Regional Gardening Reports :: National Gardening Association

In the Garden:
New England
July, 2004
Regional Report

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Annual weeds, such as this lambsquarter, are quick to colonize bare soil.

Organic Weed Control

I've never used a toxic-chemical herbicide to control weeds, but I must admit I've been tempted. Although I try to do everything right to stay ahead of the weeds, every year by midsummer at least one part of my yard or garden has succumbed to the invasion. Our local garden centers' displays of herbicides confirm that I am not alone: Weeds are tough to control. And this is especially true if you garden organically.

Protect Bare Soil
Mother Nature doesn't like bare soil, and weeds will quickly colonize any open patch -- it's nature's way of protecting the soil from erosion. Keep this in mind as you plan and plant your gardens. Protect your garden's bare soil with mulch -- before Mother Nature does it herself, with weeds. Herbicide manufacturers encourage us to look at weeds as the enemy, but they are also a good reminder that we need to protect and preserve our precious topsoil.

In garden beds. By covering the bare soil around plants with grass clippings, shredded bark, or straw, you'll keep many weed seeds from sprouting. Any that do sprout will be easy to pull from the loose mulch. Plus, the organic matter will enrich the soil as it decomposes. Protect any tilled but unplanted areas with a thick layer of straw or other mulch. If you have a large area that will remain unplanted for several months, consider planting a weed-smothering cover crop.

In walkways. If you till your garden space each spring, you'll need to create new beds and paths each year. Instead, consider creating permanent beds and walkways. That way, you can apply soil amendments, such as compost, just to the planting areas, and you can also make permanent paths. Walkways made from bricks or pavers are handsome, but also time-consuming and expensive. A simpler method is to lay down landscape fabric and cover it with decorative mulch. Carpet remnants are excellent for smothering weeds in paths, and you can often get them for free. You can also use a thick layer of newspapers, but this will eventually decompose. Avoid black plastic, because it is slippery to walk on, even when covered with mulch.

Even with thorough mulching, you'll still need to pull weeds, but you'll have far fewer than if you hadn't mulched. Try to prevent weeds from getting out of hand. Spending just 15 minutes each day pulling weeds can make a difference. There are dozens of styles of hoes and cultivators available; however, I find I spend most of my time pulling weeds by hand.

Repeated tilling between plants and in paths will control weeds, but only if you do it regularly. And frequent tilling has disadvantages. It brings up buried weed seeds, encouraging them to sprout. It breaks up soil structure -- the aggregates of particles that allow for good drainage -- so that, ironically, tilling to loosen soil actually results in compaction. It chops perennial weeds' roots into pieces, creating the potential for even more plants. It also uses gasoline and emits pollutants. Although tillers are helpful tools in the garden, they aren't the best way to manage weeds. A thick layer of mulch is a better strategy.

Herbicide Terminology
What options do organic gardeners have with respect to weed control products? There are a number of herbicides approved by the Organic Materials Review Institute: Just look for the acronym OMRI on the product label. As with all herbicides, it is very important to understand the terminology associated with their use.

"Pre-emergent" and "post-emergent." Pre-emergent herbicides prevent weed infestations by interfering with germination; that is, they prevent seeds from sprouting. They will also prevent crop seeds from sprouting, so use pre-emergents only in established plantings and NOT in new seedbeds or newly seeded lawns. Post-emergent herbicides, on the other hand, are used to control actively growing plants.

Selective and non-selective herbicides. Selective herbicides target specific groups of weeds. For example, herbicides that target broadleaf weeds kill dandelions but won't harm grasses. Non-selective herbicides damage or kill all plant matter.

Organic Pre-emergent Herbicides
A scientist's accidental discovery has brought us a surprising pre-emergent herbicide. Researchers at Iowa State were investigating the potential of corn gluten in preventing plant diseases when they noticed its herbicidal qualities. Corn gluten inhibits the growth of a newly germinated seed's feeder roots, rendering the seedling unable to take up water, thereby killing it. Corn gluten, a byproduct of the manufacture of corn syrup, is found in animal feeds, including dog food, and is safe to use around children and pets. In addition to its herbicidal properties, it has the added bonus of containing 10 percent nitrogen, so it acts as a fertilizer as well. Corn gluten is available as a powder and in pelletized form, the latter of which is easier to apply. Use only on established plants and lawns; remember that it will inhibit the growth of desirable seeds as well as weed seeds. Timing is everything: it must be applied before weed seeds germinate. Under ideal conditions, the effectiveness lasts up to 5 weeks; however, rain and soil microbial activity diminish this considerably.

Organic Post-Emergent Herbicides
Post-emergent herbicides are used on actively growing weeds. The organic herbicides currently available kill only the plant parts they come in contact with. (You'll see terms like "burndown" to describe the effect.) In contrast, some non-organic herbicides are systemic, meaning that they are taken up by the leaves and transported to stems and roots; spray it on one leaf, and the whole plant dies. In general, organic herbicides are most effective on young growth, and repeated applications are necessary to control perennial weeds.

Citric acid-based herbicides. Some gardeners report success spraying straight vinegar (5 percent acetic acid) to kill weeds. A study at Cornell University found that 5 percent acetic acid was indeed effective at controlling annual weeds, such as crabgrass, for several weeks. However, since the root systems of perennial weeds are not affected, they readily regrow. Repeat applications were necessary for continued control. Stronger concentrations of acetic acid, found in some commercial organic herbicide preparations, provided better long-term control, as might be expected. However, the material is caustic and must be applied with care. Some studies have shown that although acetic acid treatments can temporarily lower soil pH, the pH returns to normal within 48 hours.

Other ingredients commonly found in organic herbicides include potassium salts of fatty acids, found in herbicidal soaps, and essential plant oils, especially clove oil (eugenol) and 2-phenethyl propionate (extracted from peanuts).

It's best not to rely on herbicides, even organic ones, as your sole weed management strategy. Mulching, weed barrier materials, hand weeding, and cover cropping can all play a role in managing weeds. And rather than curse those weeds, remember their role in protecting and preserving your precious topsoil.

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