Weed Killing Basics

If you have a garden or yard, you have weeds. They may lurk in the lawn, thrive at the base of a shrub or mess-up flowerbeds, making weed control a constant battle. It can be a hard-fought battle requiring patience, persistence and knowledge – of both types of weeds and the weapons you have to eradicate them.

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What Are Weeds?

Weeds are any plants that are growing where they are not wanted. They usually aren't something you plant intentionally; they just appear.  However, sometimes you can plant something that is more aggressive than you planned and it can be considered a weed or become “weedy”.  Weeds grow vigorously, often outpacing and overrunning desirable plants. There are several types:

Annual Weeds: Complete their life cycle – from germination to setting seed – in one growing season; some annual weeds complete their life cycles in a matter of weeks, producing several generations in a single year.

Fact: Annual weed seeds can lie dormant from 4-40 years.

Examples: Chickweed, Crabgrass, Lamb's-Quarters, Annual Bluegrass, Groundsel

Perennial Weeds: Live for two or more years; plants grow as long as conditions are favorable and frequently die back to soil level with cold weather; new growth emerges at the beginning of the growing season, regenerating from roots or stems; in warmer regions, some perennial weeds can be green year-round.

Fact: Perennial weeds can spread by various means, including seed, underground stems, stems that root as they creep along, pieces of root or underground bulbs.

Examples: Bermudagrass, Creeping Charlie, Curly Dock, Dandelion, Plantain

Broadleaf Weeds: Leaves are wide and flat (not grassy or needle-like).

Fact: Broadleaf weeds are easiest to kill or control when they're young and actively growing. Some mature broadleaf weeds develop a tough, protective surface that makes it difficult for weed killers to penetrate.

Examples: Chickweed, Clover, Creeping Charlie, Dandelion, Henbit

Grassy Weeds: Look and grow in ways that resemble grass; leaves are produced one at a time and look like grass blades.

Fact: Many perennial grassy weeds form rhizomes or stolens, fleshy roots that regrow if left behind in soil after weeding.

Examples: Annual Bluegrass, Bermudagrass, Crabgrass, Giant Foxtail, Goosegrass, Quack Grass

 

All About Herbicides

An herbicide is a chemical used to kill plants or inhibit their growth. Weed killers contain herbicides. Some herbicides have residual qualities, meaning they continue to kill weeds for a specified time period after application. There are several kinds of herbicides:

Non-Selective Herbicide: Kills any green and growing plant, or part of it, regardless if it's a weed or not.

Selective Herbicide: Kills only specific types of plants; for example, a selective herbicide may only kill broadleaf weeds and not grassy ones. That can be a valuable advantage because you can spray it on broadleaf weeds in a lawn without harming the grass.

Pre-Emergent Herbicide: Prevents seeds from germinating or kills germinating seeds before they emerge from soil. They must be applied before weed seeds germinate. A common example of a pre-emergent herbicide is a Crabgrass preventer, which prevents Crabgrass seeds from establishing in lawns. Since they prevent germination, pre-emergents should not be used in areas where you eventually want to sow seeds. Clippings from lawns that have been treated with a pre-emergent should not be added to compost piles or used as a mulch.

Post-Emergent Herbicide: Kills growing weeds. Ideal for spot-treating lone offenders, but, if selective for broadleaf weeds, can often applied to entire lawns. Post-emergent herbicides come in two basic forms:

  • Contact herbicide – only kills the plant parts the chemical touches; ideal for treating annuals and perennial weed seedlings; more established weeds may resprout from the roots
  • Systemic herbicide – absorbed by leaves, stems or roots then moves throughout the plant, killing every part; effective on annuals and established perennial weeds; most effective when weeds are actively growing

Specialized Herbicide: To control some especially hard-to-kill weeds, like Nutsedge, Clover or Bermudagrass, you'll want to choose a specific herbicide that's been proven to be effective. Ask a local garden center or your local Cooperative Extension System office to learn which herbicides will help control your toughest weeds. 

 

Proper Timing

For success with both pre- and post-emergent herbicides, proper timing is critical.

With post-emergent herbicides, you'll get the best results when spraying young, actively-growing weeds. Tough, mature weeds may require repeated applications. Most post-emergent herbicides should not be applied to dormant lawns. If applying in spring, wait until the lawn is actively growing and has been mowed at least twice.

With pre-emergent herbicides, you'll need to apply the product prior to the time weed seeds start germinating, which can be spring or fall depending on the type of weed. For example: cool-season weeds, such as Annual Bluegrass, are usually best controlled with a late summer to early fall application, depending on where you live. If you apply too early, these herbicides may have degraded and are useless or less effective by the time the weed seeds start to germinate

Depending on the species, weed seeds germinate when the soil reaches the correct temperature. The best way to determine the ideal time to apply pre-emergents is to contact your local Cooperative Extension System office or master gardeners.

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