Managing weeds in perennial gardens and other areas of the landscape, like around shrubs, is important for maintaining healthy, attractive, and long-lived plantings. Several methods can be used to manage weeds, and in most cases, combining them will produce the best results. All of these weed control methods can be considered organic except those that utilize certain herbicides.
Hand Pulling & Digging | Cultural Methods | Cutting & Mowing | Boiling Water | Mulches | Herbicides | More Information
Hand Pulling & Digging
Cultivation and hand pulling effectively control most annual weeds. It is very important to destroy these weeds while they are small before they produce thousands of seeds, guaranteeing a weed problem for many years in the future.
Like annual weeds, perennial weeds are easy to control when in the seedling stage. Once established, they are difficult to control because of their perennial root systems and rhizomes. Repeated cultivation of perennial weeds is necessary, but be careful not to chop up or leave behind plant pieces that can root and become new plants, multiplying the problem.
When cultivating the garden, avoid deep tillage. The roots of many desirable plants grow near the soil surface. Deep cultivation will cut off some of these roots. Also, deep cultivation will bring deeply buried weed seeds to the soil surface, where they can germinate. When hand pulling, work in the garden a day or two after a soaking rain, or water the garden 24-48 hours before weeding to make pulling and digging easier.
Cultural Methods
There are several cultural or management techniques the gardener can use to reduce weeds in the perennial garden.
Proper Plant Spacing
When perennials are planted at an ideal spacing, the foliage can help shade the ground, reducing weed seed germination and slowing weed growth. When plants are spaced too far apart, weeds grow more easily.
Watering Directly
Watering plants by hand, with soaker hoses, or with drip irrigation systems can help reduce weed growth in the perennial border. Unlike overhead watering with a sprinkler, this provides water directly to the plant and does not provide the moisture needed for weed seeds nearby to germinate.
Cutting & Mowing
Cutting back or mowing weeds is sometimes used to control them in the perennial garden, though it is rarely effective. Timely or frequent cutting back stresses the weed and reduces its competitive advantage over more desirable plants. Consistent removal of the upper portion of a weed, especially a perennial weed, will prevent it from shading nearby desirable plants and slow its growth.
While weakened, weeds will regrow from their root system. Rarely will the weed be killed by mowing or cutting back alone. This is especially true for grassy weeds.
When weeds are cut back before they flower and set fruit, it can significantly reduce seed production and reduce new weeds from emerging. This is particularly helpful for annual weeds that persist year to year by starting from new seeds each season - fewer seeds mean fewer weeds in the future.
Mowing or cutting back alone rarely controls weeds completely, but it can be an effective management strategy when combined with other weed control methods, such as herbicides.
Boiling Water
Pouring boiling water on weeds is especially useful when other plants are not nearby, such as in cracks in patios or sidewalks. Boiling water acts as a contact herbicide, killing only the portion of the plant it comes into contact with. It is most effective on young, newly emerged weeds.
Boiling water for weed control is an organic option.
Be careful not to splash or burn yourself with the boiling water, and remember that it will kill both weeds and desirable plants. A tea kettle is often a good way to safely and precisely apply the boiling water directly to the weed. Use plenty of water and plan to retreat 7-10 days later, as one application rarely kills the entire plant, especially deep-rooted or grassy weeds.
Mulches
Mulches control weeds by preventing weed seeds from germinating. They can be effective at preventing new perennial weeds from germinating, but are less effective on those weeds that are already established and can spread by rhizomes and stolons. Established weeds should be destroyed prior to the application of the mulch. Wood chips and shredded bark are excellent mulches for perennial beds and areas around trees and shrubs. Apply 2 to 4 inches of material around landscape plantings. These materials decay slowly and should last several years. However, it will be necessary to apply additional material periodically to maintain the desired depth.
Learn more in this article: Using Mulch in the Garden.
Herbicides
Many home gardeners choose to avoid using herbicides in their landscape to reduce chemical use. Several factors limit the usefulness of herbicides in the perennial garden. Most perennial gardens contain a wide variety of plants in a small area. This restricts herbicide use because it is unlikely that the herbicide will be labeled for all plants in the garden. However, a gardener can use herbicides to supplement other weed control strategies in certain situations.
Pre-emergent herbicides
Pre-emergent herbicides prevent weed seeds from completing the germination process. They can help reduce annual weeds, but the timing is important. Consult the label to apply these herbicides at the appropriate time and frequency to control weeds without adverse effects.
Post-emergent herbicides
Post-emergent herbicides are used to kill weeds that have already begun growing. They must be carefully applied as they have a high potential to harm both weeds and desirable plants. Always apply herbicides when winds are calm and temperatures are cool to prevent drift and damage to desirable plants. Protect nearby plants with barriers such as buckets, tarps, or boxes to further reduce drift. Herbicides can also be applied with a sponge and wiped onto the leaves of the weed to prevent collateral damage to nearby plants. Non-selective herbicides are available to home gardeners and can be used in certain situations. Herbicides must be used according to the label instructions on the package. Pay special attention to the waiting period before replanting to avoid issues with planting new plants. Failure to follow directions may kill desirable plants or prevent other plants from being grown in the area.
Organic Herbicides
Organic herbicides can also be effective, and all act as non-selective herbicides, meaning they kill or damage any plant part they touch. Many organic herbicides use one or more of the following active ingredients: acetic acid, citric acid, clove oil, lemongrass oil, d-limonene, and ammonium nonanoate, among others.
Most organic herbicides work as contact herbicides, killing leaves and stems but not being translocated to other parts of the plant, such as the roots. Multiple applications are often needed every two to three weeks for complete control. Organic herbicides are more effective on younger, smaller weeds than larger, more established ones. They should be applied at a higher volume than most conventional herbicides, thoroughly soaking leaves to the point of runoff. Always follow the label directions on all herbicides. Even organic herbicides can harm desirable plants or people when used inappropriately.