Yard and Garden: Controlling weeds in the landscape

Aaron Steil
Iowa State University Extension
Pokeweed can grow four to ten feet tall.

Every garden has weeds, and dealing with these unwanted plants is an ongoing task in any landscape.

Control of weeds is important. Weeds are strong competitors for available water, nutrients and sunlight. Reduced air circulation created by tall weeds encourages the development and spread of foliage diseases, and a weedy garden often has more insect problems.

While a completely weed-free garden is not attainable, reducing weeds is beneficial. 

How do I control weeds in my garden beds?

Keeping ahead of weeds and controlling them when they are small is essential for good weed management. This requires persistence throughout the entire growing season to remove weeds as they emerge. There are essentially two types of weeds in our gardens: annuals and perennials. 

Annual weeds grow rapidly, flower, set seed and die in a single season. New annual weeds, such as crabgrass, velvetleaf, purslane, knotweed, lambsquarter and foxtail germinate from seeds each year. 

Perennial weeds die back to ground level in fall but send up new growth in spring. Perennial weeds, such as dandelions, quackgrass, thistle, pokeweed and plantain, reproduce by seeds or may spread by creeping above or below ground stems or by spreading root systems. 

Cultivation, hand pulling, mulches and herbicides are the primary means to control weeds in the home garden.

How do I control weeds using mechanical means, such as cultivation or hand pulling?

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 they become established, they are very difficult to control because of their perennial root system and rhizomes. Repeated cultivation of perennial weeds is necessary, being careful to not chop up or leave behind plant pieces that can root to become a new plant, 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. 

How can mulches be used to control weeds?

Mulches control weeds by preventing the germination of weed seeds. Established weeds should be destroyed prior to the application of the mulch. In addition to weed control, mulches help conserve soil moisture, reduce soil erosion, prevent crusting of the soil surface, keep foliage, fruits and vegetables clean and may reduce disease problems.

Grass clippings, shredded leaves, coco hulls and weed-free straw are excellent mulches for vegetable gardens and annual flower beds. Apply several inches of these materials in early June after the soil has warmed sufficiently. Plant growth may be slowed if these materials are applied when soil temperatures are still cool in early spring. Grass clippings, shredded leaves and similar materials break down rather quickly and can be tilled into the soil in the fall.

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. 

How and when should herbicides be used for weed control?

In certain situations, a gardener can use herbicides to supplement other weed control strategies. Several factors limit the usefulness of herbicides in the home garden. Most home gardens contain a 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. 

Herbicides 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 garden plants with barriers like buckets or boxes to further reduce problems with drift. They can also be applied with a sponge and wiped onto the leaves of the weed to prevent collateral damage to nearby plants.

Herbicides must be used according to label instructions on the package. Failure to follow directions may kill desirable plants or prevent other plants from being grown in the area. 

What organic options exist for controlling weeds in my garden?

Mechanical control of weeds through hand pulling and cultivation as well as the use of mulches to suppress weed growth are both effective organic options for weed management. 

Pouring boiling water on weeds can be used especially in situations where other plants are not nearby, such as in the cracks of sidewalks or driveways. Be careful to not splash or burn yourself with the boiling water and remember the boiling water will kill both weeds and desirable plants.

A tea kettle is often a good way to more 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 weeds.

Organic herbicides can also be effective and all of them 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, lemon grass oil, d-limonene and ammonium nonanoate, among others. 

Most organic herbicides work as contact herbicides, killing the leaves and stems, but not being translocated to other parts of the plants, such as roots. Often, multiple applications every two to three weeks are needed for complete control.

Organic herbicides are more effective on younger, smaller weeds than larger more established ones and should be applied at a higher volume than most conventional herbicides, thoroughly soaking leaves to point of run off. Always follow label directions on all herbicides. Even organic herbicides can harm desirable plants or people when used inappropriately.