Managing Lawn Weeds
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A dense, vigorous lawn is your best defense.

Weeds are opportunists. They will find bare spots or places where your grass is weak and exploit those problems to their advantage.

Perennial weeds (those that regrow from their roots every year) can spread and make a lawn unsightly. Annual weeds (those that die at the end of the season and come back from seed the next year) can leave bare spots that are vulnerable to runoff.

No matter what weeds you have, the first line of defense is preventive practices. Try these options to get at the root of the problem first, before resorting to herbicides.

Preventive practices

Mow high. Do not mow grass shorter than recommended for the species you grow. Mowing at 3 inches or higher helps grass shade out weeds and encourages a thicker, more competitive turf. See other sections of this site to make sure that you are using the right grass species, fertilizing and watering correctly, and generally doing all you can to encourage healthy grass.

Reduce compaction. Pay special attention to heavily used areas and sections next to pavement. Weeds can gain a foothold in these spots and spread to the rest of the lawn if it is weak.

Repair bare spots by raking in seed in early spring so that the new grass can compete with the weeds that are sure to come up.

If lawn is thin, fertilize to improve density.

Hand weed. This is easiest when the soil is slightly moist. Check your garden center or catalogs for tools that help get tough perennials out by their roots.

Let the weeds be your guide. If weeds dominate an area, it's likely that there is something wrong with either the growing conditions or your lawn practices. Dense stands of prostrate knotweed (Polygonum aviculare) are a good sign of soil compaction. Don't just pull out the weeds. Relieve the compaction. Violets (Viola spp.) are a good sign of low light levels. One solution might be to seed shade-tolerant fine fescues.

If you use herbicides...

  • Use the right product at the right time. Follow label directions and keep accurate records.
  • Choose products that have the least potential for leaching into groundwater. More highly water-soluble materials have the highest potential (e.g. 2,4-D, dicamba, MCPP).
  • Use extreme caution when handling materials close to wells and impervious surfaces where runoff may enter storm sewers.
  • To avoid volatilization and drift, which release pesticides into the air, do not spray when temperatures are high or it is windy.
  • To help prevent polluted runoff, do not apply pesticides when heavy rains are expected or the ground is already saturated or frozen.
  • Empty containers should be triple rinsed and disposed of properly. Unused materials should be returned in the original container to authorized hazardous materials collection sites.

The types of herbicides include:

Pre-emergence herbicides:

  • Most common for crabgrass and goosegrass.
  • Applied to soil before weeds are expected.
  • Have low solubility and bind to organic matter. Have high runoff potential if not watered in properly.
Postemergence herbicides:
  • Most common for perennial broadleaf weeds.
  • Applied after weeds have emerged and are actively growing.
  • Have high solubility and do not bind to organic matter. Have high leaching potential. Avoid application before irrigation or rain.
Nonselective herbicides:
  • Kill or injure all plants they come in contact with.
  • Used to kill vegetation before reseeding.

Annual grass weeds
Crabgrass and goosegrass are two of the most common grass weed. Both are warm-season annuals. They thrive when temperatures are hot and cool-season lawn grasses are least competitive. Still, they have a tough time invading a healthy lawn.

Annual weeds can easily gain a foothold along paved areas such as driveways and sidewalks where high temperatures can damage cool-season grasses.

You can spot treat for crabgrass with pre-emergence herbicides. These herbicides work on the seeds as they germinate. Because they are ineffective on ungerminated seeds or established plants, timing is critical.

The best time for pre-emergent treatment of crabgrass is about the time that forsythia blooms wane, when the soil temperature is between 59° F and 65° F.

Pre-emergence herbicides do not distinguish between weed seeds and lawn seeds. So you won't be able to replant grass where you've applied them for 2 to 6 months. The other drawback is that you need to apply them before you know for sure if you are going to have a weed problem.

Once crabgrass emerges, usually from early June through mid-July, you can apply postemergence herbicides. Several different herbicides are on the market that can kill plants that have not yet tillered in a single application.

Selective post-emergence control is limited once crabgrass plants develop more than three tillers. Spot treating with non-selective herbicides such as Round-up® can kill the plants and reduce their contribution to next year's seedbank. You must use care not to accidentally spray and kill other plants nearby.

Annual Bluegrass
Annual, indicates compaction

Perennial broadleaf weeds
Unlike those for annual grass weeds, herbicides for broadleaf perennial weeds are usually applied post-emergence. The advantage of post-emergent control is that you can see how many weeds you have before you decide whether or not to spray. If you just have a few, pulling them by hand might be your best option.

Most broadleaf perennials -- such as dandelions -- have their greatest visual impact in spring. But late summer to mid-fall is the time to control them with herbicides. As the weather cools, these weeds start storing food produced by their leaves in their roots, just like cool-season lawn grasses. If you apply an herbicide at this time, it will be transported along with the food and stands a better chance of killing the root.

When applied in spring, these herbicides often kill the tops but not the roots. Because the weed is more focused on producing new growth and flowering at that time, not enough of the material is transported to the roots to kill them, and the weeds bounce back later in the season.

Make sure you choose a selective broadleaf herbicide - one that kills only broadleaves and not grass. Nonselective herbicides, such as Round-up ®, can kill all plants they come in contact with.

Ground Ivy
Managing Lawn Weeds