Mulch with Weed Prevention

Each year holds new surprises for me as the Forest Health Program Leader for the IDNR.  Last year we battled with Imprelis lawn chemicals killing conifers.  The year before we had Arborvitae dying without a common theme, and we still do not know what caused it.  We constantly monitor for oak wilt, bur oak blight, gypsy moth, Asianlonghorned beetles, emerald ash borer, and Thousand Cankers disease of black walnut (to name a few).

This year brings about a new question.  Many garden stores are offering mulch with weed preventer added.  We all know the benefits of mulch:  conserving moisture, adding organic matter back to the soil, and preventing unwanted plants from competing for the available water and nutrients.  As any faithful mulcher knows, the weeds that do grow in the mulch are very easy to pull out compared to a non mulched area.

However, I am a bit concerned about adding an herbicide to the mulch to keep the weeds from growing in it.  As I read the labels on these new mulches, I see that the targeted weeds are largely broadleaves, not grass, and they are controlled through both premerger herbicide and time released herbicide.

Let’s keep in mind that the tree you are mulching around is a broadleaf.  I cannot help but wonder that if I used this mulch around one of my trees, would I not be adding a new stressor and removing some of the benefits of mulch?  After all, that tree is once again a broadleaf.  Will the chemicals volatize?  If so, that airborne chemical could have some impacts on sensitive trees such as redbud.

These products are so new that we do not have a good feel of what level of damage, if any, would be caused.   I would suspect that younger trees would be impacted more than older trees, but I still do not see the benefit of adding the herbicide.  I will keep a watchful eye on this to see if issues emerge.  Right now, we are planning to mulch around young nursery stock with two different mulch formulations and one without weed preventer to see if there is a difference.  If you have already seen some issues, please post them.  I would love to know about them.  Have a great summer. 

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  • Weed preventer in mulch is news to me. Doesn't mulch prevent weeds? At least that's what I thought. Maybe I'm wrong!  Anyway, I'll have to see if it is in the Sioux Falls area where my office is located. Thanks for the post Tivon. I also sent your post to my Community Forestry Team and Team Leader.

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