This is the time of year when lots of folks ask me about WEEDS. I discussed weeds and some weed treatments in a few earlier blog posts, mostly in response to some questions. Here, I want to “set the record straight.” I want to provide some of my thoughts and other information concerning turfgrass weeds.
Uniformity in our lawns is usually what we strive for. Weeds disrupt that uniformity. Weeds also compete with our lawns and playing fields for water, nutrients, light and space.
How do we define what a weed is? A weed is basically a plant growing out of place or where it is not wanted. A few sprigs of some nice warm weather turfgrass popping into your beautiful cool season turfgrass lawn (or vice versa) could be considered a weed. Like bentgrass growing in a stand of Kentucky bluegrass. That could be as disrupting as dandelions or thistles.
A friend of mine, who has a great deal of farming experience in the midwest, once said to me, "A beautiful rose in a field of wheat is a weed."
WEED ECOLOGY
Before developing a weed treatment regimen, let’s look at the ecology of weeds. Weeds have a unique ability to grow in a wide range of habitats and conditions. Weeds almost always possess one or more of the following traits: prolific seed production, rapid establishment, they have either rhizomes or bulbs, and they have long term seed survival. Seeds are the primary manner in which weeds disperse. Many are very abundant seed producers.
The top 6 inches of soil can contain as many as 13,000 weed seeds per cubic foot. And, one study showed how weed seed could still germinate after being in the soil for some 80 years.
Just like any other plant, weeds are influenced by climate factors. Temperature has the most effect. Moisture is the second most common effect. And, the absence or presence of light may impact weed growth. Factors with regard to your soil can also have an impact; pH level, nutrient levels, oxygen content, etc. Some other factors like mowing too low, severely dethatching in the spring, or light irrigation can have an influence. The grass seed you use can have an influence too; make sure the seed has a very low percentage of crop seed or weed seed content. Sanitary practices – like washing your equipment after use can have an influence as well.
If you have weeds, see if any of your management or cultural practices are promoting weed growth before you commence chemical, biological or nuclear warfare. There may be some simple corrective actions to take before you nuke your lawn.
There are two types of turfgrass weeds: grass like or broadleaf. As I mentioned on one of my first posts – grasses are monocots; broadleaf weeds are dicots. Weeds can also be categorized by their life cycles: annuals, biennials or perennials. Annuals complete their life cycle in one growing season, within a year. Annuals that begin in the spring and complete their cycle in the fall are summer annuals. The ones that start in the fall and complete their cycle in the spring are winter annuals.
Biennials require two growing seasons to complete their cycle; the first year they establish themselves vegetatively, the second year they form flowers, set seeds and die. Perennials live for three or more years. Strategies for controlling various weeds are determined by what type of weeds they are.
As I’ve said before – the best way to control weeds is to have a thick, healthy stand of turfgrass, by utilizing proper cultural and management practices. Many weed species are not that “competitive” with thick, healthy turfgrass. When I’m starting from scratch with one of my clients, my first step is to try to get a thick stand of grass growing. I worry about weeds later. Remember, growing a beautiful stand and eliminating weeds can take several growing seasons – it does not happen overnight. You really need to establish a “strategy” or a “program”.
Anyway, I digress…. One way to control weeds is biological control. That’s using the weed’s natural antagonists to control the weed. This is a pretty scientific and extreme method and is probably out of reach for the average homeowner. Besides, this is an area of turfgrass management that needs quite a bit more research to be done – especially for residential use.
Chemical control is something with which we are more familiar. This is the use of herbicides. Herbicides kill or inhibit the growth of plants. Although there are over 180 different types of herbicides; for turfgrass, we only need to consider a few.
Herbicides that target specific weeds are called selective herbicides. Herbicides that kill all plants they come in contact with are nonselective herbicides. Herbicides that kill only the part of the plants with which they come in contact are called contact herbicides.
Herbicides that are absorbed by the roots or shoots and are then translocated within the plant are systemic herbicides. Systemic herbicides are very good at controlling perennial weeds since they kill the underground parts of the plant. In another post, I talked about glyphosate (Commercial Name: “RoundUp”). Glyphosate is a systemic, nonselective herbicide.
Herbicides applied prior to planting or sodding are called preplant herbicides. Preplant herbicides for turfgrass usually involve fumigation with ethyl bromide. Also another method the average homeowner does not want to attempt. You want to try to put a huge tarp over your entire lawn and pump a carcinogen and a reproductive toxin under it? I recommend a different course of action.
Herbicides applied prior to weed emergence are called preemergence herbicides. These form a chemical barrier that when the weed seeds start to germinate and come in contact with the chemical barrier, they die. Preemergents are ineffective if the weeds are already growing. A good preemergent should not hurt established turfgrass. Read the label and look for: benefin, bensulide or DCPA.
Postemergence herbicides are applied after weeds have emerged or started growing. These are used to control broadleaf and perennial grass weeds. Sometimes these are used to control annual grasses. Converse to preemergent herbicides, postemergent herbicides do not control weeds prior to their emergence from the soil. Look on your postemergent labels for contents like MSMA, or 2,4-D.
ANNUAL GRASSES
Annual grasses can be controlled by either post or preemergent herbicides, although preemergent is best. As I discussed in an earlier post, put your preemergents down early – when the soil temperatures at a 2 inch depth is no more than 55 degrees F. Remember, when you do that, you have just put down a chemical barrier. That barrier will remain in place for 6 to 12 weeks. That will prevent ANYTHING – even good grass seed – from growing.
Postemergence herbicides on annual grasses are most effective when the weeds are in early stages of growth. So, this should be rather “early” in the season too. In many cases, two applications may be necessary. Space out those applications at least 10 to 14 days. Postemergence has its issues: there may be a need for multiple applications, dying annual grasses can be unsightly in your turfgrass, and, postemergent herbicides may discolor and weaken desirable turfgrass.
PERENNIAL GRASSES
These are really difficult to control. Much of the time they cannot be selectively controlled. The physiology and anatomy of perennial grass weeds and desirable turfgrasses are very similar. What kills one could kill the other. Spot treatment of perennial weed grasses is the key – trying to avoid touching the nearby desirable turfgrass with the herbicide - or as little as possible.
BROADLEAFS
These are the most common in turfgrass. Look for these chemicals on the label; 2,4-D, mecoprop and dicamba. These are systemic in nature. If you have at least two of these three chemicals on the label, you’ll be okay. If you see all three, you're in great shape. Now, be careful. These can harm surrounding vegetables, flowers, trees or shrubs. Dicamba is very mobile in the soil. Broadleafs are best controlled with postemergent herbicides. Some warm season turfs are sensitive to 2,4-D and dicamba – like St. Augustine and Bermuda.
GRASSLIKE WEEDS
Repeated applications of 2,4-D and Dicamba will provide fair to good control.
OTHER CONSIDERATIONS
Always read the label and comply with the instructions.
If using a postemergent, treat weeds at the early stages of growth and the weeds should be actively growing and not under drought or temperature stress. Air temperature should be between 65 and 85 degrees F.
Do not mow for 3 to 4 days prior to application so maximum weed topgrowth can come in contact with the herbicide.
Postemergents should stay on the foliage for at least several hours for best results. Avoid watering for at least 8 hours, ideally 24 hours, after application. Do not apply if rain is in the weather forecast within 24 hours. Delay mowing after application as long as possible.
Do not apply in windy conditions.
When using granular herbicides for broadleaf control, you will get best results if the weed is moist. You want the herbicide to “stick” to the weeds. Do not allow any traffic on the treated area for 8 to 10 hours after application.
Be patient if you see little or no results. Several days to three weeks may pass before there are any signs of success.
In many cases, newly seeded or sprigged areas are not tolerant of herbicides. Make sure you’ve mowed (using the one third/two third rule) at least three times prior to application to new areas.
Do not apply postemergents to warm season turfs as they break winter dormancy. Wait until the warm season grasses are completely out of dormancy.
After attacking broadleafs, wait 4 to 6 weeks before seeding or sprigging.
Grass clippings from recently treated herbicides should NOT be used as mulch around trees, shrubs, vegetables or flowers. Clippings collected after the turf has been mowed 3 to 4 times should be okay.
MY STRATEGY FOR WEEDS
I put my (granular) preemergent down rather early. Since I live in the mid-Atlantic region (the transition zone), I go for early to mid March. Now, this has been a very cool spring this year. But, my early approach continues to work. That usually is about a 90% solution. Some weeds still manage to pop up.
So, later, in mid April or early May, I put down my (granular)postemergent herbicide. That handles most of the ones that persist. Let’s say that’s now at a 95% to 98% solution.
Then I “spot treat” the “one-sies and two-sies” that still manage to foil my efforts with a (liquid) postemergent. I have a one gallon, pump action sprayer. It even has a strap on it so I can lug it around the yard. I spray each weed, in the center of mass with a steady stream for two or three seconds.
I realize the spot treatment phase of the operation may not be feasible for those of you with larger lawns. There are larger supplies of herbicides for larger lawns and you can get real serious with applicator equipment that can be towed behind a tractor or are self propelled, walk behind types. For example, I have a broadcast spreader that I can tow behind my lawn tractor.
Each year (or growing season) can be different. This year its chickweed. My chickweed survived every assault, until I spot treated it with my sprayer. It took at least two to three weeks to see any effect. Once it was dead, I tilled it under and planted some grass seed. That area looks beautiful now.
I recommend to my clients to do NOTHING (herbicides, pesticides or fertilizing) once the soil/air temperatures start to climb – like mid May or definitely by Memorial Day – JUST STOP; reconsolidate in your current fighting position. If you still have a weed problem by the end of May, live to fight another day. Focus on mowing and (perhaps – especially depending on rain) watering over the summer. Start planning subsequent operations for the fall.