3. Integrating Herbicides in Prairie Restorations That Require Seeding

Invasive plant management is a vital component of protecting and enhancing restored prairies, Conservation Reserve Program (CRP) lands, and remnant grasslands. Removing problem species, reintroducing fire, and possibly adding desirable seed or seedlings are management tools used to supplement existing species diversity. Increasing plant community diversity usually improves wildlife habitat value, water quality, and increasing plant community stability.

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Restoration from Cropland

Includes former cropland or other sites where prairie seeding practices are starting with bare soil.

Seeding into former crop areas is usually straightforward, and most sites can utilize one of several approaches for successful prairie restoration. Often the former cropping practice has provided all the site preparation necessary.

Restoration of Degraded Sites

Includes non-cropland, CRP, or grassland with high densities of invasive plants such as Canada thistle, and insufficient desirable grasses or forbs to recover and repopulate the site.

Sites dominated by undesirable  pre-existing vegetation should be evaluated to determine appropriate restoration activities. Often the best option is to start over with site preparation, which may include integrating herbicides, tillage, prescribed mowing and/or prescribed fire. Sites with pre-existing stands of competitive or over-dominant invasive plants such as Canada thistle, or vigorous stands of smooth brome may require multiple tillage events and/or treatment with herbicide to deplete carbohydrate reserves in rhizomes and minimize regrowth potential.

General Guidelines for Prairie Restoration Projects

Evaluate the site to determine appropriate activities for site preparation, which may include integrating herbicides, tillage, prescribed mowing and/or prescribed fire. Site preparation is the most important step in the prairie restoration process.

  • Minimize risk of soil erosion during tillage operations or consider drilling directly into non-tilled land if some desirable species remain.
  • Allow for good seed-to-soil contact at the time of seeding (e.g. thatch removal).
  • Sites with pre-existing stands of competitive invasive plants such as Canada thistle, or vigorous stands of smooth brome may require herbicide treatments and multiple tillage events.
  • Eliminate or decrease vigor and density of invasive, nonnative plants to minimize post-seeding maintenance issues.
  • Fire can be an important tool to stimulate grass vigor and improve competition with weeds. Fire can also be used to remove litter, improving seed-to-soil contact and enhances herbicide coverage on target plants.
  • On sites that are prone to Canada thistle invasion, consider planting forbs that are more tolerant to broadleaf selective herbicides such as Milestone® or Transline®.
  • Clipping and mowing is not recommended for reconstructed prairie that is more than one year post-seeding. 
  • Monitor invasive plant populations such as Canada thistle, and spot treat dense infestations with an appropriate selective herbicide. Selection of sites for spot treatment would depend on target plant species and density, and desirable forb presence.
  • On restored prairie sites with desirable forbs, do not broadcast treat invasive plant populations using broadleaf herbicides until two years following seeding.

 

Herbicide Use in Restoration

Regardless of the condition or composition of a restoration site, enriching the plant community by seeding desirable species can be accomplished by following a few basic steps/concepts:


FALL PRIOR TO SEEDING DEGRADED SITES
Apply appropriate herbicide at the correct application timing and rate to improve seeding success. For example, apply Transline® or Milestone® the growing season prior to seeding to control high-density infestations of Canada thistle (Figure 3.1). Field studies show that Milestone can be applied in the fall to control Canada thistle, followed by seeding forbs and grasses as a dormant fall planting or the following spring (Lym et al. 2011, Renz et al. 2012).


YEAR OF SEEDING CONVERTED CROPLAND
If weeds are present at the time of seeding,
treat the area with glyphosate either before seeding or 5 to 8 days post-seeding.
Monitor weed growth as native desirable seedlings begin to grow, and mow during the growing season at a height slightly above the height of the observed prairie seedlings. This helps ensure desirable prairie seedlings will have access to adequate resources for growth, especially light, water, and nutrients.


YEAR AFTER SEEDING
Maintain the site to enhance growing conditions.
Conduct vegetation maintenance practices, which may include mowing the growing season following supplemental seeding. This step is especially important in the case of supplemental forb seeding into competitive stands of native, warm season grasses. Dense infestations of broadleaf weeds can be spot treated with Transline the growing season following seeding.


TWO YEARS AFTER SEEDING
Treat invasive herbaceous weeds, trees and shrubs
using spot herbicide treatments wherever practicable during the second year post-seeding. Mowing will not control invasive plants and may impact desirable forbs (Duncan 2011). Early detection and prompt treatment of problem weeds is important.

Use this table to develop a seed mix tolerant to Milestone that meets the goal of structural diversity (height) and varied flowering times.
* Susceptible to fall herbicide treatment. See Section 4 for symptoms and injury levels associated with these rankings.


Sites that require revegetation after invasive plant control include: 1) a plant community dominated by invasive weeds with no desirable vegetation present to establish after herbicide treatment; and 2) sites with remnant desirable plant populations that are insufficient to recover after herbicide treatments.

Field studies conducted in the Midwest and West concluded that applications of either Milestone® at 7 fluid ounces per acre (fl oz/ac) or Transline® at 24 fl oz/ac could be made in the spring through fall to control broadleaf weeds prior to grass planting. Grasses can be seeded as a dormant planting (in the late fall or early winter) in the year of application, or grasses can be seeded the following spring. With a dormant fall seeding, grasses should be planted when soil temperatures are low enough to ensure that the seeds do not germinate and emerge for at least 60 days after application of either Milestone or Transline.


Summaries of seeding studies by Lym et al. 2011 and Douglass et al. 2011 are available online at
www.techlinenews.com. (see References, page 24).