(News article for July 29, 2023)
Last week, I wrote about some decisions that need to be made before planting and can affect weed management. This week’s article addresses ways to kill weeds before planting.
You can do this with an herbicide or tillage, or a combination of the two. With enough time to prepare, you may be able to kill weeds with an opaque tarp or another material that excludes light.
One of the most-effective ways to kill weeds before planting is to use a non-selective, systemic herbicide such as an appropriately labeled glyphosate-containing product. Non-selective means that the herbicide kills all types of weeds (i.e., not just grasses or broadleaf weeds). Systemic herbicides capable of moving from the shoots to the roots offer the benefit of killing roots as well as tops. Try to apply glyphosate early enough that you can wait at least three weeks before tilling. This gives it time to move into the roots and kill them.
There are also herbicides that are non-selective but non-systemic. Examples include ones that contain pelargonic acid (e.g., Beloukha Garden Herbicide, Scythe) or combinations of caprylic acid and capric acid (e.g., Bonide Captain Jack’s Deadweed Brew, Homeplate, FireWorxx, Suppress). Be aware, though, that these will not kill plant roots, and perennial weeds are likely to regrow.
Be sure to read and follow label directions when using any herbicide.
If you prefer not to use an herbicide, one option is to till the area. Tilling once will likely be sufficient to kill most annual weeds, but perennial weeds may regrow from roots or other structures.
There’s a related approach called the stale seedbed technique in which soil is tilled, weeds are allowed to grow, and while weeds are still small, soil is either tilled again or weeds are sprayed with an appropriate herbicide. If tillage is the method used to kill emerged weeds, it should only be a couple of inches deep. Tilling deeply will bring more weed seeds close to the surface and give them a chance to germinate.
Shallow tillage to kill emerged weeds and stimulate germination of more can be repeated over several weeks or months to further reduce the number of viable seeds in the soil. It’s preferable that the soil is not disturbed during or after the last killing of weeds.
Another way to kill weeds before planting is to keep light from getting to them for an extended period. People sometimes use cardboard or newspaper to do this. Heavier, reusable materials can also be employed. Unless the material is left in place for a very long time, the effect will likely be comparable to using a burndown (non-systemic) herbicide: The tops of weeds will be killed, but perennials may regrow from roots or other structures.
Regardless of what you do before planting, some viable weed seeds will remain. Ways of dealing with them will be addressed in next week’s article. (See Part 3: Non-Herbicide Practices and Part 4: Pre- and Post-Emergence Herbicides.)
Let me know if you have questions.
Contact Mary Helen Ferguson.
Reference to commercial or trade names is made for the reader’s convenience and with the understanding that no discrimination or endorsement of a particular product is intended by LSU or the LSU AgCenter. In some cases, other brands may be available.
Tillage, herbicides, blocking light from reaching the soil for a sufficiently long time, or a combination of these can be used to kill weeds before planting vegetables. (Photo by M.H. Ferguson)