Increasing Overwintering Survival of Container-Grown Perennials

Overwintering perennials that require vernalization and/or bulking is part of the nursery production cycle, but losses can occur.  We conducted six experiments in two different USDA Hardiness Zones (6a and 7a), evaluating the impact of various cultural practices on overwintering survival of container-grown herbaceous perennials  and ornamental grasses.  Rooted liners were potted in Fall and overwintered in poly-covered cold frames. Treatments included various overwintering covers, fertility levels, and substrate moisture levels.  Taxa evaluated included Pennisetum alopecuroides, P. alopecuroides ‘Hameln’,  P. alopecuroides ‘Little Bunny’, Echinacea ‘Milkshake’, E. ‘Hot Papaya’, Gaillardia aristata ‘Gallo Peach’ , and Heuchera villosa ‘Pistache’.  Data taken included survival percentage and vigor ratings. Air temperature external to and within coldframes as well as under coverings was monitored continuously, as was substrate moisture levels.  Survival percentages and vigor ratings differed by taxa; most showed increased survival and vigor with some sort of covering. Moderate to moist substrates and higher fertilizer rates also increased survival and vigor of some taxa.