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fresh sawdust as seed and weed killer

1,000 lb roll at the beginning of the crop row, and two people to unroll it.

Organic mulch materials include grain straw, fresh or old hay, fresh-cut forage or cover crops, chipped brush, wood shavings, tree leaves, cotton gin waste, rice or buckwheat hulls, and other crop residues. Hay and straw are among the most widely used organic mulches in organic horticulture. Cover crops can be grown to maturity (flowering), mechanically killed, and left on the soil surface to provide an in-situ organic mulch for no-till planting. Leaf mold (decomposed tree leaves), compost, and aged manure have also been used as organic mulches, although their crumbly texture may not provide as effective a barrier to weed seedlings as other materials.

Tree Leaves

Because straw is so much less likely to introduce serious new weed problems than hay, organic horticultural farmers located in or near grain-producing regions where straw is available and affordable often prefer straw over hay. The high C:N ratio of straw precludes much release of N from mulch to the current year's crop, but usually does not lead to tie-up of soil N, as long as the mulch lies on top of the soil and is not tilled in.

Published June 1, 2020

Usually, some weeds will eventually emerge through an organic mulch. Fast-growing, canopy-forming crop like sweet potato, squash, or snap bean often shade-out these late emerging weeds. In slower-growing, less competitive vegetables like onion and carrot, manual weeding or application of additional mulch may be required to maintain satisfactory weed control.

When you combine the recommended four parts of shredded leaves and one part green waste, it’s fairly easy for most of the dry brown material to come into contact with most of the green waste. But when you’re talking sawdust, you’d have to limit yourself to VERY small amounts to avoid going way out of whack on approximating the correct 30 to 1 carbon-to-nitrogen ratio.

BUT this does NOT mean that you can’t compost your wood waste! Wood IS a natural substance and it will become a soil-like material; just not in the average home compost pile or drum system. The easiest answer is to just pile it up and allow it to break down naturally, which will take several years. As with all compost piles, the stuff on the bottom will be ready first, so I’d check things out close to the ground after a year or two.

And finally, I think it would be a real good idea to separate out the sawdust from that walnut wood. As you probably already know, black walnut—the type most often used in woodworking—contains juglone, a naturally occurring compound that stunts the growth of (or just plain kills) many other plants, especially tomatoes and other popular backyard crops. Although the concentrations are highest in the roots, there is some juglone in every part of the tree, and compost made from black walnut sawdust might send some of your most prized plants to sing in the Choir Invisible.

It’s even tricky to compost the stuff. As I often explain, the best compost is made by combining carbon-rich “dry brown” material, like shredded Fall leaves, with “wet green” sources of nitrogen, like grass clippings and kitchen waste. Sawdust is a ‘dry brown’ material, but it’s a much more highly concentrated form of carbon than leaves.

Oh, and I would hope that this is obvious, but don’t use pressure treated wood, old railroad ties or other toxic wood in any form. Any sawdust from treated wood should be disposed of safely and legally—not in your landscape or even in the woods.