Different plants like different climates. Artichokes like mild weather year round. Okra loves heat and humidity. Apples and plums need a certain number of hours of winter chill to fruit well. To help gardeners know which plants perform best in which areas, the USDA created a map of climate zones, assigning each zone a number. Once you know the number for your area, all you need to do is look for that same number on the label when you shop for plants.
Unfortunately, these zone maps aren’t any guarantee that the plant you choose is actually suited to your area for a couple of reasons:
- It doesn’t account for microclimates—variations from the standard climate of the area caused by terrain, construction, vegetation and other variables. Concrete, for example, collects and retains heat, raising the ambient temperature of the surrounding area, while dense trees keep the ground cooler than usual by preventing sunlight from reaching the ground. Microclimates can even vary within a single yard. A sunny wall in an enclosed backyard can be almost a full number higher than a shady, open area at the front of the house.
- It focuses on lows and largely ignores highs. Just because a plant can’t take freezing weather doesn’t mean it can handle scorching heat. Take for example the artichoke, mentioned earlier. It loves the temperate Northern California coastal marine zone for its fog and even, year-round 50-60 degree temperatures but may not be happy at all in Austin Texas or the Florida panhandle (all zone 8b).
- Climates change. The current map, which was last updated in 1990, no longer accurately reflects the current climate trends. Thankfully, they are currently in the process of updating the map, but that won’t really help the plants you already have in place.
One other thing to consider, plenty of plants, given the right placement and attention, can survive outside their zone. The trick is make sure all of their other needs (sun, water, etc.) are being met and that you’ve placed them in an area of the yard with a microclimate that comes as close as possibly to their preferred zone.
NOTE: If you live in the Western United States, check out the Sunset Western Garden Book which has its own zone system designed to account for the extremely varied climates of this area.