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  • Writer's pictureDarcy Morehouse

Pre-Emergents Basics

Pretty soon we're going to start throwing around the term "pre-emergents" a lot around here. So what the heck are pre-emergents, what are they for, and how do they work?

What is a pre-emergent?

A pre-emergent is a type of herbicide (weed killer) that stops certain weeds from coming up in the spring. Hence the name. Easy to remember.

How does it work?

A common misconception is that pre-emergents stop weed seeds from germinating. In fact, pre-emergents only work on seeds that have already started to germinate. Your soil (and mine too, no judgements) has this layer of all different kinds seeds at all times, called a seed bank, many of which eventually become weeds. The seeds just kind of hang out in dormancy until the conditions are right for them to sprout.

A pre-emergent creates a barrier at the top of the soil. When the weed seed begins to sprout, they hit the barrier and it stops them from developing into full-grown big boy weeds.

What is it used for?


Ok, it can be used for more stuff, but really, that's the big one. Pre-emergents are most effective on annual plants, so the ones that reseed, die off, and regrow every year. The most common annual weed is crabgrass. Every little crabgrass cluster that you see in your lawn is a brand new weed that just grew this year from a seed*.

*A teeny tiny bit of crabgrass does replicate through the root system, but it's not a lot, so I'm only lying a little bit here. Let's call it embellishing.

That's actually what makes it so hard to get rid of. Once crabgrass has sprouted, it's up and already reproducing, spreading more seed that will come back next year. So even if you hand pull every last little spider of it, you're still going to get more next spring.

They only way to stop crabgrass entirely is to keep it from coming up in the first place. By killing off the sprouts before they get a decent shot at life (sorry little guys), you're essentially stopping the cycle. Now, keep in mind that there still may be some dormant seeds hanging out in the seed bank, so a few cycles might be necessary before your lawn is fully in the clear.

Won't it kill my grass too?

Kind of... A pre-emergent will work the same way on pretty much any kind of seed that is germinating. So yeah, it's also going do a number of emerging grass seedlings. BUT remember how pre-emergents work on annuals that die and reseed every year? Well, your lawn (unless it's all weeds) is made up of perennial grasses, which reproduce mainly through the root system which comes back every year.

So very little of your lawn is brand new just germinated seedlings. Your root system is already strong enough to deal with a pre-emergent, and (more importantly) the base of the grass blade is above that pre-emergent barrier. In other words, your existing grass is totally safe.

What is not safe is any new seed that you put down around the same time that you put down a pre-emergent. The microbes in the soil will naturally break down the pre-emergent barrier with time, so after a few weeks it's ok to put down grass seed for patch repairs or overseeding.

When should I apply a pre-emergent?

Crabgrass germinates when the soil temp reaches about 55 degrees, so the pre-emergent will need to be applied before then in order for it to be the most effective. You also don't want to be throwing it down on top of a foot of snow either. Sometime when the soil is above freezing but not quite 55 yet. Oh, you don't have a soil thermometer handy? Weird. In that case, in Northville the soil reaches an average of 55 right around the 15th of May. In Amsterdam it's about a week earlier. So if you're pre-emergent is down anytime in the month April, you should be good to go.

What do I use?

There are a lot of options for pre-emergents out there, and this is where it gets a little tricky. It comes in both a liquid and granules, and there are a handful of different active ingredients. Prodiamine and Dithiopyr are among the most common standard pre-emergents. They are relatively inexpensive and effective. There is also an organic option called corn gluten, made from a byproduct of the corn milling process, which works great, but needs to be applied pretty heavily so it can get expensive.

You can get a bag of pre-emergent at a big box store for general home use, but they're usually a bunch of herbicides and fertilizers mixed together, and it can be hard to tell exactly what you're putting down in your yard. Personally, I don't like using that pre-mixed stuff, it just makes me feel icky. They're like the gas station meat sticks of lawn care.

My real recommendation? Bring in a professional. A DEC certified applicator will be able to guide you through the process, will know exactly what to use and how much, in a safe and responsible way.

Did I mention that we're one of those safe and responsible applicators? One of the main reasons that we became certified chemical applicators is because we care so much about the environment. A lot of the damage done from pesticides and herbicides is from misuse. By partnering with the DEC we can ensure that whenever we're the ones doing the work, we are confident that it's having a minimal impact on the earth. Also, we just really love making beautiful lawns.

So like, call me maybe?




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