54 Basic Perennial Garden PlantinG & Maintenance
Findings suggest by extension that the majority of plants would benefit from a
1- to 2-in. topdressing of compost about once every 3 years, when the nitrogen
availability from the compost begins to decline to about 10 percent. The recommen-
dation often found in reference books to top-dress a garden annually with 3 to 4 in.
of compost may be more than is necessary in many cases. Keep in mind, though,
that slightly higher amounts of compost might be needed in the South than in the
North because the humus in the soil tends to break down more quickly in the heat.
Topdressing with compost appears to be the best method for providing
sufficient nutrients for perennials, and I rarely use fertilizer in my own gardens.
Most of my perennial beds go 5 years with no additional nutrients and show no
visible signs of the need for it. After 3 years, 1 bed fertilized with a general-
purpose granular fertilizer showed no noticeable difference from the beds that
were not fertilized. After 5 years, when some plants started to show decline (and
this was partly because of the need for division), I tested the soil and found that
the organic matter content had fallen to 6 percent (from approximately 16
percent after bed preparation). I top-dressed with 2 in. of compost (a blend of
composted bio-solids and leaf compost), and this had a noticeable effect on the
plants, providing more robust growth and better flowering.
Occasionally—for example, in situations where gardeners have large
expanses of naturalized gardens or informal beds of mostly native plants—it’s
possible to get an annual addition of organic matter from the plants themselves.
Roy Diblik advocates mowing perennial beds in early spring and leaving the
plant remains in place to break down over time. This is often not practical in
more conventional perennial gardens, but where feasible, it can be an option.
For a garden initially prepared with less than 20 percent compost, which also
started with an organic matter content of less than 5 percent, fertilizers can be
helpful in providing some immediate nutrients while the organics are building
up in the soil and are slowly becoming available to the plants. In such a garden,
in temperate soils, 3 consecutive yearly 1-in. applications of compost may be
added to build up the organic matter in the soil. Take care to scratch or incorpo-
rate the compost lightly into the soil. The soil should be tested before the
4th-year application. Once the soil reaches an organic content of approximately 8
percent, compost application can be spaced to about once every 3 years.
My personal experience is that perennials growing in a rich organic soil have
no great need for additional fertilizer. But just for insurance in my clients’
gardens, I use a light application of a general-purpose, quick-release fertilizer in
the spring following the first year. My belief here is that if any additional nutri-
ents are needed it will be in the spring, when the plants are going into rapid
growth and the organic nutrients in the soil may not be available due to cool
weather conditions. Sprinkle the fertilizer around the base of the plants, avoiding
the new growth to prevent burning. Watering in the fertilizer or applying it when
the soil is moist also reduces the chance of burn. Moist soil is generally not a
problem in central Ohio at the end of March or early April, which is usually when
I am applying fertilizer. I top-dress with 2 in. of compost in the 4th or 5th year after
the initial planting, along with any divisions and renovations that may be needed.
Fertilizer needs and rates of application ideally should be based on soil tests,
but tests are not always practical on an annual basis. I base my calculations of
fertilizer needs on the general recommendation for perennials, which is 1 lb. of
nitrogen per 1000 sq. ft. The quantity of fertilizer should be based on this rate
rather than on the recommendations on the fertilizer bag, because the latter
rates can be too high for most perennials.
The granular fertilizer I use has an analysis of 12–12–12 or 5–10–5. So to
calculate, for example, the amount of 5–10–5 fertilizer needed per 100 sq. ft. of
area, based on 1 lb. of nitrogen per 1000 sq. ft., start with the fact that 1/10 of the