Should I use a Pre-Emergent Herbicide?
As with all things – it depends! Start by asking yourself what your goals are for the area you’d like to treat, and keep in mind they may vary between areas in the landscape, like the lawn or flower bed.
Starting with the Goal in Mind
Pre-emergent herbicides can be an effective method for reducing the number of weeds (and hours pulling said weeds) out of your lawn and garden during the year, but before you rush out to purchase the first product you lay eyes on take some time to do your homework. Otherwise, you may wind up buying a product that won’t work for you.
First question: What type of weeds am I trying to control?
To get the most for your money, take a few minutes to identify the type of weed (or weeds) you’d like to control. (Need help identifying your weed? We recommend you head over to the University of Missouri’s handy weed ID website. Its free, easy to use, and even has a free app you can download on your phone!) Weeds have three distinct types of life cycles, and knowing what type you’re battling is a big help in choosing an effective product.
- Annuals – Annuals germinate, flower, reproduce, and die, all in the space of one short year.
- Biennials – Biennials live for two years, typically germinating and developing a healthy flush of leaves during the first, then blooming, reproducing, and dying during the second.
- Perennials – Perennials are the most difficult to control as they can live for 2+ years, and often spread both by seed and by underground ‘runners’.
Pre-emergent herbicides are most effective for controlling annuals. For biennials and perennials, a different product may work better.
Second Question: Am I dealing with a summer or winter annual?
Identifying your weed and choosing your product is half the battle – the other half is knowing when to apply it for maximum effectiveness. (Remember: these products don’t last in the soil forever, so if you apply your herbicide too far in advance, it may be gone by the time your weeds begin to grow!)
Summer annuals begin growing in late spring and reproduce prolifically throughout the warm summer months until cool temperatures or frost eventually kill them. Winter annuals in turn begin growing in the fall. The most effective way to control annuals of either type is to prevent them from producing seed. Pre-emergent herbicides take things a step further by preventing them from ever breaking the soil surface.
Here in Southwestern Illinois, we typically recommend that clients who use pre-emergent herbicide apply their product 3 times a year:
- In early spring as temperatures begin to warm, generally in late January or early February, to suppress the tail end of winter annuals growing season.
- In early summer, typically around mid to late May, to suppress summer annuals. In this region, heavy spring rains tend to wear down the efficacy of pre-emergent at a faster rate – we’ve found that a 3 month gap between these two applications works well without negative side effects to the landscape.
- In fall, about 5 months after the second application, to suppress the beginning of winter annuals. The timing for the third application can be a little more varied than the first two, depending on weather and the microhabitat of your specific property. If it rained a lot or if you watered treated areas regularly over the summer odds are the pre-emergent will wear away faster. Keep an observant eye on treated areas – if you start seeing an increase in the number of weeds popping up, it may be time to reapply!
Third Question: Am I planning to overseed my turf in the next few months?
Pre-emergent herbicides work by creating a chemical barrier along the surface of the soil which prevents germinating plants from breaking through, killing the seedling. It’s an extremely effective method to control weed populations – but many pre-emergent herbicides will also prevent grass seed germination. So if you’re looking to over seed in the next few months, double check how long your product will last (found on the label) to make sure the new grass has a chance to grow!
Still not sure if a pre-emergent herbicide or particular product is right for you?
Anytime you get stuck with a garden or landscape question, especially when it involves using an herbicide or other chemical, make a point of reaching out to your closest university extension office or agent. Odds are, you aren’t the first one to ask!