consider myself an extremist on this matter; for the first few weeks of the season, when the soil is thawing, a desert model is more appealing to me; after all, desert soils heat up fast! But desert soils are also sere and barren, and I want none of that. So I cultivate early to warm and aerate the soil and destroy sprouting weeds, but as soon as the crop plants are 2 to 3 inches tall, I cover them with a thick blanket of shredded leaves.

I prefer to do the shredding the previous October and November, when the dry leaves are freshly raked and hauled home. With the 1-inch-hole safety grate in place, they come out looking a lot like Corn Flakes or confetti, very neat and easy to spread. As much as possible, I feed them through the shredder as they come off the trailer. If there’s no time for that (if I have more loads to haul before the weather changes), I unload them into a fenced-in heap and cover them tightly with a tarp.

When I do shred them, I immediately pack them into a 6- by 9-foot wooden bin, 3 feet high in front and 4 feet high in back. Tamped down tightly, the bin holds at least two tons. Two big hatch-doors cover the top, and the slope drains off rain or snow before it can enter. This confetti must be kept dry; I don’t want it to heat up and compost, at least not yet.

Mulching a conventionally spaced garden with rows 2 or 3 feet apart is quite a chore if done with hay or straw. It is terribly time-consuming, and you still don’t get a thorough job around the plants, where it matters most. With my intensively spaced rows, often 8 inches apart, mulching with hay is nigh impossible. Yet mulch I will, and with leaf confetti it is quick and easy – and the closer planting means that less soil area requires mulch.

I climb into the bin and break up the dense-trodden confetti into a large rubber trash barrel. Since the paths between my intensive beds are all 6 feet on center, I can keep my big clumsy feet off the bed itself. Holding the barrel under my arm or over my shoulder, I use my free hand to create a blizzard of confetti over the entire bed, covering the plant rows and spaces alike. No matter if it lands on the young seedlings; it sifts right down around them, nestling up against the stems and leaving no bare spots for weeds to take hold. If a big wad of leaf shreds should inadvertently drop on a young plant or if the tiny foliage is smothered by too much at once, I simply walk down the bedside afterward, cuffing off any excess in a matter of seconds. On a 6- by 40-foot bed, I typically apply three or four trash barrels full of tightly packed leaf shreds.

A fairly level seedbed helps ensure that rain and breezes do not leave bald humps exposed. The newly applied fluffy mulch is particularly vulnerable to blowing and washing, so if showers are not expected soon, I sprinkle the bed lightly. This makes the shredded bits settle into a matted layer of interlocking flakes, which will stay put.

For repressing weeds, a half-inch of shredded leaf mulch is more effective than four inches of hay (which brings its own weed seeds with it). Compared with black plastic (I mean really, how organic is that?), leaf mulch allows raindrops to sink in right where they land and holds them there.

I often add an even heavier layer some weeks later when the plants are larger. However, lettuce and broccoli need special care when they are heading out, since errant flakes and dust may ruin the edible/salable part.

At season’s end, the mulch can easily be rototilled in, but even that may be unnecessary. The bottom layer has been decaying over the summer, and I sometimes find that a pass or two with the wheelhoe incorporates the remainder without having to deeply disturb the soil. Or, left over winter, the mulch will be nearly composted in place by spring. I have often sown a late crop (e.g., Egyptian onions for the ensuing spring) following an early crop (such as peas) by merely running a furrowing hoe down the row where the pea vines have been pulled out, the soil being already shallowly disturbed. Most seeds germinate just fine in the stirred-up soil, unless the residue is too thick and trashy or the soil too dry.

Foliage: Full of Fertility

Back to the compost discussion. Conventional wisdom has it that leaves are not particularly valuable as a fertility source, due to their lack of nitrogen. In fact they contain a considerable amount of usable nitrogen, although it’s not immediately available. A couple of personal anecdotes may emphasize this.

Even though I have a woodlot full of fallen leaves, every year I go into Farmington and haul leaves from the roadside or from people’s lawns. They are already raked up, and will otherwise go to the landfill, plus I’m doing everyone a favor. Almost everyone. I always ask the homeowner if I may haul away their piles, and they typically answer something like: “What? Is the Pope Catholic!”

One elderish lady had a real mountain of rock maple “droppings” in her yard and I cavalierly offered to rid her of them.

“No,” she replied with a lovely Austrian accent, “I use them.”

“And how?” I pried.

She explained how she turned the huge pile over once or twice (“mull them over” was how she put it) and spread them on her garden, half composted, before planting. At that late season when I saw it, her little garden was occupied by cabbage mainly, and right fine heads they were. She insisted that maple leaves were all she ever used on it, year after year. Not even much volume of grass clippings, since the garden occupied most of the area, which might have been lawn between the giant maples. Now cabbage is a very heavy feeder, so make of that what you will.

Here’s another story. After years of loading leaves from the curbsides of Farmington (where they were gathered according to newspaper announcements for pickup by the town crews), it occurred to me that I should just pay the town to truck them out to my place. The street commissioner readily agreed – no need to pay, he insisted; I was doing the town a great favor by recycling them for agriculture. (Once upon a time the town’s name was more relevant.)

I looked forward to just waiting for the annual “windfall” to arrive at my place without my assistance, but the second year the trucks failed to come and dump, so I asked the street commissioner pourquoi.

“It turns out,” he informed me in all seriousness, “that they need them out at the landfill.”

“Need them,” I echoed stupidly, certain I had misheard.

It seems they had a problem with the stable manure from the fairgrounds. The “stable manure” is so high in sawdust that even the nitrogen-laden pony doo-doo is insufficient to make it heat properly for good compost. So what are they adding to that “manure” as a balancing source of nitrogen? My maple leaves! And what does the street commissioner (who is not into veganic or organic or sustainable – for all I know he could be a Republican) have to say about that?

“It het up real good.”

Now in the bad old days before I owned a chipper-shredder, I always refrained from adding as many leaves as I wished to the compost heap, lest the unbroken leaves should make a sodden matted layer that resisted decay. The best I could do was alternate many thin layers. But now that I shred them first, I can use any amount I want in a pile, up to 100 percent leaves, like the Austrian lady.

More Ways To Shred

I could hardly manage without my chipper/shredder, which is an attachment on my Gravely walking tractor. I realize that not everyone has one, but there are other ways of breaking up leaves.

My father used to pile leaves in front of his garage door, running his rotary lawn mower through them repeatedly until they were finely shredded. That worked well with leaves meant for compost but is less satisfactory to make the uniform confetti that I use for mulch.

I remind readers of an alternative technology they already know well: If you rake the leaves into a great fluffy mound and tell your kids and all the neighbor kids: “Stay out of this,” by morning it will be as fine as any machine could make it.

I have mulched with whole leaves between more widely spaced crops, but I’ve had some problems with that, notably with ants and shrews. For some reason even coarse shredding seem to minimize those problems. For example, if I remove the grating from the shredder chute, the dry leaves are kicked out nearly unscathed, but if they are damp (as from lying in a ditch, or on the ground uncovered over winter), then the greater density gives them some inertia, and they get roughly shredded in one pass. (Wet leaves MUST be fed through with the grate removed, or else they will constantly plug it up.) A second pass, still without the grate, grinds them fine enough though less consistent; they contain some large bits of leaf, which interfere with my “blizzard” system of quick mulching. However, on the paths between my intensive beds or between wider-spaced crops such as cabbage or tomatoes, the coarser mixed stuff works just as well.

In addition to contributing nitrogen and minerals, leaves are the most efficient builders of soil humus. Even their cellulose component is a profound and stable energy substrate for a vibrant community of organisms that take the elements of earth, sky and water, and transform them into nutrients for those less energy-efficient plants we ourselves prefer to eat (and ultimately into meat for those whose tastes run in that direction).

When we add tree leaves to the soil, it is gratifying to know that we are adding something that, unlike animal manure, gives to the land a lot more than it cost the land to produce. Even more than that, it takes the millennia-old paradigm – that we must either grow food or have forest, because we cannot do both on the same land – and stands it on its head.

Instead of choosing between forests or people, we must choose between forests and starvation. You needn’t be an idealist to hug trees, merely fond of eating regularly.

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