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Preemergents and you!

Preemergent…what the heck does it do? How does it work? Is it bad for my plants? My pets? Me? Will it really keep all the weeds out of my yard? When do we have to put it down/is it too late? Can I still aerate if my service already put it down? Will it affect my bulbs? I already see weeds, now what? Do I absolutely need it?

Let’s start with what preemergents are and a simplified explanation of how they work. Preemergents are a class of herbicides that are designed to hold in place on the top layer of soil and kill young seedlings as they emerge. They come in a variety of brand names and active ingredients, but they all basically have the same job, stop tough to control weeds for a period of time to let the desirable, established plants take over the space. You may want to think of it as a chemical weed mat, like the fabric you may buy to put under your mulch in flower beds and gardens. What they don’t do is control already existing weeds*, nor are they meant for control of ALL weeds (*which we’ll discuss in greater detail down below) and they do not have an infinite window of effectiveness.

Choosing which preemergent you use is largely based on the types of weeds you historically have issue with. Each has a labeled list of weeds that it controls, for how long, at what rates, etc. etc. You’ll find that many of the products overlap each other in what they control. For example, crabgrass is pretty much a universal target for all popular turf preemergents in the market. We’ll link a few labels at the very bottom of this article (which you should open and reference while reading this article) so you can browse through and see the differences and the similarities of some of the targets, their application dates and the effective windows. What you will notice as you read these labels, is that the majority, by far, do not target cool season common weeds like dandelions, thistle, etc. This becomes important when thinking about the management of your turf with other products and cultural practices you need to be employing, especially during the cool months. It also will show you that timing of the product is not typically anywhere near as early as some of the misinformation out there may lead you to believe. Only by understanding what your target is, the length of your product’s effective window and the typical germination period for controlled weeds, can you make a decision on the if, which, when and how much when it comes to preemergents.

It is also imperative that preemergents get watered in. This not only moves the barrier across all segments you are trying to control, but preemergents are broken down by sunlight. That means if it sits on top of your turf, attached to leaf surfaces and thatch for a prolonged time, it will begin to degrade before it has even begun its job. You do have your water on if you are considering putting it down early, don’t you?

Now we’d like to cover how preemergents might work for you in your complete turf management program. What you should know right off the bat, is that there are plenty of turfs out there that do not use a preemergent at all! Usually these are very large properties (sports fields, parks, etc.) driven by cost to skip this application, but there are some lessons to be learned from these areas that, although have no preemergent, generally do not receive much in the way of weed pressure either. These are typically large open areas of well established turf, cut at a high height without a lot of cement or asphalt bordering the majority of the grass. (Cement and asphalt create high soil heat areas that cause cool season grasses to struggle. These areas also typically do not get the same amount of water, in an effort to not water the hardscape, as the rest of the turf, adding to the stress of the plants on this edge. Add to that these are also the areas typically assaulted by unskilled trimmer operators, often taking plants to the dirt, walked and driven on, increasing compaction and it becomes obvious why they are a hot bed of warm season weed potential!) Why is this important? It shows that by changing some of your cultural practices, (turf mow height, eliminating “ramp” trimming and scalping, proper fertilization, soil compaction management, water and insect management, etc.) you may be able to significantly reduce your chemical usage or even eliminate portions of it.

You should be starting to have an idea of what preemergents are and what kind of weeds they do control and don’t control, we can talk about the timing. Again, we can reference the labels for the individual products that we choose, but almost universally, we’ll see a seasonal chart that shows average crabgrass germination dates, one of our biggest control targets and a marker for many of the other warm season weeds that are targeted by preemergents. We can also view the effective window for different rates, showing how many months of control we can expect out of the product. In some cases, you can see a reference to doing a fall application, after ground freeze and having that be an effective control for the following season. The issue with this method, although it has shown to be effective with a variety of Transition Zone university research, is that here in Utah, with our wild temperature swings, there is no guarantee that areas will stay below freezing throughout the winter. We’ve all experienced the February thaw and even balmy temps in December. What happens in these scenarios is that the product begins to activate, effectively putting you “on the clock” for your control window. So if you chose a 4 month protection rate and it activates in February AND your main weed pressure window is July and August, you begin to see the problem. The same holds true for those offering advice to go with forsythia blooms. March plus 4 months leaves you short of hot August and early September days as the chart below shows. So what if you want to go so early? You’ll be doing a secondary split application later, leading to more costs and additional chemical buildup in your landscape. The target is warm season weeds, as shown on the labels, so my question to you is why would so early be so important to you?


Now that you can see how a target window may look with your program, I know what you’re asking next. Why not just do the split, double application or go to the maximum rates to get a longer window and increase my safety? Well, beyond the cost issue, we do have a few other things we’d like to consider. Although preemergents do not show turfgrass species as their targets on label, there are quite a few things that they will stop from growing. Turfgrass seed and new shoot growth are among them. What that means to you is if you have a damaged area from last season that you are rebounding during the prime cooler months, either through seed or aeration, hoping that rhizomes and stolons will spread your stand, your preemergent may block this activity. Looking to thicken the lawn to help it crowd out ALL new weeds? Again, this may work against you for the very same reasons. There is also anecdotal evidence that your preemergent application may also retard full coloring potential of the turf during its peak growth periods, whether this be through the lawn not thickening to its maximum or some other blocking activity going on, I’m not certain, but worth keeping in mind as you observe results of your efforts.

Getting a basic understanding? Good. Now we can talk about what to do with those cool season weeds, the possibility of using preemergents in bare ground or mulched areas, like flower beds or tree rings and the caution you should use with pets and children. Let’s start with the caution. Like any other substance, from a Big Mac to radiation, amount of exposure is the key to danger. Again, we can refer to the label to see precautions, reentry times, protective equipment, etc. That’s exactly what the label is there for, your safety, so please have a look and follow the recommended procedures. That’s better advice than anyone, anywhere will ever give you regarding pesticide usage. More time and money is spent researching pesticides to bring to market and gaining EPA registration than pretty much any other consumer good, including many of our medications. It’s all right there for you and you have no one to blame but yourself if you do not follow that label. If you use a lawn service and have a concern about products being used, ask for a label copy. A much better way to get factual information than succumbing to whatever is being conjured on the internet or beyond. Go with the science, that’s real and it’s all in one place for you to make your own decisions, the label. Cool season weeds , what you’re seeing early and may have been thinking you needed the preemergent for. Hopefully, you now know better, if not, go back and revisit paragraphs above and the label samples. So what to do with the pesky dandelions, thistle, etc. that are up and annoying you? Post emergent herbicides are what you are after. This is why professional lawn programs continue with post emergent, broadleaf weed control throughout the season. If one application of preemergent controlled all weeds, all season long, there would never be a need for more weed control done during the season. Let me repeat that, If one application of preemergent controlled all weeds, all season long, there would never be a need for more weed control done during the season!. So a post emergent tool is going to be part of your program. This article is already ballooning far beyond, in length, of our intial intentions, so we won’t cover post emergents here, but know that is the tool that you are going to use along with cultural practices to keep cool season weeds in check.

Bedding areas and bare ground preemergent control. A word of caution, if you have bulbs, tubers or other plant material that needs to emerge from the soil each year, you need to put off any use of preemergents until these plants are up. I’ve seen tulips stopped dead by preemergents, don’t let this be you if you have invested in these types of plant materials. That being said,this is an area where you can really save some labor this summer if you learn a little now. There are a whole host of preemergents designed specifically for bedding areas that will stop a large majority of your weeds in their tracks. Imagine cutting weeding by 90% or more! That’s what these products are intended for. Whether they go under a mulch layer, over the top of shrubs (ok’d on the label) or your established annuals and perennials, these products are some of the very most underutilized versions of a preemergent out there. They come in easy granular forms, just shake and go or liquid versions for covering large areas quickly. You can find them at the home stores , sold under brand names like preen or professional grade products at your local pro supply shop. Either way, if you don’t find bed weeding to be a zen experience, you owe it to yourself to check out some of these products in areas weed fabric or thick layers of mulch do not work.

In closing, this article is only meant to give you a basic understanding of preemergents and their role in your landscape. Please refer to labels and speak to your knowledgeable dealer for more info on products they carry.


Cavalcade (prodiamine) Label
Dimension (dithiopyr) Label
Pendulum (pendimethlin) Label