Question: In designing this year’s garden, I'd like to know exactly how many plants I need for my flower border. In the past, I've overplanted and have ended up transplanting or giving away the extras. Are there any general rules of thumb when it comes to plant spacing?
Answer: "How many plants do I need?" and "How far apart should I space them?" are common questions. Whether the subject is marigolds or maple trees, the spacing largely depends on the plant's mature size. According to Dr. Douglas Bailey at North Carolina State University, annuals such as ageratum, lobelia, pansies, dwarf French marigolds, and sweet alyssum should be spaced six inches apart. Begonias, dusty miller, nicotiana, petunias, and dwarf salvia should be spaced eight inches apart. Ten inches is a better spacing for calendulas, celosias, gomphrenas, hypoestes, impatiens, and large marigolds, snapdragons, and zinnias. Geraniums should be spaced 12 inches apart, and melampodiums and cleomes should be spaced 14 inches apart. Spacing plants too close together stunts their growth and increases the chance of disease problems.
Triangular spacing is more attractive than straight rows or a rectangular grid. For spacings of 6,10, 12, or 14 inches between plants, you will need roughly 48, 28, 16, 12, or 8 plants respectively per 10 square feet. Herbaceous perennials should be spaced according to their mature spread. Smaller, front-of-the-border plants should be to 12 to 18 inches apart. Plants of intermediate size should be placed at least 18 to 24 inches apart (four to three plants per 10 square feet) and larger plants should be spaced roughly three feet apart.
When calculating the correct spacing for woody ornamentals, use a formula recently offered by Peter Thurman in the magazine The Garden. Add the plants' ultimate spreads and divide by two to calculate the distance between them at planting time. If plants at first appear to be too far apart, you can interplant these permanent plantings with temporary "filler" plants.