Hardiness Zones

Hardiness Zones represent areas of average annual minimum temperatures. The USDA has divided the U.S. in to eleven different zones based on the lowest termperature recorded between 1974 and 1986. Other parts of the world also have Hardiness Zone maps.
This information is helpful for determining what plants may survive in your area. Hardiness Zones do not take into account rainfall, summer heat, soil type, or other factors but are still helpful when deciding what plants may grow in your area. Other factors such as lack of snow cover and lack of water can affect a plants survivability.
Plants are usually labeled with a hardiness zone and/or a minimum temperature to which they will survive. For example English Lavender Lavandula angustifolia, Hardiness Zone 5 - 8 is sometimes listed - 30 F instead of by zone.  Spanish Lavender Lavandula dentata has a Hardiness Zone of 8 - 9.  This information is more important than whether a plant is marked as a perennial.  Perennial in what zone?
My garden is a zone 6 ( -10 F to 0 F) so I can usually grow plants that can survive in zones 6, 5, 4, 3, 2, and 1. If I want to grow a zone 7 plant I may succeed by finding a warmer microclimate in my yard or growing the plant indoors. A microclimate is an area that has a slightly different climate due to wind protection, more shade, more sun, or other factors which create a climate warmer or cooler than the rest of the area.
I have been able to grow rosemary by planting it next to the foundation on the south side of my house. The foundation holds and reflects heat back to the rosemary's roots to keep it warmer through the winter.
To find your Hardiness Zone