USDA Plant Hardiness Zones
The U.S. Department of Agriculture (USDA) Plant Hardiness Zone Map is based on the average annual minimum winter temperatures, divided into 10-degree zones. There are 13 zones; each zone is further divided into northern area (a) and southern area (b). The zones have been determined from data from years 1976 to 2005. In other words, we know how cold it can get in the winter in specific areas in the United States. Therefore, we know the likelihood that a plant will survive the winter in that area.
To determine your zone, click on the USDA site and enter your zip code or your state. Virginia and Maryland can range from zones 5 to 8 (because it is colder in the mountain and warmer on the eastern shore). I am in Northern Virginia, in zone 7a. If I buy a shrub that is hardy to zone 10, it will not survive the winter here but will survive the winters in zone 10 and higher, which is to the south of Virginia. So that I do not kill the plant or waste money, I will either not buy that shrub or buy the shrub knowing that it will grow in the summer and die in the winter (treat it like an annual).
American Horticultural Society Plant Heat Zones
The American Horticultural Society (AHS) has developed the AHS Plant Heat Zone Map to indicate maximum temperatures for plants. High temperatures can result in heat damage such as destroyed flower buds, roots, or chlorophyll production. The AHS Plant Heat Zone Map is divided into 12 zones. Each zone indicates the average number of days each year that the particular area has experienced “heat days.” A heat day is when the temperature is over 86 degrees, the point at which a plant begins to suffer. However, plants differ in their resiliency, some are sensitive to high heat and some are not.
Usually a plant’s label will have the hardiness zone but if it has both hardiness and heat zone, the first range of numbers will be the hardiness zones and the second range of numbers will be the heat zones.
Average First and Last Frost
When planting seeds and plants in the spring, you want to know when there is such a low chance of frost your plants will not be damaged. Likewise, in the fall, you want to know the likelihood of your first frost. This is for two reasons. The first is to be able to harvest the summer vegetables and herbs before a fall frost damages them. The second is to be able to determine the latest time to plant in order to get a harvest by counting backwards from the fall frost date to determine when to plant the vegetable. Frost dates are not specific dates, they are days in which there is a high or low probability of a frost.
Therefore, there are charts that show percentages of the likelihood of frost on particular days. Visit davesgarden.com and enter your zip code. For example, enter “22310” and look at “Vienna Dunn Loring (Fairfax County).” There is a 90 percent chance of 32 degrees (freezing temperature) on March 30, while there is a 10 percent chance on April 22. If I plant a warm weather plant such as a tomato on March 30, I am taking a risk that frost would occur and kill the plant. If I plant the tomato at the end of April, I am taking less of a risk, probably the plant will be fine. If I wait another week there is even less chance of frost, I don’t have to worry.
For cool weather plants such as cilantro and spinach, I can plant them in March because the frost that will likely occur will not damage them. They like the cool temperatures in early spring and not sensitive to frost and very cold nights. This is why it is important to know if your plants prefer cool or warm weather.