32 Basic Perennial Garden PlantinG & Maintenance
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.Soil pH
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 glypho-
sate 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” ingredi-
ents. 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).