Tech Tip: Forsythia and Crabgrass

Spring is upon us, finally, and our lawns are slowly beginning to wake up from their long winter’s nap.  It seems like twenty minutes has elapsed from the time the last snow bank melted to when you get the first phone call from a customer telling you that the forsythia is blooming and if you don’t put the crabgrass control down right now they are going to cancel their service!  Why do people get so torqued up about forsythias?  See below for a closer look!

Customer: The forsythia is blooming and if you don’t put the crabgrass control down right now I’ll be surrounded with crabgrass this summer!

You: Not exactly.  There are many things to consider here, so lets list them out.

First, the forsythia is a very common shrub in our landscapes that has a very showy bloom of bright yellow flowers first thing in the spring.

People associate the bloom of forsythia with timing of crabgrass control because there is a relationship that can be drawn between the two.

Associating the activity of a pest with observations of plants in the landscape is called plant phenology.  In horticulture, we can measure the progress of plants based upon how much warmth has accumulated as days go by in units called Growing Degree Days (GDD).

The forsythia bloom starts from 1 to 25 GGD.  Crabgrass germination begins at 200 GDD.  You can see that there is a big difference there. Crabgrass seeds will germinate around two to three weeks after the forsythia blooms drop.

Unfortunately, people have begun to associate the application of crabgrass control with the beginning of forsythia bloom when in actuality the crabgrass will not begin germinating for up to a month later.

Here’s the problem: the crabgrass controls that we apply have a half-life in the soil, meaning it will only work for a certain period of time.  If we apply it too early, we may not have adequate control later in the season when we really need it to be there.

I say “may” because there are many factors that come to bear on the quality of control we get from preemergent herbicides – the quality of the product, the formulation, application technique, amount of rain, temperature, thickness of the turfgrass stand, etc.

Remember to tell the customer that good crabgrass control is not a one-and-done event; we scout for crabgrass outbreaks in the lawn with each application and spot spray to control these plants as we find them.

So, here’s the bottom line: Patience & Persistence.  It’s a long season and it has just started.  We have chosen excellent products for our programs that will give our customers the best lawn possible. We understand the importance of good preemergent crabgrass control timing and that it is already first and foremost in our plan!

Find more technical tips in our Technical Resource Library in the NALP Member Center.

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