The zone map into which trees, shrubs and evergreens have been cast is a temperature map. Constructed by the U. S. Dept. of Agriculture after 40 years of study it is based upon the number of frost-free days within the growing season, that is—the number between the last frost of spring and the first frost of autumn. The map is much used by nurserymen and landscape planners in working out regional planting and garden statuary placement problems. It is reliable on a broad basis.

But it must be understood that within these zones there are many variations in temperature, number of growing days, soil and other environmental conditions which affect growth. There are local differences, too, and these are the differences which concern the home landscape planner. They must be studied before any planting scheme is started. The most practical method is to first study the lists of plants, after you have worked out your plans on paper.

Select the plants desired for the scheme from under the different headings of shade, flowering trees, conifers and flowering shrubs. Note if they are recommended for your zone. Then, go further and check with your State Experiment Station, Agricultural College , or local authority—such as a reliable nursery.

Plants native to the U. S. fall pretty well into the zones, but these will have a distribution across zone lines too on either side.

You just cannot draw a line between the places where plants grow in nature. Plants ignore zone lines just as they ignore man-made international lines. The exotic or imported plants, those from Europe , the Orient, and elsewhere are more variable. At the same time, they may have as wide a distribution over territory as the most widely used native plants. Again your local nursery is a good place to study.

It must be assumed with near certainty that a plant which has been grown for several years in a nearby nursery will thrive in your garden or on the grounds. But besides those plants and trees which have a broad distribution there are many which do require certain specific conditions for growth. Typical of these are rhododendrons, azaleas, holly and similar broad-leaved evergreens. Most of these need a soil on the acid side or, at least, it must be free from lime or alkalinity.

In regions where the temperature is suitable but the soil is not (as in the Midwest and similar regions where the soil is alkaline) steps must be taken to incorporate materials with the soil which will neutralize the alkalinity. Size, height, rate of growth and similar factors are given as a guide. These growth factors are related to climatic conditions. The length of the growing season is a big factor. Red maple, for example, will grow to greater dimensions in the South because of the long growing season.

Further east, it will not attain the same stature. In the north it is still more restricted. Therefore, in interpreting the rate of growth and you should if you are setting out a tree, study local conditions for the ultimate height and the rate of growth. The conifers, those tall, spry trees which are evergreen, pines, spruces, etc., are generally more localized than the deciduous tree or those which drop their leaves. Many are extremely hardy and, of course, there are varieties which are extremely tender.