Building a Berm

When I realized I could get free fill dirt from a local company, I decided to make a berm to shelter a future sitting area from the road. The fill came from various local construction projects, mainly swimming pools, and was purported to be free of toxic contaminants and large non-organic structural debris. As the surrounding area is largely clay subsoil, the fill is mainly clay with some pockets of sand, gravel, and topsoil.

However, the berm also incorporates assorted rocks, cement pieces, and logs; all arrived in the fill and were covered as the bobcat packed the fill and shaped the berm. I like to think these "waste" objects serve a useful function, giving structure (and in some cases a deep reserve of nutrients) to the berm.

I didn't add any topsoil to the berm because I wanted to limit its fertility (to discourage germination of blown-in weeds), because I didn't want to worry about introducing weed seeds with the topsoil, and because I was eager to try the tallgrass prairie seed mix for exposed clay subsoil in the Prairie Moon Nursery catalog.

Establishing the Prairie

I sowed seeds in July of 2003. The seed mix included 23 species of forbs and six grass species. (See right column for a list of species.) I mixed it in the kitchen using a bucket and two bowls. The nursery had sent a big sack full of vermiculite, a lunch bag with seeds for a cover crop (Regreen (TM) mixed with wild oats) that would germinate quickly and hold the soil, a tiny envelope of legume seeds with their own packet of rhizobuim inoculant, two envelopes of forb seeds sorted by size, and a small sack of mixed grass seed.

In one of the bowls, I mixed the legume seeds with water, then added their inoculant.

In the bucket, I mixed together a quarter of each of the vermiculite, inoculated legumes, cover crop, grass seed, and larger forb seeds. Then I added water and stirred until the mix was damp but not dripping.

In the other bowl, I mixed together a bit of vermiculite and the smaller forb seeds, which are treated separately because they need light in order to germinate, and moistened that mixture as well.

I divided the berm into four sections and sowed one section at a time to help ensure that the seed was distributed fairly evenly over the entire berm. For each section, I raked the surface of the berm hard enough to create a thin layer of powder. I scattered the bucket's contents by hand over the raked area, then raked again to mix the seeds with the powdered dirt. I scattered a quarter of the bowl's contents (with the small forb seeds) over the top, then packed down the whole planted area by walking around on it.

I then returned to the kitchen to mix another bucket full of moistened seed and vermiculite, then back to the garden to prepare and sow the next quarter of the berm.

When the seeds were all sowed, I watered the whole berm with enough fine spray to wet the surface but not to create rivulets that would carry the seeds down the slopes.

The birds descended on the new berm and began to feast. I had forgotten to cover the seeds with a thin layer of straw or mulch, as the nursery suggested.

It rained regularly for the next several weeks, making it unnecessary for me to water the berm again.

By August, the wild oats in the cover crop were six inches high and scattered in a delightfully natural way, but there were a lot of bare spots. To those, I added some seeds of Hibiscus trionum 'Simply Love', a self-sowing annual for dry sunny places. This sowing was followed by several weeks of drought, during which I only watered the berm a few times, and then simply to aid germination. The oat grasses formed seeds and turned brown. Partridge pea and cosmos (both annuals) came up and bloomed despite the drought. Otherwise, there were few visible seedlings, though I didn't walk on the berm but merely looked from the edges, as I didn't want to crush any seedlings underfoot.

Later I added more seeds from various sources -- friends' gardens, parking lots, and so forth. I added rigid goldenrod, swamp and butterfly milkweed, hybrid asters and purple coneflowers, sweet black-eyed susan, and early sunflower.

In late September, I planted shrubs and trees on the north end of the berm. These will grow up to shelter the garden from the cold northwest winds.