One common question we get asked a lot, especially around here in Connecticut, is when is the best time to apply pre-emergent. Even within one town, there are a lot of varying "Microclimates". A microclimate is a climate of very small or restricted area, especially when this differs from the surrounding area, according to Google.
This is very true especially of the shoreline when we're talking about Madison, Guilford and surrounding towns, even up to Westbrook, Old Saybrook and Old Lyme.. Yesterday we did a spring cleanup in Madison for a client who's daffodils were about to bloom, while another client in Old Saybrook had some in full bloom.. Even within my own yard here in Killingworth, I have a microclimate. I have daffodils planted in my beds that are about halfway up. In my front, out by my stone wall where there is less sun, I'm eagerly looking for them every day, but they have not yet done much more than barely poke through the surface of the soil.
Every property is different, so how can I possibly answer that question about pre-emergent application?
Simple. Pre-Emergent is based completely on soil temperatures. Crabgrass begins to germinate when the average daily soil temperature reaches 57°-64° F at a one inch depth. Large amounts of crabgrass won't start to germinate until the soil temperature is 73° F at 1" depth. However, you don't want to wait for large amounts to germinate. You want to get in there and get it BEFORE large amount germinate and it becomes a problem
Note, we are talking about SOIL temperature, NOT air temperature. Air temperature fluctuates a lot right now. Yesterday it was in the 50's, today, only in the 30's (I know, will spring EVER get here?!). Air temperature as well as sunlight all affects soil temperature. For example, on a nice sunny day, if you walk out on to your driveway, it feels a whole lot nicer than the air does. How many times have we all heard or said "it's beautiful out, as long as you're out of the wind". Soil temperature doesn't fluctuate nearly as much. Above is a map of the current soil temperature in our region as of March 28, as well as surrounding regions.
So again, how do you know when the right time is? You can't look on a soil temperature map every day.
You could go out and get a soil thermometer and walk around all different areas of your yard, be sure you're inserting it at a 1" depth, not deeper or more shallow, and continuously do that until you consistently reach the ideal temperature every day.
Ok, so maybe the question of when to apply pre-emergent is not that simple, or is it? If all this sounds like rocket science, and you want to know if there is an easier alternative that we can change from full-blown NASA rocket to more along the lines of a model rocket you can pick up in a toy store, the answer is yes..
You could cheat. YES! You read that correctly, you can CHEAT!!
How do you cheat? Here's nature's cliff notes courtesy of your favorite landscaping blog: M&M Garden Designs.
The answer? Forsythia. Huh, you say?? What does forsythia have to do with crabgrass? Again, another simple answer, that this time actually is simple.
Forsythia blooms when the soil temperature is consistently 55° F. Oh, wait. If we go back to the top of this blog post, what temperature did I say crabgrass begins to germinate at? Hmmm, 57° F. Well, isn't this convenient! Forsythia bloom just before crabgrass starts to germinate! Takes the guesswork right out of it! Well, kinda makes you want to run right out and get a forsythia doesn't it?!
I'm sure the nurseries would be thrilled to get a rush of forsythia purchases, and they are pretty shrubs if maintained properly, but you don't really need to go out and purchase one. Another way you can cheat...just look at your neighbors or nearby homes. When their forsythia is in bloom, it's time to apply your pre-emergent.
Want yet one more easy way to cheat?? Just remember what the speed limit was forever back before they raise it to 65. Just like the speed limit on the highway today is 65, if the temperature hits 65°, you better hit the highway and go out and get your pre-emergent before the crabgrass gets to speeding along!