A few basic steps should be followed to ensure successful bed preparation. Let’s look at some key elements: (1) testing the soil; (2) eliminating perennial weeds; (3) making sure the soil drains well, yet is able to hold water and nutrients; (4) providing sufficient organic matter in the soil.
KEY STEPS TO BED PREPARATION
Testing the soil
Testing the soil is a step all too often ignored. Having worked with a variety of different sites for clients, I have become a firm believer in soil testing. It is the only way to effectively determine what soil conditions you are starting with. Experts say that in the United States alone there are approximately 15,000 different soil types. Soil should be tested to determine soil type (clay, sand, silt, loam, or a combination of these), pH (level of acidity or alkalinity), organic matter content, and available phosphate and potash. Phosphate is a form of phosphorus that assists in strong root growth in plants; it is 1 of the 3 basic nutrients in fertilizers. Potash is a form of potassium that improves flowering and fruiting.
You can test the soil yourself using soil testing kits, or send it to soil testing labs or your county cooperative extension agency. These services can run various tests on your existing soil as well as on your chosen soil amendment, and they will make recommendations. Most soil tests are performed for field crops or vegetable garden plots; be sure to specify that you’ll be growing herbaceous perennial plants or ornamentals, so that you’ll receive the proper recommendations. It is also helpful to perform soil tests after the beds have been modified so that you can see the results.
The Perennial Plant Association (PPA) provides the following standards for perennial garden soils (these figures are the minimum for an amended soil): pH of 5.5–6.5, organic matter content of 5 percent (by weight), 50 pounds per acre (25 parts per million or 0.5 pounds per 100 sq. ft.) available phosphate, and 120 pounds per acre (60 parts per million or 1.2 pounds per 100 sq. ft.) available potash. I generally work with soils that are high in clay and low in organic matter (1–3 percent), which I find analogous to working with concrete. Most of us are working with problematic, disturbed urban soils, hence the importance of amending the soils properly.
I find that perennials tend to be rather flexible when it comes to soil pH. In central Ohio the pH is neutral to alkaline, about pH 7–7.8. (A pH of 7 is neutral; anything higher is alkaline and anything lower indicates acidity.) I never attempt to lower the pH to accommodate acid-loving perennials or to raise it for alkaline lovers. I have good success with both types as long as the other requirements are met—most critically, sufficient moisture in summer and good drainage in winter. Only if you are doing everything else right and are still having trouble growing acid- or alkaline-loving plants should you consider altering the soil pH to succeed with any particular plant. Lime can be used to increase soil pH and sulfur to decrease pH; the type and quantity needed to increase or decrease soil pH will be determined by your soil type. Refer to your soil test for pH recommendations.
A few perennials that prefer slightly acidic soil include Iris ensata, Kirengeshoma palmata, and Asclepias tuberosa; Asclepias tuberosa can have problems if the pH is above 6.5. Species of Dianthus and Lavandula, Gypsophila paniculata, and most silver-foliaged plants are examples of perennials that prefer alkaline soil.
Eliminating perennial weeds
Get rid of perennial weeds before you plant a garden full of desirable perennials—it will certainly make your life more enjoyable. Eradicating weeds is the one instance in which I will resort to the use of chemicals. When working with grassed areas that are to be turned into gardens or existing gardens full of weeds and undesirable plants, I apply glyphosate (Roundup; another option might be Finale) to new planting areas. Glyphosate is a nonselective, nonresidual herbicide that is systemic in its action (meaning it must come in contact with the shoots of the plant and then travels to its roots), so it is best applied when the plants are actively growing and when temperatures are above 50°F. I usually apply glyphosate in early April in Ohio for spring installations. For those with reservations about using Roundup, keep in mind that there are formulas available with less harmful surfactants, as well as products composed of strictly glyphosate, which many would consider safer to use than other products with added “inert” ingredients.
I outline the shape of the new bed using a garden hose or a heavy-duty electric extension cord (which is lighter weight and more flexible than hose), and then spray within the outlined area to get the correct shape. After waiting about 14 days, to be certain all perennial weeds are killed, I go into the area and rototill directly through the dead vegetation, if it is not too heavy. (Seven days is the typical manufacturer’s recommendation if the weeds are annuals or grasses.) Sometimes a good number of aggressive weeds are not destroyed by the first spraying and so it is necessary to come back for additional sprayings to ensure they have been killed. This step is not one you want to rush or else you will be fighting with those weeds for the rest of your life (or at least the life of the garden).
If you do not want to use chemicals, you can cover the bed area with several layers of moistened newspaper and mulch or other light-blocking material such as black plastic. (Some materials used as mulch include wood chips, bark, and pine needles.) Then wait, perhaps up to 6 months depending on the conditions, for the weeds to be destroyed.
More perennials are killed by wet overwintering conditions than by actual cold winter temperatures. This is why well-draining soil is essential for perennials. Part of the research for my master’s degree focused on the cold hardiness of herbaceous perennials. (“Herbaceous” refers to a nonwoody plant that dies back to the ground every year.) Most of the species I studied were able to tolerate low temperatures when everything else was constant, but these same species did not survive in the field studies when exposed to excess moisture or fluctuating soil temperatures. Perennials can simply rot during the winter if the soil is not properly drained; this is often mistakenly attributed to cold temperatures, and they are labeled “not hardy.” Yes, they are not hardy—to excessive moisture.
Perhaps you are one of the chosen few who have that perfect soil for perennials—a fertile loam that is well draining but also retains adequate moisture. If so, I am envious of you and wish you happy gardening! But most of us are not so blessed. My own soil and most of the soil I work with for clients is very poorly drained. Soil texture (the relative proportions of sand, silt, clay, and loam) can be an indication of what kind of soil drainage you have. Sandy soils are sometimes too well drained, requiring constant watering. On the other hand, if puddles tend to stick around for more than half a day following a rain, or if your soil is constantly soggy, you can be sure that you have a drainage problem (and probably lots of clay). Most of you probably know your soil type and are not happy with it. (If you’re not sure of your soil type, you can run a percolation test, as described below.) In any event, let’s discuss how to improve the drainage of your clay soil or to increase the moisture retention of your sandy soil.
To ensure well-draining soil, avoid low-lying areas. Add organic matter to the beds at the rate of approximately a third by volume, or 4 in. per 12 in. of soil—this will also improve moisture and nutrient retention in sandy soils. (I will discuss organic matter in greater depth later in the chapter, as it is so vital for all soils.) Creating a slightly raised bed will increase the gravitational pull of water down through the bed.
A Simple Percolation Test to check for proper drainage:
1. Dig a 12-in.-diameter hole the depth of the amended area.
2. Fill with water and let drain.
3. Fill with water again.
4. Wait 1 hour. If the water hasn’t drained in less than 1 hour, drainage needs to be improved either with further soil preparation or with tiles.
Drainage tiles may be needed in some cases to improve drainage, but this should be considered only after you have considered all other factors. I have seen isolated instances where the compaction of the subsoil during construction of the home was so extensive that no matter what bed preparation was done, drainage tiles were still necessary to improve drainage. Tiles can be expensive and will often clog if not properly installed, so they are best used only as a last resort.
Tracy DiSabato-Aust has earned international acclaim as one of America’s most entertaining and knowledgeable garden writers and professional speakers. She has extensive experience in the United States and abroad with more than 35 years in the industry and is a gifted and award-winning designer who combines artistic vision with practical horticultural strategies.
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