Ask the Expert: Pre-emergent Herbicides
Our MSU Horticulture Garden's Annual Trial Gardens manager answers questions about pre-emergent herbicides.
How does Preen work? Is it really safe for vegetable gardens?
The active ingredient in Preen is trifluralin, a pre- emergent herbicide. The pre-emergent part means that it prevents seeds from germinating. However, it will prevent both weed seeds and desirable seeds from sprouting, so make sure to apply it after any direct-sown veggies (like beans or lettuce) have sprouted and become established.
Preen also won’t kill seedlings that have already emerged. If your veggie garden is already weedy, it won’t kill the existing weeds but will prevent further weed seeds from germinating. The good news is that means you can use it on and around established desirable plants without harming them. Since this product only works on germinating seeds, it is safe to use on fruits and vegetables. Preen also make a product specifically or vegetable gardens with a different active ingredient, corn gluten meal. It works the same way, so it’s really up to you which product you choose.