USDA hardiness zones
I’ve talked about growing your own herbs, and I’m sure I’ve mentioned that certain perennials are hardy in some zones and not in others. If you are unfamiliar with these, here is a link from the National Gardening Association which will answer some questions about hardiness zones, and will let you check out your zone. Remember that drainage can affect hardiness, and it will make a difference if the plants are in pots, rather than in the ground.
It’s not only cold that affects your plants: It’s thought that plants begin to suffer physiological damage at temperatures above 86 ° F. From the American Horticultural Society, here is a link to a downloadable heat zone map; online this map may be viewed at the Southern Gardening website. This gives the average number of days when the temperature is above 86 ° F
In using either of these (cold hardiness or heat zones) you have to consider where your plants are being grown–in the ground, in pots, the soil drainage, and water availability, among other things.