Question: I have a pair of plants that are "marginally hardy" in my climate zone. What are their chances of making it through the winter and how can I help?
Answer: If you've read or been told that a plant is marginally winter-hardy in your zone, that means that growers have had some record of success with it in your zone, but it's more reliably hardy one zone warmer. There are a few tricks you can try to ensure that the plant makes it through your winter.
Mulch. 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.
Improve drainage at planting time. For many plants, it's a combination of cold and wet that does them 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.
Take advantage of microclimates. Microclimates are small areas where the temperature, light exposure, moisture level, etc., differs slightly from that of the surrounding landscape. (All gardeners use microclimates to a degree, perhaps without calling them such, 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 it may be wet. Oftentimes winter winds are a danger to marginally hardy plants; be aware of the direction from which winter winds come and site plants so that they will be sheltered by a building or larger trees and shrubs.
One last point—microclimates apply to your broader neighborhood, too. It may sit within a certain zone on the USDA hardiness map, but 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. Valleys are generally cooler than points higher on a slope. Open hilltops are subject to the cooling and drying effects of winds.
Learn more climate-bending tricks with A Gardener's Guide to Frost.
Get advice from Colorado-based Lauren Spring Ogden in The Undaunted Garden: Planting for Weather-Resilient Beauty.
Grow vegetables year-round with Eliot Coleman's Four-Season Harvest, written from Maine.