Photo by Thinkstock

How many flowers or plants do you need for your garden bed or space?

Avoid the nursery and find out that you bought too many flowers or plants—or, discover that your number fell short of the look you want in the garden space. We consulted the New York Botanical Garden in the Bronx. This is his formula for estimating the number of plants you need. Obviously, it depends on the type of plant but this gives you a good guidelines for garden design as a starting place.

Steps for Calculating the Number of Plants

1. Determine the number of square feet in the area to be planted:

• Rectangle: Multiply length by width
• Circle: Multiply the radius by itself, and that result by pi (3.14)
• Oval: Multiply the average radius by itself, and that result by pi (3.14)
• Triangle: Multiply ½ the height by the base

2. Determine the number of square inches in the area to be planted by multiplying the number of square feet by 144 (which is the number of square inches in one square foot).

3. Determine the number of square inches a mature plant will cover by multiplying the number of inches of suggested spacing between plants by itself.

4. Divide the number of square inches in the plot by the number of square inches required for one plant. This result is the total number of plants needed for that plot.

Example A:

Step 1. The area to be planted is a 6x8-foot rectangle. 6 feet x 8 feet = 48 square feet.

Step 2. 48 square feet x 144 square inches = 6,912 square inches.

Step 3. The suggested spacing for wax begonia (Begonia semperflorens-cultorum) is 8 to 10 inches.

• At 8-inch centers, 8 inches x 8 inches = 64 square inches for each plant.
• At 9-inch centers, 9 inches x 9 inches = 81 square inches for each plant.
• At 10-inch centers, 10 inches x 10 inches = 100 square inches for each plant.

Note: The middle of the suggested range (9 inches) is usually recommended, but for quicker coverage, the low range (8 inches) can be used. The higher range (10 inches) is usually not recommended. If all plants do not grow, coverage will not be complete.

Step 4.

• At 8-inch centers, 6,912 square inches ÷ 64 square inches = 108 plants.
• At 9-inch centers, 6,912 square inches ÷ 81 square inches = 85.3 or 86 plants.
• At 10-inch centers, 6,912 square inches ÷ 100 square inches = 69.1 or 70 plants.

Example B:

Step 1. The area to be planted is a 4x4-foot square. 4 feet x 4 feet = 16 square feet.

Step 2. 16 square feet x 144 square inches = 2,304 square inches.

Step 3. The suggested spacing for French marigold (Tagetes patula) is 12 to 15 inches.

• At 12-inch centers, 12 inches x 12 inches = 144 square inches for each plant.
• At 13-inch centers, 13 inches x 13 inches = 169 square inches for each plant.
• At 14-inch centers, 14 inches x 14 inches = 196 square inches for each plant.
• At 15-inch centers, 15 inches x 15 inches = 225 square inches for each plant.

Step 4.

• At 12-inch centers, 2,304 square inches ÷ 144 square inches = 16 plants.
• At 13-inch centers, 2,304 square inches ÷ 169 square inches = 13.6 or 14 plants.
• At 14-inch centers, 2,304 square inches ÷ 196 square inches = 11.8 or 12 plants.At 15-inch centers, 2,304 square inches ÷ 225 square inches = 10.2 or 11 plants.

Note: If your calculations come out to a fraction of a plant, such as 40.4 or 80.7, always use the next whole number. It doesn’t sound like a big difference, but that one extra plant, not noticeable when first planted, may make the difference as to whether or not your design fills out.

If you are growing a vegetable garden, our online Garden Planner will automatically calculate the spacing by vegetable plant for you!  Technology is indeed amazing.  Explore the Almanac Garden Planner.

Source: 

2001 Early Spring Garden Guide. This article was originally published in 2011 and has been updated.