31. WINTER HARDINESS—SO IMPORTANT TO KNOW

One of the most important things to know about a flowering plant is its climate hardiness. Cold temperatures can be fatal to tender flowers. Certain annual flowers and warm zone perennials can only be planted outside when nighttime temperature are reliably above 55 degrees. These tender plants do not overwinter in cooler climates. They grow for one season. If you want to plant permanent perennial flowers that come back every year, first determine if they are suited to your particular climate. You can do this by knowing the plant hardiness zone for your geographic area. This is shown on the United States Department of Agriculture (USDA) Plant Hardiness Zone Map. You can find this map online at planthardiness.ars.usda.gov/PHZMWeb.

The USDA Plant Hardiness Zone Map defines 13 zones by annual extreme minimum temperature (including Alaska, Hawaii, and Puerto Rico). Each growing zone is 10 degrees warmer (or colder) in an average winter than the adjacent zone. The higher the number, the warmer the zone. If you live in USDA hardiness zone 5 and a plant is rated for warmer USDA hardiness zones 7–10, then it is not hardy in your area. These zones are broken into a (hardier) and b (less hardy) subzones.

In cold areas, snow helps to insulate many “marginally hardy” plants. Perennials that are considered marginally hardy can survive if they are covered with a solid blanket of snow for long periods in the winter. Unfortunately, icy conditions on the ground do not offer the same insulating benefits as snow does.

Lavender is native to dry hot areas of the Mediterranean. It loves heat and does not do well in humid weather or in cold, wet soils. If you live in an area where lavender is considered “marginally hardy” and want to increase its chances of survival, plant English lavender varieties in a protected, well-drained area such as in front of a southern-facing brick wall or near heat vents next to the house.

The US Department of Agriculture (USDA) Plant Hardiness Zone Map shows “hardiness zones” as strips of similar climate that run roughly east to west across the country (except in the high mountains and coasts). Zone 3 has an average annual lowest temperature of −40 to −30°F, while zone 10 has lows of only 30 to 40°F. Map by US Department of Agriculture.

The hardiness map is based on the average minimum winter temperatures. This may not be accurate in certain very cold years. For example, some plants can withstand 20-degree temperatures for a few hours overnight, but if this temperature lasts for several days, or if it occurs more than once during the season, then the plants may succumb to the cold. For this reason, keep an eye on your local area’s minimal temperatures in the winter—lower valley areas may stay cold while higher elevations do not. And areas by bodies of water can be a few degrees warmer than inland areas. These minor differences can affect a plant’s viability.

The amount of precipitation can also affect a plant’s hardiness. Some plants can survive cold temperatures but cannot tolerate winter moisture. For example, hardy English lavender (Lavandula angustifolia) varieties such as ‘Hidcote’ or ‘Munstead’ can grow in USDA hardiness zone 5, but cold wet roots can negate this rating.

Feverfew (Tanacetum parthenium), a medicinal herb, has small white daisy-like blooms above light green foliage. Feverfew’s pungent foliage emits a strong, bitter odor that repels bees. It attracts other good bugs such as hoverflies and tachinid flies. These insects eat pests like aphids, thrips, and mealybugs that ravage garden plants. Do not plant feverfew near flowers that rely on bees for pollination!

Ladybugs eat destructive insect pests, so invite ladybugs into your garden with the annual flower they love, cosmos (Cosmos bipinnatus). The flowers come in white, red, and pink, all with a yellow center. They are easy to grow and their open petals make great landing pads for ladybugs.

Sweet alyssum (Lobularia maritima), with its lovely fragrance, attracts the aphid-eating hoverfly. Hoverflies have yellow and black stripes and hover over flowers. They love to feed on the pollen and nectar of sweet alyssum and then lay their eggs on the undersides of the leaves near aphid colonies. The voracious hoverfly larvae then eat the aphids. Interplant sweet alyssum everywhere in your garden. Hoverflies also like coriander, candytuft, and pincushion flower.