Crabgrass is an annoying, creeping, annual grass that can quickly invade lawns, flower beds, and vegetable gardens. It is such a prolific nuisance that there are multiple sprays and chemical control methods specifically designed to stop the spread of crabgrass in your lawn.
Although sprays do work, they don’t solve the real issue.
Keep crabgrass out of flower beds by improving the soil and using more efficient irrigation methods. Crabgrass grows in moist areas of poor, acidic soils. Use targeted irrigation, like drip emitters, and add compost each year to make your flower beds less attractive to persistent weeds.
This multi-faceted approach of fighting unwanted plants in the landscape is known as IPM, or integrated pest management.
Yes, integrated pest management includes control of weeds (source). They can be quite a pest, after all.
The overall goal of integrated pest management is to look at the full scope of a pest problem and attack it from all angles, which can reduce current pest populations but also make the area more pest-resistant in the future.
There are 6 steps in the IPM process as it applies to weed control:
- Correctly identifying the weed causing the issue
- Understanding the life cycle and biology of the weed
- Watching for signs of growing weed populations
- Establishing an action threshold
- Taking action against the weed once it passes the threshold
- Evaluating how effective the actions were at reducing weed populations below the threshold.
The hope is that you can modify the environment to be unsuitable for pests before the population reaches the action threshold. In general, this means improving the health of the soil and using more efficient irrigation techniques.
But first, how can you make sure crabgrass is to blame for your weed problem?
How to Identify Crabgrass
Unfortunately, crabgrass has the ability to evolve and adapt to nearly any climate. It’s difficult to say for certain what crabgrass will look like in your climate because it may have thinner, thicker, lighter, or darker leaves, and it may be more or less upright and more or less disguised by your lawn grass.
However, crabgrass is easier to identify in a flower bed because it’s a grass among flowers.
In general, if a low, creeping grass is invading your flower bed, it’s crabgrass. The only exceptions may be in the South if you have a St. Augustine or Bermudagrass lawn, which can be commonly confused with crabgrass.
Another common weed grass is quackgrass, but this is a much taller, more upright grass that is easier to pull and manage in the landscape.
Here are a few more tips on how to identify crabgrass:
- Crabgrass is an annual
- Crabgrass seedlings look like corn seedlings
- Mature stems grow out from the base like a starfish
- Leaf blades have a slight fold in the center
- As the grass grows, it forms a thick, circular mat
- Blades are wider than Bermudagrass leaves
There are two varieties of crabgrass:
- Small, smooth crabgrass (typically found in the North)
- Large, hairy crabgrass (typically found in the South)
Small crabgrass is more common in lawns, while large crabgrass is more common in landscapes. However, either variety can be found across the United States and in any location within a lawn or landscape.
The Life Cycle and Biology of Crabgrass
Although crabgrass sends out runners that may root and produce new plants, the primary method of reproduction is seed.
One crabgrass plant may produce up to 150,000 seeds from September – November. (source)
Crabgrass is an annual, so a plant will germinate, grow, go to seed, and die within one growing season.
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Because the life cycle of a pest tells you when it is most invasive, and when it is most vulnerable.
Crabgrass goes to seed in the fall, which is the point when it has the potential to multiply. Spray schedules and mechanical control methods should focus on eliminating plants before they can go to seed, so you cut down on the following year’s population.
Crabgrass begins sprouting before the lawn breaks dormancy, and seeds will continue to germinate throughout the growing season. The seedlings are from the previous year’s seed heads, so weeding has no impact on new plants forming in the landscape.
Crabgrass can grow in almost any environment, but it thrives in bare patches of poor soil with plenty of moisture. Bare soil is also prime real estate for seed germination, so the first step in crabgrass control is eliminating patches of open ground.
The easiest way to do this is by using mulch. A 3” – 4” layer of good-quality mulch will smother seedlings and prevent sunlight from reaching sprouts. If the mulch is dense enough, this should stop the majority of crabgrass seedlings from breaking through and becoming mature plants.
Crabgrass thrives in areas where it gets consistent moisture. Sprinklers and hose sprayers wet the entire flower bed, which can inadvertently water weeds along with the flowers. This can create small pockets of the landscape where seeds can germinate and start to spread.
Cut down on crabgrass seeds germinating by using a drip irrigation system. Interconnected hoses with small emitters can be buried under the mulch and provide consistent, targeted moisture to each plant without watering bare spots in the landscape.
How to Monitor for Crabgrass
It’s hard to miss a mature crabgrass plant in a well-maintained flower bed. However, seedlings are much easier to overlook.
Crabgrass seedlings look like corn plants. Watch for seedlings in low pockets of the flower bed where pools of water form after watering. These low areas create microclimates that promote germination, so they are the most likely place for crabgrass to emerge.
Seedlings also need light and warmth to germinate, so look for moist areas closer to the edges of the landscape where they aren’t shaded by buildings or taller plants.
Don’t forget to monitor the lawn for crabgrass; if plants in the lawn go to seed, you may have an infestation in your landscape the following year.
Focus on identifying and removing crabgrass plants before the end of summer when seed heads start to form.
When to Take Action to Control Crabgrass
You can choose a threshold for pest control for one or more of the following reasons:
- The pest is causing economic harm (more common in commercial settings)
- The pest is causing health problems (more common for bacteria, insects, and plants that are poisonous or produce pollen)
- The pest is causing a drop in aesthetic value (more common in landscapes)
Crabgrass doesn’t cause economic or health problems in a flower bed. Therefore, you will have to establish a threshold of action based on how you want your landscape to look.
Take the life cycle of crabgrass into account when you establish your threshold. Allowing a few plants to remain in the landscape one year may mean a few hundred seedlings the following year.
Crabgrass is easy to identify and easy to remove, and leaving a few in the landscape can result in exponentially high populations. In most cases, the threshold for action on crabgrass should be 1 plant.
What does that mean?
It means that if you see one crabgrass plant, you should take action.
Remove the plant, and begin to monitor for others. Continue to remove plants and modify the environment to discourage new seedlings. If the plant population grows beyond your ability to weed and mulch for control, it’s time to take chemical action.
How to Spray for Crabgrass Control
There are two types of chemical control methods for weeds:
- Pre-emergent herbicides
- Post-emergent herbicides
The threshold for when you use each chemical may vary. In most cases, you should begin using pre-emergent herbicides in the spring if you had to pull any crabgrass the year before.
Pre-emergent herbicides are a preventative spray that kills seedlings before they can take root. These should be applied before the lawn breaks dormancy, or when soil temperatures reach 60o in the spring.
Pre-emergent herbicides can be sprays or granules. In flower beds, granules are the most effective pre-emergent herbicide. Apply your annual compost topdressing and mulch layer, and then apply pre-emergent on top of the mulch.
Post-emergent herbicides are for controlling mature plants that are too numerous or prolific to control through weeding. If you are monitoring for crabgrass, pulling mature plants, and applying pre-emergents in the spring, you shouldn’t need to spray post-emergents because the population will be small.
However, if you already have a crabgrass infestation, start spraying as soon as possible to prevent the mature plants from going to seed. Post-emergent herbicides are a method of getting the population under control, not a method of ongoing control.
Whatever method or combination of methods you choose for crabgrass control, the goal is to prevent plants from forming seed heads.
Post-emergent herbicides can be selective or non-selective, meaning they can either kill any plant they come in contact with, or they can selectively kill plants within a category.
There are two main plant categories for herbicides:
- Monocots, or grasses
- Dicots, or broadleaf plants
These two types of plants grow very differently, so selective herbicides can be applied over an entire area and kill one without damaging the other. This is usually used in lawns or in commercial agriculture to control broadleaf weeds in lawns or in grain fields.
Crabgrass is more difficult to control in lawns, because they are both grasses. Manufacturers have created crabgrass-specific herbicides that kill crabgrass without affecting the lawn.
You do not need crabgrass-specific herbicides for flower beds. Flowers are broadleaf plants, and crabgrass is a grass (obviously). Spray any selective herbicide that targets grasses to control crabgrass in a flower bed.
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How to Measure the Effectiveness of Crabgrass Control Methods
Once you take action to control crabgrass, you have to have a way to determine how effective your actions were to reduce the population.
The easiest way to do this is to take pictures and keep notes. Any time you apply chemicals to your lawn or garden, make a note on your calendar and take before and after pictures. This can help you determine the best course of action the following year.
Here are some things to watch for as you treat crabgrass:
- Where/when do seedlings germinate?
- What mulch were you using (if any)?
- What chemicals did you spray on your lawn, and when did you spray them?
- Did the crabgrass start in the landscape and move to the lawn, or vice versa?
- What kind of pre-emergent did you use, and when did you use it?
- What were your application rates for chemicals?
- What time of year did you first notice seedlings?
- What time of year did you first notice seed heads?
- Did seedlings increase or decrease with mulch?
- Did seedlings increase or decrease with a change in irrigation schedule?
Once you determine the average timing and behavior of the crabgrass in your climate, you can develop a customized control program for your lawn and garden.
Crabgrass is a nuisance. But, the good news is that as you improve the overall health of your lawn and garden, the weeds no longer feel welcome.