Best time of year for pre-emergent
Before I knew anything about weed control or herbicides, I once found a flyer at my front door for a weed control service. The offer on the flyer said that someone would come an apply a weed control herbicide to my yard, and that it was then imperative that I hose down the yard afterwards.
My first thought was that it was a trick, “Sure, they’d LOVE it if I hose down my yard. Then a whole mass of weeds will grow and I’ll need to call them back to come kill the weeds!” I passed on the service and wondered how many of my neighbors would be suckers for it. I was uninformed!
Our humidity levels are very low here and evaporation happens fast. The weed control herbicides we use need to sterilize the area of the soil where the weed seeds hang out. Usually this is about the top 1/2 inch of soil. When we apply herbicide in the form of granules, they must be watered in in order to dissolve and perform any function at all. When we apply a liquid, it needs to get through the granite and penetrate that top layer of soil, before it evaporates. You are only allowed to apply a certain amount of chemical herbicide to an area in order to comply with the product label, and hence, the law.
We calibrate our equipment to apply just the correct amount water WITH the chemical so that the appropriate amount of chemical and enough water to penetrate the soil is applied. One other method of application is to skim the surface with the application and then water it in, either with the hose or hope for some rain.
Some applicators don’t follow the product label and don’t apply the correct amount of chemical or water necessarily. Some applicators don’t calibrate their equipment and don’t know how much of anything they’re putting down. Some rely on rain as a follow up to the application. This is why you see so many weed control operations going on in around February/March and August/September. These are likely times for rain, and are also times when our summer and winter annual weeds are likely to germinate.
So when is the best time of year to apply pre-emergent? Simply, spring and fall, but not necessarily.
Pre-emergents do need to be watered in within a certain amount of time after application. If your equipment is calibrated correctly, it’s a non-issue. If you want to time the rain, good luck with that, but we might get rain in spring and fall. If you want to prevent annual weeds and didn’t apply pre-emergent six months ago or so, you’ll want to do it before the annuals germinate.
Your best bet is to treat your landscape twice a year, use a product that is effective for the types of weeds you’re likely to encounter, and make sure the application is done correctly so it works.