There are a lot of plant spacing charts out there to help you when you are designing a planting plan or even just trying to figure out how many plants to buy for an area in your garden. Many people waste a lot of time drawing circles and moving them around on paper or their computer screen. One alternative is to just measure the area you are planting, and figure it out mathematically. The plants are best laid out in the field anyway, you just need to know how many to specify or buy. I've seen a lot of plant spacing charts, all with different plants per area formulas. Which one do you use?
The best answer is, none. Don't use a plant spacing chart, unless you are farming and just want to maximize yields. If you are designing a garden or landscape, then plant spacing is a design decision. The spacing between plants will determine the ultimate aesthetic statement that your space makes. Will your trees form a cathedral-like canopy high above, or will they always leave an open piece of sky no matter how old they get? This can change the nature of a park, street, pool, or garden profoundly.
Will your cedars stand as individual vertical soldiers, or will they grow to join into a single hedge element? Will your ornamental grasses form a strong monolithic massing, or will they stand as randomly spaced spots, or evenly-spaced rows of single plants? So you see, planting design requires a knowledge of each plant you use and its size, form, and shape throughout its life cycle.
So how do you calculate how many plants to use for a given area? Well, plants are typically circular as seen from above, so we can calculate the area that a plant will take up:
Area per Plant = Pi x (Radius)^2
Radius = 1/2 of the diameter of the plant, or 1/2 the spacing from the center of one plant to the center of the next plant
If you want the plants to remain as single elements add a little extra to the mature diameter of the plant. For example, your plant has a mature diameter of 0.9 metres (3'), but we can plant with a spacing (or distance from center of one plant to center of the other) of 1.2m if we wanted wider spacing. If we really wanted a tight massing we could use 0.5m as the spacing distance.
Number of plants to use
= Area of Plant Bed / Area per Plant
= Area of Plant Bed / (Pi x (Radius)^2)
= Area of Plant Bed / (Pi x (0.5 x plant spacing)^2)
So, for example, if our Planting Bed is 5 square meters (about 50 sq ft)
and our plant spacing was 0.9m (3'),
the Number of Plants to Use is:
5 sq.m. / (Pi x (0.5 x 0.9m)^2)
= 5 / (3.14 x (0.45)^2)
= 5 / 0.64
= 8 plants
The same formula stated another way: