Successful perennial gardens start with thorough and thoughtful bed preparation. Some of the key points include: eliminating perennial weeds before turning the soil; insuring a well drained soil yet having it retain enough moisture for good plant growth; providing for sufficient organic matter in the soil; and adding fertilizer as needed.
The first step in soil preparation is to get rid of perennial weeds before you turn the first spade of soil. When establishing new beds in grassed areas or in areas with perennial weeks such as thistle, bindweed and quackgrass, apply a non-selective, systemic herbicide such as Glyphosate (Round-up) to the area. Apply this material to weeds that are actively growing, generally when temperatures are consistently above 50 degrees. Spring applications are good with fall being another time when weed control is good with this material. Follow label directions for mixing and applying.
Outline the shape of the bed with a garden hose and spray within the outline. It will take 7-14 days before you will see the weeds being killed. After the vegetation is brown, you can till the area. For weeds that are particularly aggressive, the first spray of Round-up may not control all of the plant. It is suggested that after tilling to leave the bed remain unplanted for a few weeks to see if any of the perennial weeds regrow. If they do, a second application of Round-up will control the remaining weeds. It is a good idea to not be too much in a rush to plant without getting all the weeds under control otherwise you end up fighting those weeds while trying to grow perennials. If you do not want to use herbicides, you can cover the bed with several layers of wet newspaper and mulch or some other light blocking material such as black plastic. Then wait. It make take up to six months for weeds to be destroyed depending on weather conditions and types of weeds.
Well-drained soil is essential in order to grow perennials successfully but is most critical when it comes to overwintering perennials. More perennials are killed by soils that stay wet over the winter than by the actual cold temperatures. To ensure a well-drained site, avoid planting in low-lying areas. During bed preparation, add organic matter at a rate of about 25-30 percent by volume of soil. This translates to adding about 3-4 inches of organic matter on top of the bed and working it into about 10-12 inches of soil.
In areas that tend to have less than good drainage, raising the bed either with timbers, rocks, landscape bricks or similar materials will greatly improve drainage and your chances of growing and maintaining a perennial bed. Drainage can be checked by simply digging a hole 8-12 inches deep and filling it with water. Let it drain and fill it again. If this water drains in less than 1 hour, drainage should be satisfactory.
Organic matter is the key to improving less than great soils. There is no easy short cut and no magic soil preparation material that can take its place. Organic matter helps to improve the physical and biological properties of soils when added in sufficient amounts and to sufficient depths.
The bottom line is, don’t short cut this part of bed preparation. Organic matter improves the structure and aeration of clay soil and improves moisture and nutrient retention in sandy soil. There are a variety of organic matter materials that can be used depending on availability, preference and cost. Materials to consider would include compost, peat moss, composted barks, leaf compost, mushroom compost, and composted manure.
Generally, the fertilizer requirements for new beds consists of adding about two pounds of 5-10-5 fertilizer per 100 square feet of garden bed area. Till this in at the time of bed preparation.