planting after weed killer

Asked September 1, 2016, 3:12 PM EDT

I have some large areas that are all weeds and I want to plant either grass or wildflower seeds this fall. What I am thinking is to use weed killer that prevents any new growth for 3 months and then in the fall plant the seeds and cover with seed mats. My understanding is that the weed killer now would not effect germination of the seeds or growth of the seedlings in the spring. Can you confirm this or offer other ideas as to how to take care of these areas? Thank you and have a great day!

Wright County Minnesota

2 Responses

Thank you for the question. Fall is a good time to apply herbicides to perennial weeds because they are storing food for the winter. The herbicides are more efficiently absorbed by the plant and therefore are more effective. I don't know what product you are thinking of using but the best thing to do is read the entire label about how to use it and when it is safe to plant afterwards. If I understand your question correctly, you want to lay down seed/seed mats before snow flies and you are thinking that they won't germinate until next spring, long after the 3 month time period has expired on your pre-emergent herbicide. I don't know how winter temperatures, frozen soil, and freeze thaw cycles effect the breakdown and residual effects of your product.
You may want to call the customer service line for the chosen product and ask them specifically about the long term action of their product and how it affects the specific seeds you want to plant.
It's a lot of work to remove seeds, prepare a seed bed, and plant. If there is any chance of residual herbicide in the soil, you will have to replant in the spring. To be on the safe side, please call the company if the label doesn't provide the information you need.

An alternative idea is to kill all the weeds now with a non selective herbicide like Round Up. It works well and is not a pre-emergent herbicide. Therefore, it won't prevent your seeds from coming up next year. Remember, it is non-selective and will kill any plant it comes in contact with. Turfgrass and wildflower seed can do quite well planted just before winter sets in. This is called dormant seeding. Read more about it here: http://www3.extension.umn.edu/county/stearns/county-horticulture-educator/article/dormant-seeding-your-lawn

Thank you for contacting Extension.

Thank you for your help and have a great day!