To stimulate the garden planning process and help avoid expensive mistakes, landscape designers usually match flowering perennials at the nursery.
Arrange plants on a nursery cart to see which ones work well together.
Pair flowers for color - whether hot and very summery (red, orange, and yellow) or cool and springlike (blue, lavender, peach, and pale pink).
Check plant tags to find out how big the plants will grow and whether they like sun or shade.
Choose plants that will bloom together, and - to prolong the bloom season - include early and late bloomers. In a border, for example, choose two or three great combos for each season.
Consider plant size. Gallon-size perennials (in 6-inch pots) will take hold faster in the garden than smaller stock, but they're more expensive. Buy smaller plants (4-inch pots) for container planting - you can really pack them in for a full display - or when you need larger quantities for massing in garden beds.
Check plant health. Avoid buying plants that have leggy stems, or lots of roots protruding through drain holes of the nursery pots - signs that the plants are probably rootbound and may perform poorly.