Guide To Weed Control For Cool Season Grasses

If you live in the Northern part of the United States then you may be familiar with what are known as cool-season grasses. In the uppermost third of the country, cool-season grasses are the only type of grasses that grow while the mid-portion is known as the transition zone where both warm-season and cool-season grasses grow and overlap.

As opposed to warm-season grasses that thrive in a warmer climate, cool-season grasses are grass types that grow well in areas that have a cooler climate and are able to endure the colder temperatures better. However, while they have a better tolerance for the cold weather, they do tend to brown out during the summertime so they will require extra watering and maintenance in the warmer times of the year.

The most common types of cool-season grasses are Kentucky bluegrass, perennial ryegrass, fine fescue, and tall fescue. Cool-season grasses are usually divided into two broad categories, ones that need a lot of sun and others that can handle shade or are a shade-tolerant species that you can plant under trees. If you have a cool-season lawn however, you still have to deal with the possibility of weeds appearing and uglying up your yard.

In this guide, we will look at how to manage weeds on cool-season grasses using proper cultural practices and the help of herbicides which will keep your cool-season lawns looking great without the blemishes brought on by unwanted invasive weeds.

Identification

There are a number of common weeds that show up on cool-season grasses, so before you proceed with a treatment program, you need to first identify the problem weed. Here we will share a few of the common varieties:

Carpetweed

This low-growing broadleaf annual weed found in lawns forms a thick mat and grows horizontally rather than upward which is why they get their name since they look like a carpet. It can grow out of control everywhere if there is no intervention.

Knotweed

Common knotweed is a low to the ground spreading annual weed where it'll start in just a small little area and they'll be a plant or two. Their thin stems form a thick mat separated by little knots or joints, which is how they earned their name.

Henbit

Henbit is a lightly hairy weed that grows annually in the wintertime and has greenish purple stems that are delicate. Its other leaves are oval-shaped with blunt points and veins on the underside of the leaf.

Dandelion

Dandelion is a common weed known for its dark green leaves that bunch up in a rosette pattern close to the soil surface. They grow large bright yellow flowers that gradually mature into puffy white seed heads. When the seeds are ready to be released, which can happen fairly quickly, the seeds can easily be blown off by the slightest gust of wind.

Canada Thistle

Canada thistle grows in thick clusters and can choke out turfgrass where established. Canada thistle can grow between 1 to 3 feet tall and blooms prominent flowers that look like light-purple colored puff balls.

Crabgrass

Crabgrass is an annual and perennial invasive grass that forms slender stems that grow out from a central root underground that resembles the pinchers of a crab.

Sedge or Nutsedge

Nutsedge is a perennial weed that is neither a grass nor a broadleaf weed. Distinct characteristics include a triangular stem and yellowish-green color.

Inspection

It is important to do a thorough inspection on your lawn when you notice weeds and invasive plants emerging on your lawn. An inspection will help to determine what weed you have on your property so you know where they are concentrated, what conditions may be helping the weed to thrive and what herbicides may best be able to control the weed.

Where to Inspect

Scan your lawn and look for the weeds we described in the identification section. There are weeds that appear at certain times of the year, known as summer annuals, or winter annuals, and perennial weeds which are persistent from year to year. Timing is important when it comes to inspecting for these weeds because catching them early when they are young and tender makes them more vulnerable to herbicide applications.

What to Look for

Closely observe the traits of the invading plant and determine what weed it is. You should also look at the surrounding environment for anything that can tipoff why the weed is thriving. Is the soil compacted? Are their drainage issues? Is your grass undergoing a lot of stress. Finding these answers can determine how you approach control.

Treatment

Once you have identified the weeds on your cool-season lawn, the next step would be to eliminate them through the use of herbicides. It should be noted that there are two types of herbicides, post-emergent, and pre-emergent.

Pre-emergent herbicides are essentially a sort of birth control for weeds on your lawn and are used for prevention. These are best applied when you are aware of a particular weed that regularly grows at a certain time every year on your lawn and you lay out a pre-emergent to make it so the weed doesn't germinate.

Post-emergence herbicides are used to control weeds which are actively growing and are visible. The herbicide is sprayed on the foliage where the plant will absorb the chemical which will travel down to its root system, killing the entire plant.

Timing is important no matter what herbicide you use. Pre-emergents should be applied to the lawn before plants have grown. If the target weed has already been established, the pre-emergent herbicide is ineffective. For post-emergent herbicides, its best to use when weeds are in active growth which is usually in the fall or spring. Applying herbicides outside of these seasons will result in the herbicide performing slowly and can be a waste of product.

Pre-Emergents For Cool Season Weed Control

Our top recommendation for pre-emergent control of most cool season weeds is Prodiamine WDG. Prodiamine works by stunting the development of seeds at the beginning of the germination phase. It is labeled for most grasses but it may be harmful to Annual Bluegrass so if that is a desired grass on your lawn, use with caution.

Determine how much Prodiamine you will need by calculating the square footage of your yard (length x width = square footage). The maximum application rate can range anywhere from 0.185 oz. to 0.83 oz per 1,000 sq. ft. per calendar year. Mix the appropriate measured amount with water based on your calculations and apply over your entire lawn uniformly.

Pre-emergent herbicides should be applied between March and April depending on what part of the country you are in to control summer annual weeds and in the late summer or fall to control winter annual broadleaf weeds and grasses.

To know exactly when to apply, check the soil temperature. Once temps have reached 55F at a 2” depth and after the risk of a hard frost has passed, this should be the best time. Depending on factors like weed history, weather and soil type, a second application may be necessary to get control all season long.

Post-Emergents For Cool Season Weed Control

Our top recommendation to deal with post-emergence broadleaf and grassy weed control in cool-season turfgrasses is 2,4-D Selective weed killer. This is a selective herbicide meaning it will only harm target weeds and spare desired cool season grasses.

For small applications with a hand sprayer, mix between 0.72 to 1.1 fl. oz. in a gallon of water to treat 1,000 square feet. Fill your sprayer halfway with water, add the appropriate measured amount of 2,4-D and then fill the rest of the way with water. Apply the 2,4-D mixture to areas where unwanted vegetation is currently growing, preferably on a fan spray setting to ensure even coverage.

For summer annual weeds, post-emergent herbicide applications are most successful when weeds are young in late spring and early summer. For winter annuals and broadleaf perennials weeds, the best time to apply post-emergent herbicide products is in the fall, typically around the first frost.

Prevention

Cool-season lawn weed control requires a combination of good cultural practices and choosing the right herbicides necessary to tackle the weeds and give your warm-season grass the advantage in keeping the weeds from thriving. Some lawn owners would rather not use chemicals on their lawns for weed management, so it is possible to eliminate weeds without the use of herbicides.

Weeds that appear on turfgrass, no matter if they are cool season or warm season, usually rear their ugly seedheads because something that the lawn maintainer is or isn't doing. In other words, the lawn maintenance practices required for a healthy cool-season grass that will resist or choke out invasive weeds are either not done properly or are neglected.

For example, some reasons for the appearance of weeds include mowing with a blunt blade or mowing at a height that isn't conducive to the health and benefit of your lawn. Other issues can be soil compaction due to foot traffic or poorly drained areas.

Correcting these practices and issues (via applying the right nutrient combination of fertilizer for your lawn, proper mowing, and watering) can go a long way in promoting a thick, lush nutrient-rich turf which will choke out invasive weeds trying to establish themselves and prevent weeds from growing on your property.

With this being said, there are some species of broadleaf weed where improving cultural practices will not be enough to eliminate the weeds. This is when herbicides may be necessary to add to your control plan to sufficiently get rid of whatever weed is plaguing your cool season lawn.

Key Takeaways

• Cool-season grasses are not immune to pesky weeds. Invaders like Carpetweed, Henbit, Dandelion and Crabgrass commonly plague cool-season turf.
• We recommend Prodiamine 65 WDG for pre-emergent control of most cool-season weeds and 2,4-D Amine Selective Weed Killer for post-emergent control.
• Putting in place proper grass management practices and being persistent with using the right herbicides at the right times will successfully get rid of weeds from your cool-season grasses.