The USDA hardiness zone you see listed with a plant should be considered a recommendation, not an absolute. Hardiness zones measure the average annual minimum temperature, but they do not account for soil moisture, duration of cold weather, and other factors affecting plant survival. Intrepid gardeners love to roll the dice and advance their zones and their fortunes. Here are some tricks that can help you avoid this.
1. Mulching too much
A herbaceous plant only needs protection for its roots and crown, and a thick layer of mulch can often provide additional hardiness to half the entire zone. Just remember to put back that heavy mulch layer in the spring.
2. Planting against a wall
The British have a term that is falling out of style, but I find it useful. If a plant is listed as a “wall plant” in the references, it is not entirely hardy, but the protection from wind and radiant heat provided by the wall will help it survive.
3. Take advantage of a slope
In recent years I have been amazed at the success of planting on just one slope. Moderately hardy plants appreciate the heavy cooling air that moves down and away, something fruit growers have used for generations when planting their orchards.
4. Wrap it up.
If all else fails, real denizens will wrap plants in burlap or plastic, insulate them with foam or fallen leaves, and even wrap them in string lights to keep them warm. will
Mark Wethington is director of the JC Raulston Arboretum in Raleigh, North Carolina.