FORMULA #1: Calculating Amount of Needed Amendments

Adding 4 inches of organic matter per 12 inches depth of soil is ideal. Work it well into the bed. The soil should be dry, but not too dry, when you work it. Avoid working wet soil. Four inches of organic matter works out to be 2 inches of Canadian sphagnum peat moss and 2 inches of a compost blend. It’s useful to check your soil pH to know what you’re starting with.

You will need to know the square footage of your bed to accurately calculate the amount of organic matter needed. In an example of a 100-sq-ft (10 ft x 10 ft) bed, 2 inches (or 0.166 ft) of compost and 2 in. of sphagnum peat moss is required to get to 4 inches of amendments.

Step one: 2 in. (0.166 ft) of compost per 100 sq. ft of bed = 0.166 ft x 100 sq. ft = 16 cubic feet

To calculate how much compost is needed in cubic-yard units, divide by 27 cubic feet (1 cu. yd): Thus, 16 cubic feet divided by  27 cubic  feet = 0.6 cubic yards.  Round off and use one cubic yard of compost here.

Step two: 2 in. (0.166 ft) of sphagnum peat moss per 100 sq. feet of bed = 0.166 x 100 sq. ft. = 16 cu ft  Now divide by 4 cu. ft. to determine the number of 4-cubic-ft bales of peat needed to cover the 16-cubic-foot area: Thus, -16 cu. ft. divided by 4 cu. ft. = four 4-cubic-foot bales of peat moss

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 cubic feet. Based on the 6-cu-ft coverage, you would use 2 1/2 or 3 bales of peat.

Formula 2: Spacing plants

Most herbaceous plants, perennials and annuals, are best planted in odd-numbered groups or drifts, although a large plant may stand alone as a feature. Guidelines: small plants (under one foot in height) or plants at the front of the border are spaced 8-12 inches apart. Medium-sized plants (1-2/1/2 feet in height) are best spaced 15-24 inches apart with 15-18 inches best for the majority. The large tropicals like grasses and cannas are spaced 2-3 feet apart.

How many plants do you need?

Know the square footage of the area you’re planting (easy when you use graph paper). Multiply this square footage by the number of plants per square foot for the desired spacing. This will tell you how many plants you need.

Equation:  square footage of planting area x number of plants per sq. ft. = number of plants needed  for drift.

Spacing              Plants per Sq Ft.

  4 in.                         9

  6 in.                          4

  8 in.                          2.25

12 in.                          1.0

15 in.                          0.64

18 in.                          0.45

24 in.                          0.25

36. in.                         0.11

Grid paper: Use 8-1.2 x 11 paper with a scale 1/4 inch = 1 ft (one square on the paper is equal to 1 sq ft of bed space). Overlay with tracing paper or vellum to do actual drawing.

In my plan, the grid paper plan showed the actual layout of plants on the grid blocks and I could multply length by width where I wanted to plant a particular plant and arrive at square feet for each of those separate planting spaces. For instance, most perennials are planted about 15 inches apart. One area in front was five feet long by 2 feet wide or 10 square feet. Read down the ‘spacing’ column to ‘15 inches’, then  read across and see 0.64. Multiply by square feet (10). Thus 10 sf  x 0.64 is 6.4 plants to fill that space. If you know the plant is narrow and upright you would know to buy 6 plants, but if you know this plant will fill out and sprawl a bit, you would decide 5 plants would be right.

Structural plants: These are the trees, shrubs, roses, large grasses, etc. that are the ‘bones’ of the garden and should be planned for first. In a mixed bed, try to buy a 5-6-foot tree if it’s a small tree so it doesn’t require much pruning when young and  isn’t overcrowded by fast-growing perennials which could cause it not to thrive.