Tips for Weed Control in the Home Landscape and Garden

Byline: Tony Bratsch, Extension Educator, Horticulture

Weed control is an issue for just about anyone with landscape plantings around the home. Weeds invade and compete with plants for nutrients, water and sunlight, and are unsightly in the landscape. When the season first begins, most of us do a great job of keeping unsightly weeds controlled. This is important in the short term, but the long-term battle with weeds is often lost as the season progresses and we weary of the task.

Part 1: Understanding weeds

There are many tools the gardener can use to keep ahead of weeds, and probably the best tool is gaining an understanding of weed biology and life cycles to maximize the effectiveness of control methods.

Most weeds are classified as either annual, biennial or perennial. Annual and perennial weeds can be a grass or broadleaf species, while most biennial weeds are broadleaf. Annual weeds are prominent in the fall, winter and early spring (winter annuals), or in the late spring, summer and early fall (summer annuals). These weeds sprout from seed when conditions such as soil temperature are ideal for their growth. Biennial weeds grow just two seasons. They grow vegetatively the first year, send up a seed stalk the second year and then die. Perennial grass and broadleaf weeds usually go dormant in the winter and grow back each season from permanent underground plant parts such as a crown or rhizome. Knowing the type of weed is important for determining when and how it should be controlled. For example, herbicides that affect weed seed germination should be applied well before the seeds start to grow. Perennial weeds are best controlled by herbicides when they are actively growing and storing reserves in late summer.

Success is improved when we consider control from a weed ecology perspective that involves both environment and weed biology. Like all other plants, weeds are affected by environmental factors of light, air, temperature and moisture. An important weed biology concern is limiting new weed seed introduction. Weed seeds in the soil can be compared to making deposits and withdrawals from a bank, or a "weed seed bank". Our actions (or lack of) lead to either a withdrawal or deposit of seed in the soil bank. Over time, enough seed withdrawals can be made to significantly reduce weed pressure, but if not managed, weed problems can increase due to new deposits. Weed seeds can have significant longevity in the soil (years and decades) and can be high in number depending on the site. When weeds are allowed to go to seed, a large percentage of these seeds cause problems the next year; and another percentage will remain in the soil to periodically germinate, creating problems in future years.

Gardeners lose ground when weeds are allowed to go to seed at any time or if they are introduced unintentionally. Manure and other un-composted materials are good organic matter additions to the garden; but they can also contain weed seeds. Late season weeds can be a major source of new weed seed deposits. By mid- to late summer, when it is too hot to work in the garden or we go on vacation, weeds can take over and go to seed unnoticed. Some weeds have the botanical characteristic of flowering when days get shorter in the fall. This day-length response results in weeds that quickly go to seed soon after they germinate, producing seeds on very small and young plants. For example a spring-germinating pigweed will get very large by the time seed heads form, while a late-summer germinating pigweed will form a seed head when the plant is much smaller.

Part 2: Methods to Control Weeds:

Both cultural and chemical techniques can be used to control weeds. Cultural (and organic) methods include mulching, hoeing or hand-pulling weeds, using stale seed beds and solarization.

Mulching with organic (wood chips or newspaper) or inorganic mulches such as weed mats and plastic mulch do a good job of creating a barrier to weed emergence, but can make transplanting and dividing of perennials a more difficult task. Organic mulch in combination with weed mats provides exceptional weed control, though weeds from wind-blown seed can sprout in decaying mulches on top of the plastic.

Cultivating, hoeing and hand-pulling can be used in gardens, annual and perennial bed areas with exposed soil. It is important to remove weeds when they are young and have not developed a strong root system. Most weeds, especially newly sprouted weeds, can easily be killed by removing just the top, while some species must be pulled by the roots or they will re-sprout. Large-leafed garden plants such as sweet corn and broccoli and perennials such as hostas compete well with weeds by shading. Using transplants can also provide a competitive advantage compared to seeding flowers or vegetables directly.

Additional cultural methods include stale seed bed use, solarization, using weeder geese and flaming. Utilizing a stale seed bed simply involves repeated cultivation or tillage in an area to encourage new flushes of weeds. At least a couple weeks is allowed between tillage events to allow time for weed seeds to germinate and grow. This method helps reduce the weed seed bank. Solarization or covering bare soil areas with clear plastic, heat sterilizes the soil and kills a certain percentage of weed seeds. This must be done in the heat of the summer. It is mostly used in the SW and desert climates, but can be effective in the Midwest if the summer is hot and the plastic stays in place at least 6-10 weeks. The soil should be worked very finely and leveled out, and the plastic should be laid tightly against the soil for maximum contact. Its main disadvantage is the site being taken out of production for the main growing season. Pond geese, when allowed to range over the garden, can do a surprisingly fair job of removing weed seedlings from between garden plants, though small garden plants will also be at risk. Flaming involves using small portable propane burners to torch new weed seedlings soon after they emerge.

There are several chemical weed control tools available to home gardeners. Pesticides that kill weeds are herbicides. Herbicides are either classified as "pre-emergence" or "post-emergence" in their action. Pre-emergence materials are applied to the soil to prevent weed seed germination. As the name indicates the material needs to be in the soil before weeds germinate. Thus depending on whether weeds are winter or summer annuals, timing the application is important. Apply too early and the herbicide dissipates before weed seeds start to grow; apply too late and the seed has already germinated and will not be affected. There are several pre-emergent herbicides recommended for home garden use sold under different trade names. Look for bensulide, DCPA, napropamide and trifluralin as "active ingredients" on the label. Preen™ is probably the most commonly available pre-emergence herbicide and it is made from trifluralin. Each pre-emergence material has certain weed species it is most effective against. When two of these chemicals can be combined, a broader spectrum of control is achieved. Remember that some established garden and landscape plants can be sensitive to pre-emergence herbicides, and the label should be carefully consulted.

The second type of herbicides is post-emergence, which is applied to green tissue of living plants. These herbicides can be selective and not harm certain species of plants, or non-selective, with the potential to kill whatever they are sprayed on. They can also be classified as contact herbicides which burn and kill above ground plant parts, or translocated, meaning it is moved from the leaves to the roots by the plant, effectively killing the entire plant. This is important when considering perennial weeds which emerge from underground root structures. Round-up™ is an example of a non-selective, translocated post-emergence herbicide. As related to weed biology, these herbicides are the most effective on perennial weeds when applied late summer as the plant translocates nutrients downward for winter storage.

Regardless of the control method used, try to think about how weed biology and ecology are affected. For example, some weeds need brief sunlight exposure to germinate, and mulching prevents that exposure. More importantly, minimizing soil disturbance reduces weed seed exposure to light and other germination stimulus. Shallow cultivation or hoeing keeps new seed from being brought up as does "gently" pulling large weeds. Flaming weeds and post-emergence herbicides limit soil disturbance. Always hoe weeds when they are just emerging and during the hottest part of the day, so they are less likely to re-root. When watering, use drip or soaker hose methods that put water near the plants and not over the entire garden to limit weed germination between rows. Never allow weeds to go to seed.

Knowing the biology of weeds and applying best management tools at appropriate times helps ensure a weed free garden and landscape, and reduces time spent on this chore. For additional information about weed control in the home garden, consult the "Home, Yard and Garden Pest Guide" (publication C1391) available from your local University of Illinois Extension office.