You are right to take time to learn to properly space your perennials. For a modest investment up front, perennial plants will bring years of color and interest to your garden or landscape. A little time and effort spent in proper planting will pay off in the future. Consult a good book on perennials before planting. The Complete Perennials Book by Ortho and the Miracle-Gro Guide to Growing Beautiful Flowers provide information about spacing as well as soil, sunlight, and water needs.
If you are planting nursery-grown plants, refer to the label. The preferred spacing is usually a range, such as 18-24 inches. Planting closer will give you a fuller garden more quickly, but you will probably need to divide the perennials sooner. Plants with closer spacing are more susceptible to fungal diseases caused by poor air circulation. Most perennials benefit from division after 3-5 years.
If you can't find information about a specific plant, a general rule is to space small perennials 6-12 inches apart, 2-3-foot-tall perennials 12-18 inches apart, and taller perennials 18-36 inches apart.
