Weed control is the biggest problem facing wildflower establishment for direct-seeded projects and one which has no easy solution. Weed seeds are present in many situations and lie dormant, but viable, for long periods. A weedy area converted to wildflowers will have a large reservoir of weed seeds in the soil, ready to germinate when conditions are favorable. In most cases, it is advisable to consider weed control in two phases – as part of site preparation prior to planting, and as an important component of a post-germination maintenance program.
Weed Control Methods
Before planting, existing weeds and unwanted vegetation can be removed in a number of different ways.
- Using the no-till method
- Applying a non-selective, non-residual herbicide such as a glyphosate product
- A combination of tilling and an herbicide.
- For additional weed control after site preparation, a soil fumigant that kills weed seeds may be used
Smothering of existing weeds and vegetation is an option for small-scale planting projects and can be done with black plastic that is UV stabilized. This method “cooks” the vegetation and weed seeds in the topsoil. The edges of the plastic should be covered with dirt to prevent airflow underneath the plastic. This is a good option if you wish to avoid the use of herbicides. However, smothering usually takes a full growing season to successfully kill perennials that can regrow from the roots.
Repeated tilling is another good option if you wish to avoid the use of herbicides. The initial tilling removes the existing vegetation, and then repeated tilling every three weeks for a full growing season will help to deplete the weed seeds in the soil by killing newly sprouting weeds. In dry areas, providing supplemental water will encourage weed germination and regrowth so that repeated tilling can be effective.
An effective, no-till method to prepare a seed bed is to apply a glyphosate herbicide and allow the vegetation to die back (usually about 10-14 days). Use a scalper to remove the dead thatch and scratch up the soil surface. The top surface of the soil is lightly roughened by the scalper and makes a good seed bed. Once seeds are sown, go over the area with a roller or cultipacker to cover the seeds.
Methods for Extremely Weedy Areas
- Till soil or spray vegetation with glyphosate herbicide. When using an herbicide, allow vegetation to die, then rake out the dead debris. If aggressive, perennial weeds such as bindweed are present, using an herbicide is more effective than tilling.
- Irrigate to encourage germination of weed seeds near the surface; most seeds will germinate within two weeks if consistent moisture is available. Do not till the soil again because this will bring even more weed seeds up to the surface.
- Spray any new growth with glyphosate herbicide.
- After raking out dead vegetation, allow soil to recover for 3-4 weeks before planting seed.
Once the seeds have germinated, further weed control is usually necessary. If practical, pull all weeds as soon as they can be identified. Other successful techniques are spot-spraying with a general herbicide or selectively cutting weeds with a string trimmer. Be sure to remove weeds before they reseed.
Many unwanted annual and some perennial grasses can be controlled with the herbicides Grass-B-Gon®*, Ornamec®* and Fusilade®*. These post-emergents do not affect broad-leaved plants so they can be applied over existing flowers; they are most effective when sprayed on new growth and young plants. Take care to avoid treating areas with desirable native grasses or fescues.
*Observe all precautions and follow manufacturer’s recommendations for application.