In 2011, the City of Scottsbluff was awarded a grant through the Greener Nebraska Towns Initiative. The grant is funded by the Nebraska Environmental Trust and administered by the Nebraska Forest Service and Nebraska Statewide Arboretum. This grant has made several demonstration projects possible throughout the City, and one of these projects was recently completed. This project included an extensive tree planting demonstration and a series of three rain gardens designed to capture and infiltrate polluted stormwater runoff.
The area for the demonstration project is a long, narrow strip of land alongside a one-lane, one-way street. This area used to be underneath a two-way street, but when the street was reduced to one-lane as part of a quiet zone project, it left this long, oddly shaped area between the lane of traffic and the existing buildings and parking lot. The west half of the project is essentially the entryway to our downtown area, and this made an excellent spot for a tree planting project to frame this entryway and make it more attractive.
The east half of the project, which is lower in grade, is situated to receive the runoff from the adjacent buildings and parking lot. Over 10,000 sq. ft. of impervious area drain to this portion of the landscaping. This made it an excellent site for a series of rain gardens.
The landscape plan was designed by Amy Seiler, Community Forester with the Nebraska Forest Service. The tree portion of the project included 96 trees and shrubs, and the series of rain gardens included 492 perennials and grasses. The area around the rain gardens was seeded to a buffalo/blue gramma grass mix.
The City hired Anita’s Greenscaping to install the landscape. Because the existing soil was poor, the City provided compost to be worked into the soil in order to increase nutrients and organic matter.
The trees and shrubs were planted first. We wanted to plant the trees before it got too late in the year, since trees do best when planted in spring or fall. Some of the trees had to be staked due to the high winds that we have here in Western Nebraska. While staking can be necessary to keep trees from blowing over, the staked trees should still be free enough to rock back and forth in the wind. This will help the tree develop a good root system. The stakes should also be removed no more than one year after planting.
The City then installed a drip system to keep the trees irrigated. This is especially important for this first summer after the trees are planted, while they are still developing their root systems.
The Rain Gardens
The site was graded and rock inlets and outlets were installed to slow runoff and reduce the risk of erosion within the rain gardens. The next night, we got a small amount of rainfall that filled up the rain gardens. While we were happy to see that our inlets and outlets were functioning as we had hoped, we were disappointed when over the next couple of days it became apparent that the area was not going to drain the way we had hoped.
Because the soil in this area was extremely sandy, we determined that our problem was most likely compaction. Once the area dried out, the City Wastewater Department brought a bobcat to the site and dug through the compaction, which allowed the gardens to drain within about 16 hours, which was within our goal drainage time of 12-24 hours.
After the garden was dug out and draining again, it could be clearly seen which areas had been dug out. Everywhere where the water was able to get through the previously compacted top layer had sunk down a few inches. Once it was dry, we raked it out and it was ready for the plants!
The 492 plants we used included 187 grasses and 305 perennials. The grasses included Switch Grass, Little Bluestem, Indian Grass, Sideoats Grama, Big Bluestem, Prairie Dropseed, and Palm Sedge. The perennials included Liatris, Goldenrod, Penstemon ‘Rondo’, Penstemon ‘Pineleaf’, Jacob Kline Beebalm, Agastache ‘Sunset Hyssop’, Agastache ‘Blue Fortune’, Coneflower, Woods Aster, Butterfly Milkweed, False Indigo, Moonshine Yarrow, and Purple Poppy Mallow.
An organic mulch is essential for maintaining soil moisture, suppressing weeds, and improving soil nutrient content. We used a wood chip mulch around the trees and shrubs. When mulching around trees and shrubs, the mulch can be placed fairly deep (4-6 inches). When mulching around perennials, mulch should be placed more thinly. We chose not to use a wood mulch around the perennials, as wood mulch can sometimes be abrasive to the stems of perennials, and with the water flowing through the area, we didn’t want the wood mulch to float to one end and cover the plants there. Instead, we decided to try a mulch made of cocoa bean hulls. We minimized the amount of mulch we used in the bottom of the rain gardens, using just enough to surround each perennial. So far the cocoa bean hulls seem to be staying in place very well, but of course, the real test will be when we get our first major rain event.
The Completed Project
Below are some pictures of the completed project. As with any green infrastructure project, the first day after installation is the worst day for functionality. Be sure to check our website for updates as these plants become established and begin to do the work of filtering and cleaning our stormwater runoff!