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. Before they die, most annual weeds produce large quantities of seeds. If annual weeds are controlled every year before producing seeds, they will eventually become less of a problem as the seed population gradually decreases.
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.
Winter annuals germinate late in the growing season, often grow slowly over the winter months, then quickly establish, grow, flower, and set seed in spring before dying in summer. New seed germinates again in the fall starting the cycle over. Common winter annual weeds include common chickweed, henbit, annual bluegrass, and Persian speedwell.
Summer annuals germinate in the spring or early summer, grow and flower through the summer months, set seed in late summer and early fall, and are killed by the frost in the fall. New seed germinates the following spring, continuing the cycle. Common summer annuals include crabgrass, foxtail, purslane, lambsquarters, spurge, and prostrate knotweed.
Preemergent herbicides are effective at preventing annual weeds. They interrupt the growing process of a newly germinating plant, preventing that plant from becoming established. This works well for annual weeds because they can only establish themselves year after year in the garden by growing from seed.
Timing is critical when using preemergent herbicides to control annual weeds. If applied too early, the herbicide will be leached or washed away before it can impact the germinating seed. Applied too late and the growing process the herbicide interrupts will have passed allowing the plant to take root and become established.
To control winter annuals, preemergent herbicides must be applied in late summer or early fall. To control summer annuals, preemergent herbicides must be applied in spring or early summer. Precise timing of application will vary by species, soil temperatures, and/or weather conditions. Consult the label to determine the appropriate application time. Always follow the instructions printed on the herbicide label.
Cultivation and hand pulling should be done periodically throughout the growing season to prevent weeds from flowering and setting seed. Start early in the season when the weeds are small and repeat the process frequently during the season. It is much easier to destroy small weeds than large weeds. Avoid deep cultivation as it brings more weed seeds to the soil surface where they are more likely to germinate.
Tilling can help uproot and destroy existing weeds, but it also brings buried and dormant seed to the surface where they are exposed to the sunlight and warm temperatures they need to germinate. The stale seedbed technique utilizes this phenomenon to reduce weeds, especially annual weeds, for the remainder of the growing season.
Till the area two to four weeks prior to planting to destroy existing weeds. This purposefully brings weed seeds to the surface and forces them to germinate early. Right before planting, hoe, lightly cultivate, or use another weeding method such as herbicides or flame weeding to kill the newly emerged weeds. Do this second weeding with minimal soil disturbance. Utilize a mulch so the soil does not have to be disturbed again during that growing season to prevent new weeds from germinating.
The seeds of many annual weed species require light for germination. Adding a layer of mulch reduces the number of weed seeds that germinate by preventing light from reaching the soil surface. Mulching is especially effective in controlling annual weeds because they must germinate from seeds each year.
Organic mulches are an excellent way to prevent weed growth while conserving soil moisture and improving the soil organic matter content. Organic mulches that might be used include clean weed-free straw, old hay, shredded leaves, grass clippings, crushed corncobs, wood chips, shredded bark, leaf mold, sawdust, and well-rotted animal manures.
For established annual weeds the best time to apply herbicides is early spring when the weeds are actively growing but before they go to seed. Winter annuals should be targeted in early spring as they often begin setting seed by mid to late spring. Summer annuals that have emerged in spring should be treated with herbicides when they are small and more easily controlled.
Several factors may 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.
Broadleaf, selective herbicides can be utilized in lawn settings to control broadleaf weeds while not harming the turf grass. Broadleaf herbicides will not control annual grassy weeds like annual bluegrass and crabgrass.
Non-selective herbicides will kill any green plant and can be used to spot-treat annual weeds. These 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. Herbicides 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 the label instructions on the package. Failure to follow directions may kill desirable plants or prevent other plants from being grown in the area.