FOR THE LAWN AND GARDEN
Written by: David Johnson, Washington Toxics CoalitionWeed management is probably the most time consuming and frustrating task facing the home gardener. No matter what we do, it seems there are always weeds to remove. Plants we define as weeds will always be part of our landscapes. It is not realistic to strive for complete elimination. Attempts to do so usually involve frequent herbicide applications that may have unacceptable adverse effects on the environment. However, we can manage these pest plants in a manner that minimizes their impact. In order to develop strategies to outwit them, we need to learn what weeds are and how they grow.
Weed Classification
A weed may be defined as a plant that is not valued where it is growing. Thus, a plant may be considered a weed in one situation and a desirable plant in another. For example, Dutch White Clover is usually a weed in lawns, but it can be a desirable groundcover used to improve soil conditions and prevent erosion on slopes in new landscapes. The leaves of dandelions, among the more notorious lawn weeds, are valued in salads.
Common chickweed is an annual plant that is easily hand-pulled in gardens.
We can classify weeds by placing them into one of three plant groups according to their life cycle. Annuals germinate, grow, flower, and set seed within one year. Biennials produce leaves and store food the first year; the second year they flower, produce seed, and die. Perennials live on from year to year. In many cases the tops will die to the ground, but the roots persist.
Annuals tend to be opportunistic. They are most often seen colonizing bare soil that has recently been disturbed. Examples of annuals are chickweed, snapweed, and groundsel. Their seed is either blown in or tends to persist in the soil for years until conditions are right for germination. Many annual weed seeds will germinate after the soil is turned and they are exposed to light. Once they germinate, annual weeds grow very quickly and in a short time are flowering and producing seed for new generations. Biennial weeds, such as foxglove and money plant, are easy to spot in the winter garden because of their overwintering basal leaves, which form flat clumps without a visible stem. The food reserves stored in their thick roots promote rapid growth in the early spring.
Perennial weeds provide the most difficult challenge. Morning glory, horsetail, quackgrass, and buttercup are perennials which have achieved considerable notoriety. The most successful perennial weeds have a thick fleshy root system that stores extensive food reserves. When the leaves are removed, these reserves quickly provide the energy to produce new ones.
Weed Control: Philosophy and Tactics
An integrated approach to weed management begins with analyzing the problem. Identify the weeds and determine whether they are annual, biennial, or perennial. Establish a level of tolerance. You need to determine: (a) if the weeds are affecting the growth of desirable plants, (b) if their presence is an aesthetic problem, and (c) what their potential is for spreading over the entire garden or lawn. You must develop a clear picture of the problem in order to formulate a strategy to solve it. At this point in the process, we begin to consider specific weed control tactics: (1) mechanical and physical controls, which include mowing, cultivation, burning, and paving; (2) horticultural controls, such as mulching and the establishment of desirable competitive plants; and (3) chemical controls, including both selective and non-selective herbicides. Chemical controls should be used only if the other management techniques are not adequate. Since most problems involve a mixture of weed types, a mixture of tactics is often required to achieve effective control
Techniques and Strategies
When considering control options for annual weeds, one has to move quickly to successfully interrupt the cycle of seed production. The presence of a mulch will deter annual weeds in several ways. It creates a physical barrier that blocks light from reaching the soil, prevents seed from coming in contact with the soil, and smothers germinating seed underneath. In areas where a thick mulch is not practical, such as in a vegetable garden, it is necessary to remove weeds frequently before they develop seed and use a light mulch of grass clippings or leaves. Annual weeds can be easily pulled by hand.
Biennial weeds are in many ways easier to control than annual weeds. Their life cycle, extending over two years, provides ample opportunity for effective control. The overwintering leaves may be rapidly removed with a hoe or by hand pulling. As their flower stalks shoot up in the spring, they may be easily pulled. If you wait too long, however, the seed will be dispersed and another cycle will begin.
The key to controlling perennial weeds lies in the destruction of their root systems by physically digging them out, repeatedly pulling the tops to deplete the food reserves stored in the roots, or, if nothing else will work, by using the least toxic and most effective herbicide. Once the weeds have been killed or removed, long-term management relies on establishing conditions which do not favor weed growth.
In a vegetable garden, a mix of control tactics may be used. Because of soil preparation and harvesting, vegetable gardens frequently contain large areas of bare, disturbed soil perfect for weed establishment. To inhibit weed growth, it is necessary to shallowly cultivate the soil frequently to kill weed seedlings. Use grass or leaf mulches when possible, and cover unused areas with a living mulch of crimson clover, vetch, or annual rye grass. Frequent turning of the soil should be discouraged since it can bring dormant weed seed to the surface. Soil amendments can be spread on the surface and scratched in rather than turned. Use of herbicides is inappropriate near food crops.
Two techniques can reduce the spread of weeds from neighboring yards. Invading roots from morning glory or quackgrass can be turned away by burying a barrier of aluminum flashing under your fence. A depth of about 18" should be adequate in most cases, but 24" to 36" may be required for horsetail. A windbreak of trees or shrubs around your yard will help keep out windblown seeds while it encourages birds.
Plan for the Long-term
The permanent solution for weed control lies in designing out weed habitats, treating the cause of the problem rather than just the symptoms. Avoid creating conditions that optimize weed growth. Once weeds are cleared from an area, apply a 3" to 4" mulch layer and install plantings which will eventually cover the mulch. Mulches are really a short-term solution to weed control, since they eventually break down to form soil in which weeds can grow. But they do make a good interim barrier to weeds while plants are getting established, and they can be replenished. Permanent weed management lies in having a landscape covered with desirable plantings that form a multi-layered groundcover within which weeds cannot get established.
Tools for Weeding
Using the proper tools will make hand-weeding easier and more effective, reducing your need to resort to chemical control.
Hoes
Hoeing is recommended in vegetable gardens or annual flower beds. One effective type is called a stirrup hoe. The blade is a flat metal loop attached to the handle at an angle that places the blade parallel with the soil. The blade is moved forward and back just under the soil surface, slicing off weeds at the root line. Any hoe that minimizes soil disturbance and cuts weeds just under the soil line is more effective than a hoe which chops them. Chopping weeds tends to pull them from the ground, but the roots often remain attached and reestablishment occurs.
String trimmers
These machines are appropriate for cutting down the vegetation in rough areas. They are often used to mow down seed heads before they ripen to prevent seed dispersal.
Black Plastic
This material is best used on the surface to smother perennial weeds. If used during the summer with the edges sealed with soil, it blocks light and heats the soil underneath to the point where most roots will die. Its use as a permanent barrier under bark mulch is discouraged, since it effectively seals the soil from the atmosphere, which can cause a decline in plant health.
Herbicide Application Tools
If you must resort to an herbicide, don't spray it over whole areas. Use a sponge-type paint brush to apply herbicide directly on the plant leaves. Special wiper tools are also available which do an excellent job. The small, hand-held squirt bottles that come with ready-to-use herbicides can also be used to direct herbicide onto individual plants. Do not use hose-end sprayers. They tend to leak, are difficult to control, and release herbicide too rapidly, increasing the chances of inhalation and contact with desirable plants.
Tips for Difficult Weeds
Dandelion is a common perennial weed that invades both lawn and garden beds. It becomes established through wind-dispersed seed. Dandelions have a long, thick tap root that stores food reserves. Control is achieved by hand digging, making sure that the whole root is removed. In large lawns, this is easiest with a tool which allows pulling the roots rapidly while working in a standing position. Watering the area first makes it easier to get the entire root. If the root breaks while pulling, the plant can resprout from the piece remaining in the soil. To minimize spreading of dandelions, the flowers should not be allowed to go to seed. Mow often to keep the flowers from maturing. In lawns, the removal of dandelion plants leaves small bare areas where new weed seed can germinate. Be sure to spread a little grass seed in those spots to prevent weed establishment.
Morning Glory. This weedy vine will twine all over the garden, covering your ornamental plants to the point of smothering them. It is usually introduced by seed or invasive roots from under the neighbor's fence. Its success as a weed lies in its thick fleshy roots which travel long distances just under the soil surface. It prefers loose organic soils. Since morning glory is a perennial weed, control lies in removing the root system. Hand weeding can remove large quantities of roots, but any broken pieces are capable of sprouting new growth. Repeated, persistent rototilling or flame weeding as the new growth sprouts can deplete the food reserves and allow a groundcover to compete successfully. If chemical control is required, cut back the growth and apply the material to leaves or stems in as localized a fashion as possible.
Moss. Moss is a natural part of the Pacific Northwest. It is our native bottom-story groundcover. If moss is thriving in your lawn, consider leaving it there. Most mosses prefer shade, moisture, and poor, acid soils. Immediate control is achieved by raking it out of lawns or by applying an iron-based product. For long-term control, correcting the conditions that encourage moss growth is more effective than chemicals. Water infrequently but deeply, making sure the water is penetrating and not running off. Proper aeration and thatching of lawns will ensure good water retention. Do not apply water faster than the soil can absorb it. Soil should be limed and fertilized to encourage the growth of the desired plants or turf. Consider letting the moss spread in planting beds to form an attractive groundcover.
Clover. Several types of clover are often found in lawns and gardens, and they do provide certain benefits. The small blossoms form an attractive mass of color over large lawn areas. If left in the lawn, clover provides some soil fertility and attracts bees, which can present a hazard to those using the lawn if stepped on. Clovers are legumes and are encouraged by phosphate, potash, and sulfur fertilizers. They can be controlled somewhat with fertilizer high in nitrogen and low in phosphorous. Do not over-fertilize, however, because excess chemicals can run off and contaminate water.
Tips for Least-Toxic Weed Management
- Don't attempt total eradication. Set your weed tolerance at a realistic level.
- Identify the weed. For most annual and biennial weeds, mechanical or horticultural methods should be adequate. Hand pull as much as possible.
- Hire your children or have a weeding party.
- Don't use herbicides as a preventative tactic. Avoid "weed and feed" mixtures.
- For long-term control, design out weed habitats and concentrate on proper plant selection and horticulture.
- Do not use herbicides in food gardens. Hoe or hand pull, use grass or leaf mulches, plant living mulches, and minimize turning of the soil.
- Chemical control is the last resort. If chemical control is necessary, choose the least-toxic product possible.
- Buy the smallest amount of herbicide you can, even if it costs more per unit. Do not use at greater than recommended concentration or application rate. More is not better.
- Do not use broadcast or spray applications. Paint or squirt product directly onto individual plants or leaves.
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