“I have trouble buying the right number of plants and amount of mulch for my garden. What is the best way to determine how many plants and how much mulch and compost I need?”
—Justin Sorel, Grayslake
Many gardeners are out purchasing plants, compost and mulch for their gardens at this time of year. Different types of plants will require different spacing, so there is not one standard formula to use.
In general, spring annuals such as pansies will grow less, due to the short season, so if you want a full bed, you will need to space them closer together. Groundcovers like pachysandra that spread can be spaced closer or further apart, depending on your budget and how soon you want the bed to fill in.
To determine the amount of soil, compost, mulch or plants needed for your garden, first calculate the square feet of the bed you are working with. Use the following calculation to determine the square feet of a bed that is 10 feet, 6 inches long and 4 feet wide. 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 10.5 feet in bed length by 4 feet in bed width, which equals 42 square feet of bed space.
If you want to fill this 42-square-foot bed with ground covers or annuals, then use the following calculation. The number of plants will depend on the spacing used, which, in turn, will vary, depending on the type of plant used and how quickly you want the bed to fill in.
To fill the bed with pachysandra planted on 8-inch centers, first convert the spacing distance from inches to feet by dividing by 12 inches per foot, which equals 0.67 foot centers (8 divided by 12).
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 42 square feet of the bed by the 0.45 square feet each plant will fill to calculate the number of plants to purchase, for a total of 93 pachysandra plants.
Pachysandra is commonly sold in flats of 24, so you would need four flats to fill the bed on 8-inch planting centers. Installing the pachysandra on 6-inch centers would require 168 plants, or seven flats of 24 pachysandra.
I generally top-dress perennial beds with about an inch of mulch, and shrub beds with about two inches if needed. Established garden beds generally need very little to no mulch, so an inch of mulch may suffice for shrubs too.
Divide 1 inch by 12 inches to convert the depth of mulch or other materials to feet instead of inches (0.08 feet of mulch). Next, multiply the depth of the mulch, 0.08 feet, by 42 square feet of bed space to determine that 3.36 cubic feet of mulch is needed. Mulch is sold in bags containing 2 or 3 cubic feet, so purchase either two 2-cubic-foot bags of mulch or one or two 3-cubic-foot bags.
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 that you need by 27 to determine the number of cubic yards of mulch, soil or compost to order.
Sod is generally sold in a 10-square-foot roll that is 2 feet wide and 5 feet long, so you would need to purchase five rolls for a total of 50 square feet of sod to fill the bed.
It is a good idea to increase your sod order a little over your square feet calculation, as 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 in a bed that is square or rectangular.
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.
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