How late in the season can I plant newly purchased perennials? How late can perennials be divided and transplanted?
The nice thing about nursery-grown perennials is that they are already “hardened off” when you purchase them. Perennials that have been container-grown and have gone dormant through one winter season in the pot are considered “hardened off” plants; they will be winter hardy. This makes your success rate greater when planting perennials late into the season. You can plant hardened-off plants up until the ground freezes. When planting later in the season, be sure to prepare the soil properly depending on the particular plant’s needs. Also use a root stimulator like Root ’n’ Grow to give the plant a jump start to a healthy root system.
After the foliage of the perennial dies back from frost, cut back the dead leaves to about 3 inches from the ground and cover the entire plant with a light mulch about 3 inches deep. Using pine straw mulch is recommended because it is light enough to allow air circulation at the plant’s crown in order to prevent rotting and at the same time it will insulate the root system for the winter. Winter mulching perennials is an important maintenance step every year, but it is especially important with newly planted perennials.
As general rule, perennial division can be done after the plant is finished flowering for the season. Since division greatly stresses the plant, it should be done early enough to give the plant at least one month in the growing season to reestablish its root system. Dividing on a cool day is also recommended. A first-year perennial should not be divided as it needs at least one growing season to become hardy and healthy enough to handle division.