Managing Weeds with a Light Touch (Part 2 of 2)

comments (3) April 19th, 2009

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For weed-free paths, layer tar paper over the soil, then cover with gravel or other paving material.Click To Enlarge

For weed-free paths, layer tar paper over the soil, then cover with gravel or other paving material.

Photo: Kim Jaeckel

Once you have acquired some biological background on the weeds that grow in your garden and have established realistic weed tolerance levels, as I discussed in Part 1 of this article, you are ready to implement strategies that will rid your garden of excessive weeds and prevent new ones from taking their place.

This approach to weed management is often referred to as Integrated Weed Management or IWM. IWM focuses on understanding the conditions that allow weeds to grow and on reducing those conditions. By addressing the basic causes of weed growth, not just treating the individual weeds themselves, you can help your garden develop weed resistance, keeping weed numbers low enough to prevent excessive competition with food crops, yet retaining the benefits of low numbers of certain beneficial weed species. An important feature of the IWM approach is that herbicides are rarely, if ever, necessary.

Weeds need life-support systems in order to grow. These include adequate soil type and condition, water, nutrients, sunlight for photosynthesis, and space for growth. When designing (or redesigning) your garden, you can build in weed-prevention components that deny weeds one or more of these essential life-support systems and thereby prevent unacceptable levels of weed growth.

Defend paths against weeds
Many gardens have unpaved paths or paths covered with a permeable layer of gravel or loose bricks or stones. These paths are wide open to weed invasion.

Weed-free paths
  To discourage weeds from growing in your garden paths, lay several layers of tar paper over the soil, then cover with gravel or other paving material. Tar paper is slow to decompose, so it will block light and deter weed growth for many years.
 
To prevent or reduce the likelihood of weeds growing through a permeable paving, you can place several layers of heavy building paper or roofing paper on the soil before installing the paving material. Tar-impregnated roofing paper is very durable and does not decompose rapidly. By blocking light, tar paper will prevent weed growth for many years. Synthetic weed barrier mats, discussed below, serve the same function, but tend to cost more.

You can weed-proof unpaved paths by covering them with 4 to 6 inches of readily available mulch materials that deny weed seeds the light or nutrients needed to grow. Our favorite mulch for garden paths is sawdust. Not only is it usually free for the hauling, but microorganisms attempting to decompose the sawdust tie up soil nitrogen, rendering it unavailable for weed growth in the path. In this sense, sawdust acts as an herbicide; remember to use it only on paths, not on your planting beds. As a bonus, the rough surface of dry sawdust also helps deter slugs and snails, whose soft bodies are vulnerable to abrasion when they try to navigate over this material.

Tip the balance in favor of food crops
As I explained in Part 1, disturbed soil is a highly favored habitat for many weeds, whose rapid germination and rate of growth enable them to get a head start on slower-germinating food crops.

Since soil cultivation is a regular activity when preparing beds for planting, disturbed soil is a common feature of most kitchen gardens. The trick to overcoming this seeming dilemma is to combine a number of horticultural strategies to tip the ecological advantage to the food crops instead of to the weeds.

Limit light. When weed seeds germinate, they have a finite amount of energy available to them to push up through the soil and reach sunlight. Weed seedlings denied access to light will die without ever making an appearance above the soil. The most effective way to do this is to cover the soil with a mulch of organic plant residues, such as compost or weed-free straw or hay, or with a synthetic weed barrier fabric.

To be effective in preventing or limiting weed growth, mulches must be applied immediately after soil cultivation or other soil disturbance; for example, after pulling weeds or readying the soil for planting. This timing is critical to prevent sunlight from reaching weed seeds brought to the surface when the soil was disturbed; it also prevents migrating seeds from settling in. It is best to have mulch on hand before you begin digging or weeding so you can cover cleared sections before moving on.

Organic mulches must be applied deep enough to overcome the attempts of germinating seeds to get to sunlight. But too much organic mulch can smother the roots of desirable vegetation and impede water penetration and gaseous exchanges in the soil. On vegetable beds, apply at least 3 inches but not more than 6 inches of organic mulch. In perennial beds containing such plants as artichokes, berries, grapevines, or fruit trees, be careful to keep the mulch several inches away from the stems of the plants; mulch mounded against plant stems retains moisture and promotes disease.

Weed access to light and growing space can also be limited by a technique called close planting. If your garden soil is loose, well drained, and full of rich organic compost, you can plant vegetable seeds or transplants very close together without sacrificing yields. The close spacing will enable the transplants to occupy most of the soil habitat, inhibiting weed germination and shading out those weeds that do manage to lift their leaves above the soil line.

Another technique, called interseeding, involves sowing seeds of low, fast-growing annual or short-term perennial flowers, such as sweet alyssum (Lobularia maritima) or scarlet flax (Linum grandiflorum var. rubrum), in the bare soil around vigorous crops such as potatoes or berry vines that eventually produce a significant amount of top growth. The flowers serve as colorful temporary fillers that germinate quickly and occupy the soil spaces in and around these slower-growing crops, protecting the soil from weeds until the flowers themselves give way to the more aggressive food crop.

Another method for excluding light from weed seeds is to cover planting beds with a synthetic weed barrier fabric. These fabrics, typically made from woven polyethylene, are widely available at garden centers. Their fine weave excludes light while allowing the passage of water and the exchange of air above and below ground. You should prepare the soil for planting before covering the cultivated area with a weed barrier fabric. Cut holes into the fabric at appropriate intervals and insert transplants into the soil through the holes. Then cover the barrier fabric with straw or other light mulch to protect it from degradation by sunlight. This approach is especially efficient with perennial plants, where the weed fabric can lie undisturbed for the 5 to 10 years of its functional life.

Limit water. Where feasible, use drip, ooze, or furrow irrigation to water your garden. Drip and ooze irrigation systems place water directly in the root zone of plants and apply it slowly enough for plants to absorb most of it soon after it reaches the roots, so little moisture is left over to support weeds. By contrast, overhead irrigation systems apply water indiscriminately over the soil surface, providing water to both garden plants and weeds.

The amount of water available to weeds can also be limited by planting vegetables on raised soil mounds and irrigating them via furrows dug beside the mounds. The furrows and the edges of the mounds where weeds get enough water to grow can be cultivated. Meanwhile, the tops of the mounds remain relatively dry, so few weeds germinate there.

This technique may not be effective if the mounds are shallow (less than 4 inches high) or in clay soils where water may move upward by capillary action. When this happens, the tops of the mounds become wet after a furrow irrigation, a phenomenon known as subbing.

Manipulate soil fertility. To prevent weed seedlings from getting a jump start on slower-germinating vegetables, try holding back on nitrogen fertilization at planting time. Nitrogen encourages fast growth of leaves and stems, and weeds will make use of it ahead of most vegetables. Instead, make sure there is adequate available phosphorous in the soil. This nutrient is key to root growth, which temporarily slows above-ground growth of foliage while a healthy root system is being developed. The vegetable plants will welcome ready access to phosphorus during the initial growth stages (as will the weeds), but the temporary absence of excessive nitrogen might help level the playing field for vegetables and weeds at the soil surface.

  If all else fails and weed you must, get more info on weeding tools and methods at FineGardening.com:

Short-Handled Weeding Tools

Weeding Made Easy

Video: Controlling Weeds
   
The Integrated Weed Management strategies I have just described will help you to prevent or significantly reduce the presence of weeds in your garden for long periods of time. To the degree that these strategies become permanent components of your garden planning and operation, your battle with weeds will require less and less effort and expense. With these methods in place, the occasional weeds that crop up can be tolerated or easily pulled, hoed, mowed, or cultivated out. In addition, you will enjoy the fruits of increased biodiversity in your kitchen garden. In most cases, herbicides will not be needed in order to keep weeds at bay. And if they are, they can be applied in very limited amounts in specific locations as a one-time transition tool until these long-term weed prevention methods are implemented and fine-tuned.

by Sheila Daar
February 2000
from issue #25


posted in: weeds

Comments (3)

lucyg22 writes: I don't recommend synthetic weed barrier fabric at all. 1) I used it a number of years ago in a new peony bed, where I didn't expect to move plants for a long time. I cut X's in the fabric as directed on the pkg, but they turned out to be too small (needed to be around 18" in diameter, and actual holes, for mature peonies.) The fabric prevented new shoots from coming up in the spring. 2) Worse, though, is the fact that weeds sprout in the decomposed much on top of the fabric. This happens every year after the first. I have to do about the same amount of weeding in this bed as any other. 3) Deep-rooted vigorous weeds will sprout in the mulch and send their roots down through the fabric, so it gets pulled up when I pull the weed. 4) Whenever I take a shovel to this bed for dividing, transplanting, etc, I chop up the fabric and I have to fish pieces out of the dirt I've dug. Don't use this stuff!
Posted: 10:38 pm on May 13th
Blewbury writes: Why recommend tarpaper when paper works just as well? Most tar paper is treated with coal-tar derivatives which induce acute to chronic disease in animals. Many gardeners today wish to avoid toxic substances to their gardens and there is no information in this article about the toxicity issue.

Alternative: Water the ground well, place down 8-12 layers of newspaper watering well with each addition. Water when you are done. (Today's newspaper inks are soy based, so this is an organic weed barrier.) Then place down the mulch/gravel of your choice when you are done.


Posted: 12:22 pm on April 28th
nancynursez637 writes: To control weeds in your garden beds you might grow things more closely, For instance in the bush beans, i grow them very close together (broadcasted across a 4' X 12' bed) so when they grow up they shade the soil completely. No weeds at all there. Likewise with carrots, i do have to thin the carrots a bit but they grow up and no weeds. Low vine peas also are grown in this way, no rows just a solid bed of peas. They not only shade the soil to keep it cool, weed free but then lean on each other to keep the vines off the soil.

Nancy
Posted: 8:30 pm on February 17th
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