Saturday, March 23, 2013

What does phenology have to do with crabgrass!?

By Alison O'Connor, Horticulture Agent, Larimer County

Since our CSU Rams just lost in the NCAA basketball tournament (and my Cyclones were just robbed by my other alma mater on a buzzer beater), I texted the Turf Man, Tony Koski, about when to apply my pre-emergence herbicide (yes, slow day here in northern Colorado).  Turns out that there's a really cool trick you can use to time your application: using the blooming time of certain landscape plants.  This, boys and girls, is what's known as phenology.  Don't you love science?

Phenology is officially defined as, "The scientific study of periodic biological phenomena, such as flowering, breeding and migration, in relation to climatic conditions" (courtesy of 

There's been several scientific studies, summed up nicely here in this Purdue article, about what plants can be reliable indicators of when to apply your crabgrass preventer.  One is the glorious golden yellow blooms of forsythia...another is the white blossoms of Callery pear...and another are those dainty pink (or white) redbud flowers.  When these plants are in bloom, or near bloom, it's time to apply your crabgrass preventer.  To me, this is SO much easier than calculating degree days or measuring soil temperature.  Or asking Tony every year...

I was just in Portland earlier this week visiting family, and their forsythia were in full bloom.  I hope my brother put down his crabgrass preventer.  But somehow, since he has a one-year-old, I'm guessing it was put on the back burner...

The wonderful Oregon Garden in Silverton.  Look at those magnificent forsythia!
It's important to remember that you have a window of time to get this done.  Tony recommends that you apply your crabgrass preventer by April 1 in Denver, April 15 in Fort Collins and ASAP if you're further south or on the west slope.  Just remember, these products need 0.5 to 1" of water after application to make 'em work.  

In the meantime, enjoy the snow and look for those blooms...a sure sign that spring is coming!


  1. Great info Alyson. I always know my time is short to get my pre-emergents down once I see the yellow forsythia buds. We have been able to extend your crabgrass control season by using dithiopyr (Dimension) as it has some post emergent control.

  2. Thanks, Mike! When in your position, trying to get all your customers pre-emerge completed, I'm glad there are products, like Dimension, that can help with post-emergent control. And they work best on young plants. Regardless, it's good to have a bit more leeway.

  3. what is the best Pre - Emergent product to apply to a lawn in the Ft Collins area to prevent future and kill already growing crabgrass