Planting a Wildflower Meadow? Site Preparation Comes First!

by Cathy Neal

Successfully establishing a meadow from seed is a three-year process, with the first year devoted to good site preparation. This may be a hard sell to your clients (or yourself), but time spent eliminating competitive vegetation before you plant is essential to long-term success. Seeding in the fall after a full season of site preparation is my preferred strategy when starting a new wildflower meadow. The best preparation method depends on the site.

Summer-long site preparation methods were compared in this 2017 trial at UNH. Temperatures underneath black plastic reached a maximum of 92◦F, compared to 108◦F under clear plastic, at a 2-inch depth.

If starting with rough turf or lawn areas, it is essential to completely kill existing grasses and other perennial weeds before planting wildflowers. Keep in mind that our lawn and pasture grasses are predominantly non-native species that are extremely competitive because they spread from strong underground roots or rhizomes. If left to compete with a germinating wildflower mix, the grasses always win. As an alternative to using non-selective herbicides, they can be effectively killed by a process called “smothering” over the course of the summer. Steps to take:

  1. Mow the area as short as possible once or twice after it greens up in the spring. Scalp it! Then, rake off any excessive organic matter to create a uniform surface. Leaving a light layer of clippings is okay. Do not till the soil.
  2. Lay sheets of thick (4- or 6-mil) black plastic or other opaque material over the entire area, overlapping the edges by about a foot if you use more than one sheet or roll of plastic. Bury all the outside edges with soil or hold the plastic down with rocks, cinder blocks, sandbags or other available materials. The objective is to exclude light from the grasses and weeds trying to grow underneath the plastic. Plants are unable to photosynthesize without sunlight and will eventually run out of energy and die. Any seeds that germinate under the plastic are likewise unable to survive for long.
  3. Leave the soil covered from mid-June until mid-September. When you remove the plastic or other covering, you will have bare soil on which to plant. Avoid disturbing this clean seed bed; do not till the prepared area or you may stimulate weed growth from seeds. Do not apply compost, manure or other nitrogen-rich material, because wildflowers do best in soil that is low in nutrients. If needed, rake lightly to remove dead grasses and surface debris just before spreading the wildflower seed.

Smothering is an easy, inexpensive, no-till method for preparing small to medium size areas.  The plastic can be cleaned and reused. Alternatively, in turf areas you could remove the top layer of sod with a sod cutter and then apply one of the site preparation methods described for cultivated soil, below.

If starting with a cultivated field or garden, perennial grasses and weeds are apt to be less of a problem; however, there is always a weed seed bank lying dormant in the soil. Options for reducing the weed seed bank include repeated tillage, use of herbicides, a cover crop, or solarization. Any of these strategies should be implemented for the entire growing season prior to planting the meadow mix.

The test plot shown during site prep in 2017.

Unlike smothering with black plastic, which works by light exclusion, effective solarization depends on trapping solar radiation as heat, raising the soil temperature high enough for long enough to kill weed seeds. Solarization is most reliable in hot, arid climates and there are conflicting reports on the effectiveness of soil solarization in northern climates. Results may vary from year to year but may yield some success here in the northeast if the site is fully exposed to sun. In a cloudy, cool season it is apt to be unsatisfactory. Use 4- or 6-mil UV resistant plastic, such as growers use to cover high tunnels or greenhouses, and bury the edges all the way around to trap and build up heat. This method works best if soil is tilled before applying the plastic and the soil should be moist when covered. Seal up any tears and holes which occur as soon as possible with heavy-duty clear repair tape. You will often see water condense on the lower surface of the plastic during the day, and you may even see heat-loving weeds such as purslane survive and grow underneath the plastic. Once the plastic is removed, summer annual weeds will be killed by frost and there is no need to till before seeding the wildflower mix.

The test plot in 2018, the meadow’s first year.

Broadcast your seed mix in the fall over the surface of the prepared site. Mulch lightly with clean straw to help keep the seed and soil in place over the winter, ready to germinate in the spring. Other than black-eyed Susan, most forbs will stay vegetative and low to the ground the first growing season.

Some persistent weed species will doubtless still thrive and a vegetation management plan should be in place. Mowing high during mid-summer, right over the top of the wildflower seedlings, cuts back the tall weeds and allows light to reach the seedlings underneath. Once past the site preparation year and the first growing season, the established meadow will require very little additional maintenance. You and the bees will reap the rewards of your labor for many years to come.

The effects of site prep treatments are clearly evident in the balance between forbs and weeds, both in 2018 and 2019.

For a complete description of site preparation alternatives and wildflower establishment strategies, see the UNH Extension fact sheet Planting for Pollinators: Establishing a Wildflower Meadow from Seed. It is available here, along with additional resources on wildflower meadows.

Site preparation, seed selection, and related research will be discussed and demonstrated at a Univ. of New Hampshire field day on July 30, 2019: Connecting the Dots for Pollinator Conservation: Wildflower Meadows and Pollinator Habitat.  This event is co-sponsored by ELA, the Xerces Society, NH Ag. Experiment Station, and Northeast Climate Hub. Due to high demand, online registration closed on Friday, July 12. Contact cathy.neal@unh.edu or call 603-862-3200 to see if space is available.

About the Author

Cathy Neal came to UNH Cooperative Extension from Univ. of Florida in late 1999. She works with commercial horticulture clientele in the state to maintain professionalism in the green industry and to encourage sustainable practices and designs. She is well-known for her educational leadership in landscaping for water quality and in planting for pollinators.

Complimenting her Extension work, Cathy has a split appointment as a Researcher with the NH Agricultural Experiment Station (NHAES). Her research program focuses on methods for establishment of wildflower meadows for provision of ecosystem services including pollinator and wildlife habitat, biodiversity, water and soil conservation, human connection to nature and beauty, and more. Other research areas include work with nursery production systems and root cold tolerance of woody shrubs. Cathy plans to retire at the end of September.

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