Well folks, spring is here and you’re going to start hearing the phrase “crabgrass pre-emergent” or “crabgrass preventer” when it comes to your lawn. You may be asking yourself, “what is a pre-emergent and how does it work?”  Green Side Up is going to break it down for you to better understand.

What is a pre-emergent

A pre-emergent is an herbicide that is applied to the lawn prior to crabgrass seed germination and forms a physical barrier at, or right below, the soil surface.  It can be applied in both granular and liquid forms.  This barrier doesn’t stop the seeds from germinating, as commonly believed.  However, once the seeds do germinate, the chemical prevents the cells from dividing and the seed dies. In this way, the seeds are destroyed.  There are potentially millions of crabgrass seeds in your lawn.  They just need to get to the surface, get some warm weather and sunlight and they can take off.  A homeowner must keep in mind that pre-emergents will have the same effect on most lawn grass seeds as well.  Do not over-seed directly before or within a few months after herbicide application or your seed may be ruined.



Understanding crabgrass

Let’s talk a little more about our target so we can really understand what we’re dealing with.  Crabgrass favors sandy, compacted soil where the grass is in a weakened, thinned condition.  Bare, thin lawns allow sunlight to directly hit the soil, which is an ideal condition for crabgrass germination.  It has a prolific tillering or branching habit and a single plant is capable of producing 150 to 700 tillers and 150,000 seeds.  Crabgrass plants are very adaptive to mowing height. Plants can produce seeds at mowing heights as low as 1/2-inch.  Un-germinated seeds can wait in the soil for up to ten years before sprouting. What a nuisance!


When does crabgrass germinate

So, when does crabgrass germinate?  The old rule-of-thumb is of course, when the forsythia bloom.  That’s not a bad way to gauge it, but the biggest factor is soil temperature.  You can count on crabgrass germinating when soil temps reach 55 degrees or higher for 4 days in succession.  There’s also air temperature, which we would need to discuss the intricacies of ‘degree days’ if we want any quantifiable data. Trust me, neither of us want that.  Another thing to keep in mind is that crabgrass germinates over a long period of time, not just when the forsythia blooms.  That’s why pre-emergents are designed to last a 4 to 5 months regardless of when they are applied.  However, factors such as heavy rain, high summer heat, and foot/pet traffic will degrade the preemergent and lower its life expectancy.  So, under normal circumstances a pre-emergent applied in early March will likely begin to fail sometime in July.  That’s not good enough with a good chunk of summer remaining.

Rolling with the knowledge that crabgrass germinates over a long period of time and that our pre-emergent has a finite viability, the issue of too soon or too late arises. With a large window of when the pre-emergent can be put down for the best results and that decision is tough.  That is why we split our pre-emergents into 2 applications, it extends our coverage as long as possible.  This allows us start applying in early March even though it might not be the “best” time to do so.  It also ensures no early germinating crabgrass if the weather is unseasonably warm, and the issue of “too late” is avoided.  Our follow up application is about 6-8 weeks later for protection over the entire summer.   We have used that large window to our advantage and with the hopes of ideal conditions we will not see a single shred of that unsightly weed.


Other key factors to help prevent crabgrass

Outside of Green Side Up’s pre-emergent game plan there are other keys to crabgrass prevention to stack the playing field in favor of your turf grass.  In other words, give it ideal growing conditions, and your grass will choke out weeds.  Providing ideal growing conditions for your turf grass is very important and most are simple cultural changes.  First, adjust your mower’s height to allow your grass to grow to its optimal level.  The higher your turf grass is above ground, the longer the roots are underground; this means less room for anything else to take root.  Aeration and overseeding can also decrease the presence of crabgrass, because it adds grass seed and favorable turf-grass conditions to your lawn.  Lastly, ensure your lawn’s soil has the proper amounts of nutrients via a simple soil test.  Soil testing will help you determine what compounds and nutrients your lawn is lacking so you can fortify your lawn with a fertilizer or other soil amendments, which will correct any inequities.