Weed Control

Ann Lovejoy’s Handbook of Northwest Gardening, of which this is an excerpt, is full of ideas for creating and maintaining a sustainable garden. Though written for the gardener in the Pacific Northwest, many of the ideas, such as those for controlling weeds, are perfectly appropriate for gardeners in other parts of the West.

Weeds rank right up there with slugs and deer as ongoing annoyance factors in the garden. The problem is that this is one of the best places in the world to garden; plants from all over the world love being part of the lush Northwestern scene.

Unfortunately, this means weeds as well as imported border beauties. Some of our worst weeds are deliberate imports that were brought here by misguided (if well-intentioned) gardeners. Scotch broom (Cytisus scoparius), ox-eye daisies (Leucanthemum vulgare), and English ivy (Hedera helix) were imported as elegant ornamentals.

Other common weeds are termed “seeds of disturbance,” plants that mostly show up where the ground is broken. Fireweed (Epilobium angustifolium) is a classic example. This handsome, hot pink native is found in much of the temperate world. In the wild, fireweed is often the first plant on the scene after forest fires, but it is less successful at infiltrating healthy stands of native plants. In gardens, fireweed is a wicked runner that can tear through beds and borders in a single season.

Many of our annual weeds, from sow thistles (Sonchus) to dock (Rumex), spurge (Euphorbia) and chickweed (Stellaria media), hail from other shores. Most came as passengers (often in seed form) on deliberately imported plants. Until fairly recently, nobody worried or cared that weed seeds might come along for the ride. Now we know better, but the Euro-trash weeds will always be with us.

Let Mulch Do the Work

Many common weeds can be kept at bay with mulch. Long a tradition in organic vegetable gardens, mulching is less common among ornamental gardeners. However, a good, thick mulch is effective anywhere and can be quite attractive. Besides suppressing weeds, mulches conserve moisture and add insulation to soil, keeping our plants cooler in summer and warmer in winter.

Smother mulching is the answer to more serious weed infestations. It is also one of the best ways to work toward a large soil-clearing project. Anywhere you have an infestation of annual weeds, you simply heap a mixture of green grass clippings and shredded dry leaves or straw on top of the area. Be generous here; a two-foot-thick layer is not too much. This makes a small, static compost pile that will work away quietly while you think about what you want the bed to look like. Almost anything already growing there will be smothered by that heavy blanket, and no new weeds will grow through it.

Worse weeds, such as English ivy (Hedera helix), nettles (Urtica spp.), and blackberries (Rubus discolor), need coarser cover. For these, I use deep (two- or three-foot) layers of very coarse bark chips. My supply comes from the road crews who trim trees around power lines. These chips are not suitable for garden use in the ordinary sense, but they do make a superior smother mulch. As it breaks down, the coarse bark draws a lot of nitrogen from the soil, leaving it depleted and very lean. Coupled with the weight of the bark and the lack of light and air, this is enough to discourage almost any weed from growing. Before dumping the bark, cut back any weedy top growth (be sure to wear heavy gloves and long-sleeved, protective clothing), and heap it over the vulnerable root zone of the problem spot.

If the weeds are especially thuggish and well established, consider removing as much root as possible before applying the smother mulch. The more you take away, the less you need to starve out. Short-handled mattocks are great tools for root removal, as are heavy picks. If the work is too difficult, just use more chips and keep a watchful eye on the area. You will need to be vigilant about removing any and all shoots and stems as they appear, but by cutting the tops and starving the soil, you will win out in time.

The most effective way to use a smother mulch is in combination. Begin by mowing or cutting back all weeds, and then layer on a two- or three-foot blanket of a hot-composting mulch such as grass clippings and dry shredded leaves in equal quantities. The heat helps to discourage the weeds from making a comeback, and, when this layer has broken down, the soil beneath it will be loosened, making it much easier to dig out any persistent weed roots. Now you can add the smother mulch of coarse chips, making it at least two or three feet deep. Any weeds that do emerge will be easy to pull out, and the heap will rot peacefully for as long as you like. When you want to garden in that spot, pull the chips away (they make a fine path material), and replace them with compost and manure, with a booster of alfalfa. This will restore the nitrogen level of the underlying soil in a hurry. Within three months, there is an average of eighty percent kill.

Wild and ferocious spreaders like bindweed or wild morning glory (Calystegia), tap-rooted terrors like Canadian thistle (Cirsium arvense), and relentless roamers like creeping buttercup (Ranunculus repens) are to be avoided at all cost. Where aggressive weeds proliferate, it’s a good idea to clear the ground well before trying to garden. We now know that fast-acting chemical killers like Roundup are not a benign solution (visit the website of the Northwest Coalition for Alternatives to Pesticides at www.pesticides.org to learn more about Roundup and its relatives). Methods that involve soil disturbance generally make matters worse. The seeds of many weeds can lie buried for decades, waiting patiently until some chance movement stirs the soil, bringing them up to the light and air. These opportunists love it when we till the soil for a fresh bed. Other weeds also enjoy the chance to proliferate that tilling offers; each scrap of weed root strives to become a whole new plant. Instead of tilling, consider soil solarization with clear or black plastic or a deep smother mulch.

More Techniques

To tame rough grass and open meadows, mow them high, and they can be whipped into reasonable shape surprisingly fast. Even rough-cutting a meadow a few times a year will control all but the very wildest weeds. Frequent mowing of weeds and wild meadows keeps them from flowering; without a steady seed crop, many weedy plants are easier to control. High-mow a rough meadow regularly for a few seasons and you will have a not-that-bad lawn. Because mowing selects for fine-textured grasses and perennials, it gets rid of coarse grasses, reeds, and taller weeds quite quickly. Tap-rooted weeds that make flat basal rosettes, such as dandelions, hawkweeds, and many thistles, can slip beneath the mower and persist for years, unless they are dug out by hand or controlled with a broadleaf weed killer (several safe versions are now available from companies like Safer and Bioganic). To control tap-rooted weeds like dock and dandelions by hand, use a Japanese farmer’s knife (a hori-hori), a long, thin-bladed weeder, or an old kitchen knife to remove at least four or five inches of the root; any remaining root usually rots away.

Try plain old boiling water for spot weeding. Though most of us don’t want to lug vast cauldrons of boiling water into the garden, a daily visit with a teapot can keep a small area weed-free all year-round. Here, too, persistence pays: where persistent weeds such as thistles and morning glory are sprouting from narrow sidewalk cracks, spot-watering with boiling water will eventually overcome the weeds.

A by-product of the manufacture of cornmeal, corn gluten is nature’s own pre-emergent herbicide. Since it also provides a good source of nitrogen (most forms have an N-P-K ration of 10-1-0), you could call it the original, natural weed-and-feed. When spread over bare soil and wetted down, corn gluten releases substances that cause weed seedlings to dry out before they can fully germinate. Tests conducted at Iowa State University showed that corn gluten performance improves over time; crabgrass was reduced by eighty-six percent the first year and by ninety-eight percent the second. Dandelions were reduced by one hundred percent over a period of four years of seasonal applications.

Where weed suppression is desired on bare earth, the raw or powdered form of corn gluten is far more effective than the pelletized form, because it provides better coverage. Corn gluten does not discriminate: it will kill the seeds you sow as well as nature’s choices. Thus, it should be spread on vegetable beds after all vegetable and flower seeds have germinated. Those allergic to corn may experience a reaction to corn gluten if they inhale the dust while spreading it.

Both citrus oils and vinegar concentrates are effective herbicides and can be used to rid beds, borders, and lawns of persistent weeds. Some, such as Blackberry Block, work by lowering soil pH until plants can’t survive. This acidifying effect lasts from several months to a year, depending on soil type and weather. All work best on warm, dry days, when signs of damage can be seen within minutes of use. Over-dilution of concentrates or application before rain or watering may cause these products to act like fertilizers instead of herbicides.

Products such as BurnOut, which combine both citrus oils and vinegar concentrates, kill broad-leaved weeds and certain grasses on contact. These products are usually top-killers that strip protective, waxy coatings off foliage, allowing the plants to desiccate. Tap-rooted weeds like dandelions will need several applications, but many young weeds will be killed with a single exposure.

These products may be extremely irritating to eyes and skin, so use eye protection, wear gloves, and wash well after use. They are safe for mammals but may harm beneficial insects that are accidentally exposed to direct spray. They are most effective when temperatures are above 65° F. Citrus- and vinegar-based products are not safe for use on food crops but can be used around the perimeter of vegetable gardens or orchards.

Blackberry Block and similar vinegar-based concentrates are best used in small amounts for spot-killing persistent weeds such as black-berries, horsetail, and bindweed. Never use these on a septic field, or you will destroy it. Do not use near natural water, as these products can damage all forms of aquatic life. Do not use in the root zone of established trees, or you may accidentally kill or damage the tree.

Excerpted and adapted, with permission, from Ann Lovejoy’s Handbook of Northwest Gardening, written with a strong commitment to preserving the health of our environment by Ann Lovejoy and published by Sasquatch Books in 2004.