It is important, as a gardener, to understand plant zones and how they help us to grow the right plants. Every gardener should be aware of the general lows so as to grow plants which have the hardiness to take the temperatures found in their area.
You should choose plants according to the zone map first, and then adjust the map to fit your own locale after you have observed the lows for a few years. Neighbors and gardening friends are also useful advisers in choosing plant material. Plants grown successfully within a few blocks should do well for you.
The plant zones seen on the map are very important to know and to follow, but not every plant listed with a particular zone number will thrive in that zone. These zones are hardiness zones and do not take into consideration other climatic factors which affect plant growth such as heat, length of the growing season, and rainfall. (The growing season is the average length of time between the last killing frost in spring and the first frost in autumn). You will find that many vegetables which are not generally possible to grow in other areas farther north prosper in the South because of its longer growing season. Likewise, many plants like Brussels Sprouts are not really satisfactory for the South because their best crops are in the fall; being long-season plants, they must grow during the highest temperatures of southern summers, which are not conducive to their proper growth.
Two zone numbers are given for each plant: zones 5-8. The first zone designation is intended as an estimate of winter tolerance, that is, the coldest zone in which the plant should be expected to survive outdoors. The second zone number is an estimate of mild climate tolerance, that is, the warmest zone in which the plant should be expected to do well outdoors.
The Silver Maple is a fine tree in New England, but in the southern long growing season and high temperatures it grows too rapidly and becomes weak wooded.
You can extend a plant's hardiness zone with a protective cover. A 3-inch mulch over plant roots will buffer extremes of soil temperature. And snowy regions have snow cover as a natural hardiness zone extender.
We must consider plant zones when choosing what plant material has the best possibilities for our area, but hardiness is only one of many factors which determine how successful a plant will be. All climatic factors must be considered.