To avoid over-crowding, Sullivan suggests researching your plants first so you know exactly how big they will get and how to space them out accordingly. “Typically perennials, plants that live for more than two years, should be spaced approximately 18 inches apart,” says Dailey. “This allows enough room for new growth and will usually make the garden look filled right away.”
It’s also important to know how high your plants will grow, he says. “Shorter and creeping ones should be planted toward the front and edges of the garden bed, with the taller plants in the back.” This is where knowing about sun exposure comes in handy—be mindful of taller plants that would block smaller ones, or the varieties that prefer a lot of sun or shade.