42 Basic Perennial Garden PlantinG & Maintenance
the soil is moist, to avoid compaction. Walk around the outside of the beds or stay
on the designated access paths, if possible.
The soil is said to have sufficient organic matter when you can work it with
your hands. An abundance of earthworms is also a sign that the organic matter
content is good. Our amended beds generally test out to approximately 16–18
percent organic matter, improved from the original 2–3 percent of the native soil.
Remember, the PPA recommendation is a minimum of 5 percent organic matter.
Most authorities say 6–8 percent is good for increased perennial plant growth. I
have had great success with this higher amount of organic matter for both plant
establishment and growth in later years. I find that it is about 4 to 5 years before I
need to start adding more organic matter to the soil. But a word of caution: If you
get above 20 percent organic matter, you are really working with what is consid-
ered more of a container mix, and your watering practices need to be monitored.
You could be back to an overly moist soil because of the excess organic matter.
Some horticultural references suggest adding as much compost as you can get
your hands on. This is unfounded advice—too much of a good thing is still too
much. Too much organic matter can cause soil to become spongy. Also, excess
quantities of certain composts can lead to increased disease and insect problems,
and even death of the plants. From a business and economical view, the law of
diminishing returns comes into play as well.Calculating amounts of needed amendments
Adding the proper 4 in. of organic matter (30 percent by volume) requires a little
of what I call “perennial gardener’s math.” It’s no use to look at an area and guess
how much organic matter to use—even after 40 years in the industry, I wouldn’t
trust myself to do it. Proper calculation helps ensure accuracy and takes little
time. On landscape installations, where efficiency is profit, the proper amount of
material must be ordered from your supplier. In most circumstances, you are not
able to walk over to a compost pile and get more organic matter when you need it.
To accurately calculate the amount of organic matter required you will need to
know the square footage of your bed. Square footage is determined by multiply-
ing length by width. If you have an irregularly shaped bed, it is helpful to sketch it
out on some type of grid paper (as discussed in the first chapter) so you can add
up the grids to get your square footage. Following is an example of how I calcu-
late the soil amendment needed for a 100-sq.-ft. (10 ft. × 10 ft.) bed. In this
example I am incorporating 2 in. (or 0.166 ft.) of compost and 2 in. of sphagnum
peat moss to get my 4 in. of needed amendments.
2 in. (0.166 ft.) of compost per 100 sq. ft. of bed =
0.166 ft. × 100 sq. ft. = 16 cu. ft.
Then, to calculate the amount of compost needed in cubic-yard units, divide
by 27 cu. ft. (1 cu. yd.):
16 cu. ft. ÷ 27 cu. ft. = 0.6 cu. yd.
In this case I would go ahead and bump up my estimate and bring in 1 cu. yd.
of compost.
2 in. (0.166 ft.) of sphagnum peat moss per 100 sq. ft. of bed =
0.166 ft. × 100 sq. ft. = 16 cu. ft.
Then divide by 4 cu. ft. to determine the number of 4-cu.-ft. bales of peat
needed to cover the 16-cu.-ft. area:
16 cu. ft. ÷ 4 cu. ft. = four 4-cu.-ft. bales
The total cubic footage can also be divided by 6 cu. ft. because a 4-cu.-ft. bale
of peat moss is compressed and can actually cover 6 to 8 cu. ft. Depending on how
poor the soil is, I make my calculations based on 6-cu.-ft. coverage, so in this
example I would use 2 1/2 or 3 bales of peat.