Let's define what marginally hardy means when talking about a plant. When growers have had some record of success with the plant in your USDA Zone, but it's more reliably hardy one zone warmer then the plant will be described as marginally hardy. The great challenge to growing these plants successfully is getting them through the winter.
Key #1 to Helping Marginally Hardy Plants Through the Winter
There are a couple things you can try to increase the odds that your marginally hardy plant makes it through your winter.
First, take advantage of microclimates when first planting. Microclimates are small areas where some growing condition (temperature, light exposure, moisture level, etc.) differs slightly from that of the surrounding landscape.
For example, all gardeners note and use microclimates to a degree just by choosing plants for sun or shade!
Generally, cold air pools in depressions, so low-lying areas of the yard will be cooler than more level areas. Areas adjacent to the house or other buildings tend to be a bit warmer, because structures absorb heat during the day and radiate it back out at night. The same can be said for hard-paved surfaces such as walks, driveways and patios, but because pavement does not absorb water, the soil adjacent to it may be wet.
Key #2 to Helping Marginally Hardy Plants Through the Winter
Second, aside from finding a warm microclimate for your marginally hardy plants, improve drainage at planting time. It’s often the combination of cold and wet that does plants in over the winter. If your soil tends to hold water, improve its drainage by adding organic matter such as compost to your planting hole.
After the ground freezes, apply an extra few inches of mulch over the crown of the plant. This will help keep the soil temperature regular.
Keep in mind that microclimates apply to your broader neighborhood, too. It may sit within a certain zone on the USDA hardiness map but actually be slightly warmer or cooler.
Urban areas tend to be warmer than rural areas because buildings and pavement radiate heat, as noted above. Areas adjacent to bodies of water also stay a bit warmer in winter and cooler in summer because water moderates air temperature.
This excerpt was taken from the November/December 2011 issue of Horticulture. Back issues are available at GardenersHub.com. Join thousands of gardeners just like yourself and get smart gardening advice from experts delivered right to your home—Subscribe to Horticulture.