I ended up either buying too many plants or too few last year and would like some advice on how to more accurately calculate the materials I need for my garden. It seems like I am always miscalculating what I need.
— Justin Barz, Skokie
There’s no need to guess the amount of soil, compost, mulch, or number of plants needed for your garden. The garden buying season is about to begin in earnest, and with a few calculations in mind, it’ll be easy for you to purchase the right amounts.
First, calculate the square feet of the garden bed or beds that you are working with. I generally top-dress perennial beds with about an inch of mulch and shrub beds with 2 inches. Use the following calculation to cover a bed that is 13.5 feet long and 5 feet wide with 2 inches of mulch — all units of measure need to be the same, so convert any measurements that are in inches to feet by dividing by 12 (inches in a foot). Multiply 13.5 feet in bed length by 5 feet in bed width to get 67.5 square feet of bed space. Divide 2 inches by 12 to convert the depth of mulch or other materials to feet instead of inches (0.17 feet of mulch). Next, multiply the depth of the mulch, 0.17 feet, by 67.5 square feet of bed space to determine the volume of mulch that you need (11.5 cubic feet). Round this figure up to 12 cubic feet. Mulch is sold in bags containing 2 or 3 cubic feet, so purchase either six 2-cubic-foot bags of mulch or four 3-cubic-foot bags of mulch. You could also buy one less bag to save money and still have plenty of mulch for the bed. Consider buying materials in bulk for larger areas to save money. Bulk mulch, compost, and topsoil are sold by the cubic yard, which equals 27 cubic feet. Divide the cubic feet of mulch you need by 27 to determine the number of cubic yards of mulch to order.
The mathematical formula for calculating the volume of a column will work to estimate how much growing medium to buy for your containers. The result will be close but not exact; most containers have angled sides, so they’re not a perfect column. Multiply the radius times the radius of the container times 3.14 (pi) times the height of the container. A container that has an 18-inch diameter and is 24 inches high needs approximately 3.5 cubic feet of growing medium (0.75 feet (9-inch radius), multiplied by 0.75 feet, multiplied by 3.14 (pi), multiplied by 2 feet (height)).
If you want to fill this 67.5-square-foot bed with groundcovers or annuals, use the following calculation. The number of plants will depend on the spacing used, which will vary depending on the type of plant and how quickly you want the bed to fill in. To plant the bed with pachysandra planted on 8-inch centers, first convert the inches of spacing to feet by dividing by 12 inches per foot, which equals .67-foot centers.
Calculate the square feet each plant will cover by multiplying 0.67 feet by 0.67 feet, which equals 0.45 square feet per plant. Divide the square feet of the bed by the square feet each plant will fill to calculate the number of plants to purchase — so, divide the 67.5 square feet of the bed by 0.45 square feet per plant for a total of 150 pachysandra plants. Pachysandra is commonly sold in flats of 24 plants, so you would need six or seven flats of plants to fill the bed.
Sod is generally sold in a 10-square foot roll that is 2 feet wide and 5 feet long, so you will need to purchase seven rolls for a total of 70 square feet of sod to fill the bed. It’s a good idea to increase your sod order somewhat over your square foot calculation since there will be some unusable pieces after cutting the sod rolls to fit an area. Beds that have a lot of curves or irregular sections will result in more unusable pieces of sod than you’ll get in a bed that is square or rectangular. If the 67.5-square-foot area to sod has an irregular shape, I’d go ahead and purchase eight rolls of sod.
For more plant advice, contact the Plant Information Service at the Chicago Botanic Garden at plantinfo@chicagobotanic.org. Tim Johnson is senior director of horticulture at the Chicago Botanic Garden.