Perennials -- the lazy gardener’s best friends -- grow for at least three years in areas where they are hardy. The day comes, however, when even modest perennials grow too large for their space and beg division -- or maybe a division from a neighbor’s plant comes to stay. Success in transplanting any perennial hinges on timing the move to fit the plant’s growth pattern in your U.S. Department of Agriculture plant hardiness zone.
About USDA Zone 5
The USDA publishes plant hardiness maps based on data collected by the National Climatic Center. The new map, published in 2012, moved USDA zone 5 approximately half a zone north throughout its swath across the country, starting from southern Maine around the Great Lakes; looping across Iowa and Nebraska; running up the foothills of the Rockies in Colorado, northeast Utah, northern Idaho, and western Montana; and ending along the Washington-Canada border. USDA zone 5 gardeners experience their last killing spring frost from late April through mid-May. The first hard frost of fall occurs during October. Winter’s coldest average temperature may register between -10 and -20 degrees Fahrenheit.
Transplant herbaceous perennial divisions in early spring, including daylilies (Hemerocallis spp.), hardy from USDA zone 3 through 9. Move hosta (Hostas spp.) divisions, also hardy from USDA zone 3 through 9, in June as early as the ground is comfortable to work and before plants leaf out so that the crowns don’t grow unevenly. Plant summer bulbs such as allium (Allium cernuum), hardy from USDA zone 4 through zone 8, after the soil has warmed above 50 F for best results. Plant purchased perennials throughout spring until hot weather sets in anytime from mid-June through July.
Some perennials, notably daylilies, are so hardy that they can be moved throughout the summer in USDA zone 5, when it is relatively mild and humid. Best results follow planting in spring, however, unless spring is when the perennial typically blooms. Summer transplants need extra attention and faithful irrigation, because root growth is slow and summer heat and drought places stress on plants. Tender perennials, woody perennials or perennials that bloom during summer, such as bigleaf hydrangeas (Hydrangea macrophylla “Tokyo Delight”), hardy from USDA zone 5 through zone 9, should never be moved in summer -- delay transplanting them until fall.
Transplant at will again after mid-August, when USDA zone 5 temperatures begin to moderate, until mid-October. Always transplant peonies (Paeonia lactiflora), hardy from USDA zone 3 through zone 8) in September so that they have time to establish their roots. Red peony buds often pop up through the snow in USDA zone 5 and may bloom around the end of May. Spring and early summer bulbs such as tulips (Tulipa spp.) and lilies (Lilium spp.), both hardy from USDA zone 3 through 8, receive necessary chilling when planted in the fall at least six weeks before the ground freezes. Plant them by early November in warmer parts of USDA zone 5 and mid-October in northern areas.