Yard and Garden: Weeds

AMES, Iowa —Weeds are a constant problem for home gardeners. Weeds compete with garden crops for nutrients, moisture and sunlight. In large numbers, weeds can reduce the quality and yield of fruits and vegetables. Horticulturists with Iowa State University Extension and Outreach answer questions about weeds in general and a couple of specific weed issues. Horticulturists are available through the ISU Hortline to answer additional questions. Contact them at hortline@iastate.edu or 515-294-3108.

How do I control weeds in my garden? 

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

Cultivation and hand pulling effectively control most annual weeds. Perennial weeds are often more difficult to control. Repeated cultivation or the use of herbicides may be necessary to destroy some perennial weeds. When cultivating the garden, avoid deep tillage. The roots of many vegetables, fruits and flowers 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. Hoe or till around plants or between rows and pull weeds close to plants. To effectively control weeds, cultivation and hand pulling must be done periodically through the growing season. Small weeds are much easier to control than large weeds. It’s also important to destroy weeds before they have a chance to go to seed.  

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 fruits and vegetables clean, and may reduce disease problems.  

Grass clippings, shredded leaves 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 relatively 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 retain the desired depth.  

In some situations, herbicides may be necessary to control difficult to control perennial weeds. 

How can I control weeds in my raspberries? 

Cultivation and mulches are the most practical weed control measures for home gardeners.  Cultivate the raspberry plantings frequently during spring and summer. To prevent injury to the roots of the raspberry plants, don’t cultivate deeper than 2 to 3 inches.  

Mulches help to control weeds and conserve soil moisture. Possible mulching materials include straw, shredded leaves, lawn clippings, crushed corncobs, sawdust and wood chips. The depth of the mulch needed depends upon the material. The depth ranges from 3 to 4 inches for fine materials, such as sawdust, to 8 to 10 inches for straw. Since mulches gradually decompose, apply additional material each year. Avoid mulching poorly drained soils to discourage root diseases. 

How do I control quackgrass in my iris bed? 

Quackgrass is a cool-season perennial grass. It spreads rapidly by underground stems or rhizomes. Quackgrass is extremely difficult to control. To effectively control quackgrass, all of the rhizomes in the soil must be completely destroyed. Any rhizomes (or parts of rhizomes) that remain in the soil will eventually root and produce leafy growth.  

My suggestion is to carefully dig up the irises in early summer, place the plants in flats or boxes, and set the irises in a shady location. Then slowly and methodically dig up the area and remove as many of the quackgrass rhizomes as possible. Periodically check the area over the next several weeks and promptly dig up and destroy any quackgrass sprouts that appear. In late summer, replant the irises back in their original location.