Amending soils for perennial beds

Quick facts

  • A well drained, sandy loam soil with plenty of organic material is ideal for perennials.
  • Most soils need amendments for growing perennial flowers.
  • Commonly used organic materials are compost, composted animal manure, peat moss and perlite.
  • Organic matter increases a sandy soil's ability to hold water and nutrients.
  • Organic matter improves a clay soil's structure, improves drainage and reduces stickiness.

Perennial flower beds can add beauty and value to any landscape. Unlike many shrubs, perennials are less adaptable to poor soil conditions. Fortunately, perennials have small root systems. It is possible to amend soil conditions for perennials.

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 edge of bed
  • Rake for smoothing soil, removing lumps and rocks

Site considerations

An ideal soil for growing perennials is loose and easily workable. A sandy-loam soil that is well drained, fertile and has a lot of organic material grows fabulous perennials. This type of soil is uncommon in gardens.

Most soils need amendments for perennials to thrive. Consider the site's drainage, pH, fertility, texture, structure and moisture content at the time you plan to work.

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Adding organic matter

Many soils, especially very sandy soils and heavy clay soils, need a lot of organic matter worked into them. This organic matter makes an ideal soil for perennials.

Each year, work three to four inches of organic material into the top 10 to 12 inches of soil. You can add up to six inches of organic material initially. Do this before planting perennials.

If plants are established, spread the organic material evenly between plants and carefully work in as much as possible. Material left on top works as well as mulch. Worms will help mix more material into the soil.

Apply a nitrogen fertilizer if you use organic materials that will continue to decompose, such as straw, shredded bark, wood chips or fresh sawdust. Microbes need nitrogen to decompose this material.

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Amending clay and sandy soils

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Step-by-step process

  1. In the spring, wait until your soil is dry or slightly moist.
  2. Mark out the area you wish to dig using stakes and string, spray paint or the garden hose.
  3. Slice off any existing sod by sliding a spade under the roots.
    1. If you have to clear a large area, a sod cutter will work well.
    2. If available, a Bobcat can scrape off the sod in a large area very quickly.
    3. Compost sod pieces or use to patch bare spots.
  4. If the area is infested with weeds and weedy grasses, first apply Roundup.
    1. Wait a couple days to remove excess top growth as you would remove sod, or till under with a garden tiller or small tractor.
  5. 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.
    1. Shovel 3 to 6 inches of organic material on top of the overturned soil.
  6. Dig the organic material and sand into the soil, mixing well, to a depth of 10 to 12 inches.
    1. This may also be tilled if prepping a large area.
    2. 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.
  7. Directions for double digging:
    1. Dig a trench along one side of your bed, 1 to 2 feet wide. Pile this soil in a wheelbarrow or alongside the bed where you do not intend to dig.
    2. 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.
    3. Dig a second trench alongside the first. Place this soil into the first trench.
    4. Continue as with the first trench until the whole bed is dug. The soil from the first trench will fill the last trench dug.
    5. 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.
  8. 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.
  9. Once settled, your bed is ready to be planted, mulched (compost works well) and edged.
    Black and white diagram showing the four steps of the double digging method.
    The double digging method

Jennifer Kaczmarski

Reviewed in 2018

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