Q&A: Planting Potted Perennials
Q: If I buy perennial plants in four-inch pots for my garden, will they flower this summer? I am interested in Rudbeckia, Echinacea, fleabane and poppies.
A: Perennials in four-inch pots, planted early in optimum growing conditions and cared for well, will often manage to bloom the first year. However, since they are so young and need to devote energy to establishing themselves, they will not make anywhere near the type of show they will the following year. Many gardeners interplant a new perennial garden with annuals so that there will be a display that first year. (Note that this works only if the annuals are not allowed to overshadow and overwhelm the young perennials.) Be sure too to plant the perennials far enough apart to accommodate their future growth.
— National Gardening Association