Does Mowing Weeds Kill Them?

While it might seem like mowing the lawn when weeds are present is giving in to them, cutting the grass is actually an important way to keep weeds down and out of your yard. In fact, it is one of the main lines of defense against weeds because regular mowing not only keeps those weeds from taking up space and putting out seeds or runners, but a healthy lawn will be able to better compete with weeds.

While some unwanted growth may still show up here or there, your mowing strategy makes a difference in how hospitable your lawn is for local weeds.

Weeds And Your Regular Lawn Care Routine

It might be obvious to say that the way you maintain the lawn sets the stage for whether weeds will be able to grow freely or not. A lawn that doesn’t get mowed frequently enough and has thin coverage, unbalanced nutrients, and/or compacted soil leaves lots of opportunities for weeds to sprout and grow uninterrupted. Weeds grow where there’s space and favorable conditions, and, in fact, they can tell you a lot about the condition of your lawn.

Dandelion, for example, likes to grow in compacted soil that has low calcium. It has a deep taproot that can reach minerals in the ground that your grass may not be able to. In nature, this results in rebalance of the soil when the dandelions decompose and reintegrate into the soil, but in your lawn, you can correct for calcium, and the dandelion is a competitor for your grass.

Crabgrass is another weed that likes soil depleted in calcium, and clover likes soils that are more alkaline than most grass species prefer. A well-fertilized lawn with a slightly acidic pH, on the other hand, encourages good grass growth and doesn’t leave nearly as much opportunity for weeds to sprout. Fertilizing, aerating, and trimming the lawn all contribute to healthy grass cover.

While many weeds can be suppressed by healthy grass growth and good lawn care, some can still show up. Low-growing weeds like broadleaf plantain and bull thistle stay close to the ground and might escape the mower blades entirely or grow back from minimal damage.

These weeds will likely need close attention to get them out of the yard, but their presence shouldn’t be used as a reason not to cut the grass: never skip mowing just because weeds are present.

Is It Better To Pull Weeds Or Mow Them?

While it depends on the type of weed and whether mowing will truly eradicate it, pulling weeds from the lawn usually isn’t the best way to remove them (unless you use specialized weeding tools in a healthy lawn). If any roots are left in the ground when you pull one, the weed can frequently regrow.

When you mow annual weeds down, like tree seedlings or crabgrass, you’re cutting off their life supply, the leaves and stem. For these types of weeds, the roots will start to wither and die.

Perennial weeds, however, like clover and dandelion, have hearty roots that can sprout anew from root fragments. Cutting these down may help your grass crowd them out, but since your lawn’s ground isn’t as soft and loose as a garden bed, it isn’t as easy to pull weeds effectively by hand.

Will Mowing Weeds Cause Them To Spread?

Mowing your lawn when weeds are present is a great way to keep them from spreading. Even if the root system isn’t destroyed immediately, cutting plants like dandelions and clover and keeping them from flowering will keep them from spreading.

A key to controlling weeds is to prevent them from seeding and stopping the next generation. Regular mowing won’t only keep flowers down, but damaging the leaves will, over time, weaken and destroy the roots.

If you notice weed flowers in the lawn, use a bag on your lawn mower to prevent weed seeds from being left on the lawn with your grass clippings.

Do I Need To Mow Extra Low To Kill Weeds?

Cutting grass too low can damage the grass itself and leave the right conditions for weeds to take hold and spread. Depending on the type of grass, most lawns like to be kept between 2 and 4 inches tall, although this varies by species.

In general, the 1/3 rule of thumb for cutting grass says not to cut more than one-third of the grass’ height at the time of mowing, so it has enough blades left to use its stored energy and recover. Some grass likes to be cut short, while other kinds like to remain on the taller side:

  • Bermuda grass – Cut when it reaches 3 inches down to 1.5 or 2
  • Zoysia – Cut from 2 inches to 1 or 1.5
  • Buffalograss – Cut when it reaches 3 inches down to 1.5 or 2 inches
  • St. Augustine – Cut from 5 or 6 inches to 3.5 or 4 inches
  • Kentucky bluegrass – Cut from 3.5 inches to 2
  • Ryegrass – Cut when it reaches 3 inches, down to 2 inches
  • Tall Fescue – Cut from 5 inches to 3 or 4
  • Fine Fescue – Cut from 3 inches down to 2

When grass is cut too short, more sunlight, air, and moisture reach the surface of the ground. Thatch and grass blades usually provide some shade and cover that keeps weed seeds from germinating, but the open space and extra sunlight give an opportunity for weeds to take hold.

In addition, if the grass is cut too short, it will be more stressed trying to repair itself with less available energy to do so (when blades are cut too short, chlorophyll stores are significantly reduced), and the lawn may easily dehydrate and dry out.

When Should I Use Herbicide On Weeds?

While mowing regularly and keeping your lawn in as healthy a condition as possible will help keep many, if not most, weeds out of the yard, you may still need to use an herbicide to really keep the lawn clear of unwanted growth.

A pre-emergent herbicide helps prevent weeds from sprouting in the first place, while post-emergent herbicides are available to be used when hearty weeds do show up.

When you apply a pre-emergent herbicide, it’s usually done in the earliest parts of spring before you’re even mowing the lawn since it hasn’t grown in yet. This helps keep weeds from sprouting before your grass does so your lawn can grow in without competition, grow in densely, and provide the tight cover that prevents weeds from popping up.

When you need to apply a post-emergent herbicide for weeds that made it through, don’t mow right away. 2 to 5 days are required for the herbicide to be absorbed by the weed’s leaves and work its way to the roots. This short period of time can be fit in between your weekly scheduled mowing. Be sure to collect weed remnants with your mower’s bag to prevent any spread!