Garden Planning: How to Assess a Plant's Hardiness

By Ann Whitman, Suzanne DeJohn, and The National Gardening Association

10 of 12 in Series: The Essentials of How to Plan a Garden

When choosing plants for your garden, pay attention to its hardiness, which determines how well it handles climate extremes, such as cold and heat. Plant catalogs often use the term rather loosely to indicate whether you can expect a particular plant to live in a cold-winter climate, but hardiness really is a measure of a plant's ability to survive all the aspects of a particular climate.

Many factors influence a plant's cold-hardiness:

Genetics: The genetic adaptability of plants to specific climates and soils is called provenance. Provenance is a major factor to consider in choosing landscape trees and shrubs, as well as some perennial plants.
Stage of growth: The timing of winter readiness, varies with each species and depends partly on growing conditions, such as soil moisture and fertility.
Health: Environmental stressors (such as drought, flooding, storm damage, diseases, and pests) weaken plants and can make them more vulnerable to cold damage.
Plant parts: Flower buds are often less cold-hardy than the woody stems of trees and shrubs, and they may be damaged or killed before stem damage occurs. That's why a late-spring cold snap often kills frost-tender flowers but does little harm to other plant parts.

Climatic factors that influence plant survival include the following:

Duration of winter: Genetic programming signals some plants to begin flowering and growing after a particular number of hours of cold temperatures followed by warm temperatures. Even before winter really ends, some plants break dormancy and are damaged by spring frost.
Duration of extreme cold: Prolonged periods of extreme cold usually cause more damage than a single night of unusually cold temperatures.
Wind: Wind increases moisture loss. Unfortunately, plants can't replace lost moisture while the soil is frozen and plants are dormant. Evergreens, which keep their leaves year-round, are especially vulnerable to the effects of drying winds in winter.
Snow: Snow provides an insulating blanket that protects plant roots and stems from extreme cold. In areas that receive little snow, the soil temperature gets much colder than in areas with snow cover. (You can use a thick layer of loose mulch to mimic the insulating effect of snow.)
Sun exposure: The sun can increase moisture loss from winter foliage and stems. The winter sun can cause frost cracks: The bark on young or thin-barked trees like beech and maples thaws during the day, then freezes at sunset, causing the bark to split.

Heat tolerance is a common limiting factor for plants, and in many parts of the country, this factor is the primary concern. Plants native to desert and tropical regions are naturally heat-tolerant, whereas plants from cooler regions may show little tolerance. Some plants can withstand high daytime temperatures but suffer if nights stay too warm. Sun exposure, humidity, and soil moisture can influence a plant's ability to thrive in a hot climate.

If you're not sure how to pinpoint the cold, heat, or climatic extremes that affect your gardening, hardiness maps can help you identify your specific challenges.