Table of Contents
Amending Soils for Perennial Beds
Perennial flower beds can add beauty and value to any landscape. Unlike many shrubs, however, perennials are less adaptable to poor soil conditions. Fortunately, perennials have small root systems, as compared to large shrubs or trees, and it is possible to amend the soil in beds that have less than perfect soil conditions for perennials.
Amounts, Specifications, and Supplies:
For clay soils:
- 3 to 6 inches of organic material
- Perlite if desired
- Coarse sand as an option
For sandy soils:
- 3 to 6 inches of organic material
- Perlite if desired
For both soils:
- Fertilizer – the type and amount of fertilizer needed can be determined by a soil test. Fertilizer comes in various forms (pellets, powder, liquid) and many nutrient ratios. Nursery and garden store employees can answer questions and make a recommendation.
- Lime or sulfur additions – only needed if a soil test indicates an excessively low or high pH, respectively.
Tools and Equipment:
- Soil test kit
- Small trailer for hauling amendment materials (compost, peat, sand, etc.) or have them delivered, depending on how much you need
- Non-selective herbicide, such as Roundup, if bed area is infested with weeds and weedy grasses
- Sharp spade, sod cutter, or Bobcat for removing existing sod
- Spade and pitchfork, rototiller, or small tractor for digging in amendment materials
- Wheelbarrow or garden cart if using the hand-tilled method called double digging
- Stakes and string, flour, spray paint, or something similar for marking the edge of the bed
- Rake for smoothing soil, removing lumps and rocks
A soil that is ideal for growing perennials is loose and easily workable. A sandy-loam soil that is well-drained, fertile, and contains an abundance of organic material would grow fabulous perennials; however, many of us do not have this type of soil in our garden. Most soils need to be amended somewhat to provide an environment in which perennials will thrive. To turn the soil in your planned site into the envy of the neighborhood, you must consider the site’s drainage, pH, fertility, texture, structure, and moisture content at the time you plan to work (2).
The most important consideration before adding any amendments to clay soil is internal drainage. If the subsoil does not drain well, drain tiles may need to be installed (8,10). Surface drainage is also very important. The site to be amended should be graded to a 1% to 3% slope so surface water will drain. The area should also be leveled so low spots do not collect water (2). Another essential step in amending clay soils is thoroughly (uniformly) mixing the amendments with the existing soil to a depth of about 12 inches. If amendments are not thoroughly mixed with the clay, layering in the soil could occur and interfere with water movement in the soil (8,10). If re-grading your bed does not solve the problem, and subsurface drainage cannot be done, raised bed gardening is an option.
Fertility and pH
A soil’s pH and fertility should be determined by a soil test. Most ornamental shrubs, flowers, turf, and vegetables grow best in slightly acidic soils. At a pH level of 6.5, most nutrients are readily available for plants to use (3). Levels of pH can be adjusted if a soil test indicates your soil is too acidic or alkaline to grow attractive, healthy perennials. In addition to determining your soil’s pH, a soil test will also indicate your soil’s fertilization needs. This is very important for sustainable landscape gardening because it eliminates spending money on unnecessary fertilizer. This reduction of unneeded fertilizer also reduces contamination of the environment.
Texture and Structure
Soil texture refers to the size and distribution of particles in a soil. Sand is comprised of larger particles (grains) while clay is comprised of small (microscopic) particles. Soil structure refers to how the particles are organized. Unorganized particles create poor soil structure, as in a heavy clay soil which has poor drainage and poor air porosity. When organic material is added to clay soil, the clay particles clump together in much larger particles. This allows water to drain more thoroughly and increases air porosity (4).
All soils benefit from the addition of organic matter. “Problem soils” require the addition of organic matter every year. Coarse, sandy soils dry out very quickly because of their rapid drainage. This rapid drainage also causes leaching of important nutrients, which makes it difficult to maintain fertility. Organic matter helps improve sandy soil’s ability to hold needed water and nutrients. Clay soils often remain wet well into the growing season, which is problematic because clay can readily compact if worked when wet. This compaction, which can also be caused by heavy rains, makes it difficult for air and water to reach plant roots. Clay soils also become sticky or slippery when wet and then form hard clumps when dry. The addition of organic matter improves a clay soil’s structure, improves drainage, and reduces stickiness. The addition of organic material also reduces the amount of compaction that occurs when clay soils are worked.
The more commonly used organic materials are compost, composted animal manure, peat moss, and perlite. They can be used alone or in combination. Occasionally, coarse sand (0.25 to 1.0 mm in diameter) is added with compost to heavy clay soil (8). This is more commonly done during soil preparation for heavy traffic turf areas rather than perennial beds (10).
The type of organic material used depends on the type of soil, what is to be accomplished, availability of materials, cost, and personal choice. Compost is probably the most commonly used because it is readily available and affordable (sometimes free of charge). Most communities have public compost sites that are open seven days a week. Check your local newspaper or call your city for locations and hours. The big disadvantage of using compost from a public site is the lack of control of what goes into it. It is only as clean as the material donated to the compost site and may contain many weed seeds and perhaps trash. You may need to try several sites. Personal compost bins are easy to construct and maintain if you’d like an alternative to the public compost site.
readily available in bulk in urban areas but is easy to find in rural areas. It can also be bought in bags from nursery or garden stores. The most notable disadvantage of using animal manure is that it might not be thoroughly composted. In this case, it may be odorous and could possibly burn plants. This is not a problem with packaged, composted manures.
Sphagnum peat moss is a wonderful soil amendment but can be expensive if you need large quantities. Peat moss is a naturally occurring substance that must be harvested from peat bogs. It can be purchased in bags at nursery or garden stores. To reduce costs, you can add peat to compost to make a rich mix of organic material.
Coarse sand can be purchased from nurseries or garden stores in smaller quantities, or from sand and gravel companies or landscaping firms in bulk. Call several companies to compare prices and delivery charges as they can vary.
Perlite is used to loosen soil, improving its air porosity and drainage. It is more commonly used for small beds and perennial or annual gardens in pots or planters. It can be purchased in packages from garden stores. Check the package for recommendations on the amount to use.
Many soils, especially very sandy soils, and heavy clay soils need a lot of organic matter worked into them to produce a soil best suited for perennials. Each year 3 to 4 inches of organic material should be worked into the top 10 to 12 inches of soil (up to 6 inches of organic material can be added initially). This can easily be done before perennials are planted. If plants are established, spread the organic material evenly between plants and carefully work in as much as possible. Material that is left on top works well as mulch. Worms will also help to mix additional material into the soil.
If you use organic materials that will continue to decompose, such as straw, shredded bark, wood chips, or fresh sawdust, you should also apply nitrogen fertilizer. Microbes need nitrogen to decompose this material, which will cause a temporary nitrogen deficiency in your soil if additional nitrogen is not added at the same time as the organic material (2).
Moisture Condition at the Time You Plan to Work
This is a greater concern with clay soils than with sandy soils. You should not work clay soils when they are wet because their soil structure can be easily damaged, and they will readily compact. To test your soil’s moisture condition, dig down six inches, grab a handful of soil, and squeeze it. If the soil can be crumbled after pressure is released, it can be worked. Otherwise, wait for several days without rainfall to begin digging (2).
- In the spring, wait until your soil is dry or slightly moist. Mark out the area you wish to dig using stakes and string, spray paint, or the garden hose.
- Slice off any existing sod by sliding a spade under the roots. If you have to clear a large area, a sod cutter will work well. Also, if available, a Bobcat can scrape off the sod in a large area very quickly. Compost sod pieces or use to patch bare spots. If the area is infested with weeds and weedy grasses, first apply Roundup. Wait a couple of days to remove excess top growth as you would remove sod, or till under with a garden tiller or small tractor.
- If sod or top growth was scraped off, turn over the soil in the entire area to a depth of 10 to 12 inches. If prepping a large garden area, use a garden tiller or small tractor.
- Shovel 3 to 6 inches of organic material on top of the overturned soil.
- Dig the organic material and sand into the soil, mixing well, to a depth of 10 to 12 inches. This may also be tilled in if prepping a large area. If your soil is very heavy, needs improved drainage, or you plan to plant deep-rooted plants, the double digging method can be quite effective. Directions for this hand method of digging follow. Also see diagram, based on information in “Conran’s Basic Book of Home Gardening” (2).
- Dig a trench along one side of your bed, 1 to 2 feet wide. Pile this soil alongside the bed where you don’t intend to dig or in a wheelbarrow. After the soil is removed from the trench, use a fork to break up the bottom to a depth of the fork. Add some compost to the trench and work it in.
- Dig a second trench alongside the first. Place this soil into the first trench. Continue as with the first trench until the whole bed is dug. The soil from the first trench will fill the last trench dug.
- Spread chosen soil amendment materials on top of the entire bed and dig those into the top 10 to 12 inches of soil. Any additional soil amendments, such as fertilizer, should be worked into the soil at this time also.
- If you dig your perennial bed in the spring, water it several times to settle the soil before planting. The soil will settle on its own over the winter if it was dug in the fall.
- Once settled, your bed is ready to be planted, mulched (compost works well), and edged.
- Anoka County Extension Service. http://www3.extension.umn.edu/county/anoka/
- Stefren Buczacki, Conran’s Basic Book of Home Gardening, A Complete Guide for the First-Time Gardener, Viking Penguin, Inc., N.Y.
- Ohio State University Extension Service, Online Fact Sheet, “Improving Soils for Vegetable Gardening,” HYG-1602-92, Marianna Riofrio and E.C. Wittmeyer. http://www.ag.ohio-state.edu/~ohioline/lines/hygs.html
- Ohio State University Extension Service, Online Fact Sheet, “Soil Testing is an Excellent Investment for Garden Plants and Commercial Crops,” HYG-1132-97, Gary Gao, Joe Boggs, and Jim Chatfield. http://www.ag.ohio-state.edu/~ohioline/lines/hygs.html
- Ohio State University Extension Service, Online Fact Sheet, “Physical Characteristics of Growing Mixes,” HYG-1251-97, Claudio C. Pasian. http://www.ag.ohio-state.edu~ohioline/lines/hygs.html
- The Reader’s Digest Association, Reader’s Digest Illustrated Guide to Gardening, 1994.
- Ellen Phillips and C. Colston Burrell, Rodale’s Illustrated Encyclopedia of Perennials, Rodale Press, Ammaus, PA, 1993.
- Rosen, Carl J., Professor, Department of Soil, Water, and Climate, University of Minnesota, personal interview.
- University of Minnesota Extension Service, Online Information, “Compost, Using It,” #278, David Whiting, 1995. http://www.extension.umn.edu/info-u/plants/BG278.html
- White, Donald B., Professor, Department of Horticultural Science, University of Minnesota, personal interview.