There are innumerable plants in the world, and knowing which to choose can be a daunting task. A plant may be labeled an annual in one climate, a perennial in another or an evergreen in a tropical environment. Plants’ needs vary by sunlight, water, fertilization and soil types, and a key component to survival is winter hardiness.
The USDA developed a plant hardiness zone map in 1960, revised periodically based on weather data collected over time. The 1990 edition divided Arkansas into three separate zones: Zone 6 (lows between minus 10 to zero degrees) in the northern tier; Zone 7 (lows between zero and 10 degrees) in the center of the state; and Zone 8 (lows of 10 to 20 degrees) in the southern tier.
Keep in mind this map gives one piece of the puzzle — the average low winter temperature over a period of years. It does not tell average high summer temperature, rainfall amounts or humidity. It is a guide, but not the ultimate tool in choosing plants.
Over the past 20 years in Central Arkansas, we have seen a few single-digit temperature days, but they were rare. And Northwest Arkansas still has some brutally cold days. But overall, we have found our winters to be a bit milder.
The most current edition of the plant hardiness zone map was released in 2012 and can be found at garden.org/nga/zipzone. Enter your zip code, and it will give your zone.
The current zone map divided Arkansas into four zones. Little Rock and points south are, for the most part, 8a (minimum temperature of 10 to 15 degrees), with other areas being 7b (5 to 10 degrees), 7a (zero to 5 degrees) and the far northwest corner 6b (minus 5 to zero degrees). Remember these are not the lowest temperatures we have experienced in our areas, but the average lows between 1976 and 2005.
Every year there are fluctuations. In Central Arkansas last year, we had a killing frost in October; this year it was Dec. 1. The southern tier was similar. The northern tier gets colder sooner; temperatures are lower all winter, and the cold lasts longer.
Many catalogs today list plants’ hardiness zones, but you shouldn’t rely strictly on that information. Most zones begin in the northeastern United States and end up in Washington state. Other regions’ winters may be similar to ours, but their summers are quite different, and that affects plants’ success.
The key to gardening is trial and error. Most of the plants sold as shrubs and trees in Arkansas nurseries should be considered hardy in Arkansas for both summer and winter, but extreme weather can still cause damage.
Cold-hardiness can also change with the season. Plants are often most susceptible to cold damage as they are going dormant in the fall and emerging in the spring. Abrupt temperature drops versus a gradual cooling can make a difference, as can soil moisture. As the days get longer and we experience a few bouts of warm weather, the buds begin to swell on some plants, and they think spring is here. As plants leave their dormant state, they become less cold-hardy.
Try new plants. If you have plants that are only moderately winter-hardy in your area — gardenias, for instance — place them in a more protected spot in your landscape. When we think of a protected spot, it is often an eastern or northern exposure next to the house. South- and west-facing exposed areas may seem warmer, but often can lead to more winter damage. Your entire yard could be warmer or cooler than your neighbor’s yard because it is more sheltered with trees and shrubs or more exposed. Low-lying areas typically get colder than hilly yards. In Arkansas, huge fluctuations in temperatures typically cause more problems than low temperatures do. Plants exposed with full afternoon sun will experience even more fluctuations.
I often say that you are not a true gardener if you have never lost a plant, because you aren’t experimenting and trying new things. There are so many new plant introductions that it is hard to know what will survive. And we have such fluctuating weather, we can’t know all the rules. If a plant dies, think of it as an opportunity to buy a new plant!