As you know, in the late winter and early spring I talk a lot about pre-emergent applications and we watch the soil temps rising until it’s time to throw down.

Pre-Emergent Herbicides are those that prevent weeds from growing in the lawn. They are our best defense against invaders that can severely slow down our progress. The key is to get your pre-emergent down and watered in BEFORE weed seeds germinate.

But how does it actually work? Let’s look into the science.

Prodiamine is the pre-emergent herbicide we use. Prodiamine is an active ingredient. It used to have a brand name called “Barricade” so you may hear that sometimes too. When you hear me say “it’s time to apply prodiamine” I’m referring to that active ingredient. It’s the one that I use most often because it’s accessible, affordable and it works very well. It’s literally the gold standard of pre-emergent active ingredients.


Prodiamine is a herbicide. The word “herbicide” comes from the latin roots: herba (vegetation) and -cide (killer). Herbicides kill plants, in this case, our targets with prodiamine herbicide are crabgrass, goosegrass, signalgrass, foxtail and several others.

Herbicides are classified into “modes of action.” The “mode-of-action” is the means in which a herbicide kills the target plant. Each mode interrupts, disrupts or mitigates the plant in a different way.

For prodiamine, its mode of action is group 3. Two other familiar active ingredients in this group are dithiopyr and pendimethalin. All of these attack/kill plants with the same mode of action.

Active ingredients in this group are known as “microtubule assembly inhibitors.” This mode of action works by interrupting cell division in very young seedlings.

The process is called “mitosis.”

Prodiamine, when applied and watered into the soil, interrupts mitosis in young crabgrass plants, goosegrass, signalgrass and many others. If plant cells can’t divide (mitosis is interrupted), the plant will not grow and instead dies. Prodiamine interrupts this process by cutting off the plant’s ability to produce tubulin. Tubulin is used to create microtubules that are essential to dividing cells (daughter cells). Without tubulin there are no microtubules and therefore no cell division can occur. Mitosis is thwarted.

This is why it’s so important to get prodiamine down and watered in before weed seeds germinate. Once a plant is more mature, it’s too late.

The good news for you and me is we know that nature has programmed into crabgrass, goosegrass, and signalgrass seeds a trigger window for germination. That window is in the spring and it’s when soil temperatures are between 55 and 70F. That is the time when all of this cell division will be taking place. So what do we do?

We apply prodiamine once in early spring or late winter as soil temps are crossing 50F and heading to 55 and then we apply a second application (like a second coat of paint) to cover us up from around 60-65F heading to 75F. These two applications will give us the protection we need against these germinating weeds.

One thing to keep in mind is that not all of the crabgrass and signalgrass seeds will germinate right as the window opens at 55F. Some will come later and others may come a bit earlier - nature is funny that way. Some seeds are also buried at different depths that others which mixes things up. The reason I point this out is to let you know that if you happen to be late getting started with your pre-emergent prodiamine, that’s ok because apply 2 weeks too late is better than apply 4 weeks too late. Just like applying 4 weeks too late is better than applying 5 weeks too late. Make sense? Weeds are germinating all throughout this window so if you are in it, it’s time to throw down!

Want to know when you should be applying prodiamine to your lawn this spring? Get my free app here and it will give you the soil temperature in your neighborhood, right down to the street level. It’s super accurate and gives you a very good indication of when your window is opening and closing. Download the app here.

Science is pretty cool when you look at it from a cellular level. I hope this little dive into the science has provided you with a better understanding of how the strategy works so that you can be a better Lawn Care Nut this season!

I’ll see you in the lawn!