The production of good quality plants is a major concern of growers. Environmental conditions play an important role in achieving successful growth and development of any kind of nursery commodity. Among many factors that will determine the growth of the plants (like soil type, latitude, rainfall, winds, act.), temperature is of the first rank, and has to be taken seriously under consideration.
Each plant has a range of temperatures, confined by the minimal and maximal thresholds under which plants can perform its vital functions. Beyond that range, temperatures become lethal and either cold or excess heat is fatal to the plant. It is critical to meet the optimum temperature requirements of selected plants.
Areas of commercial production are most often limited by:
The USDA (United States Department of Agriculture) provides a general indicator that is widely used by growers, in the form of the map and accompanied table. In that system, the US is divided into 11 regions. Divisions are based on average minimum temperatures during the winter season. The fact that it is the average minimum and not the absolute minimum temperature is worthy of notice. Data has been collected over many years (since 1974), and the temperature in your zone can get much cooler than what the table is showing! However, the zone map is a great tool when evaluating the feasibility of a new crop production enterprise.
A plant’s assigned hardiness zone is the lowest temperature they are expected to survive with any reliability. Zones are numbered from the harshest (lower #) to the mildest (higher #).
How to use zone map?
There are several other plant hardiness zone maps published, so when comparing a plants ratings versus a zone rating make sure that they correspond. Occasionally plants assigned to, for example zone 7, can suffer freeze damage in zone 8. Apparently the hardiness zone system is not totally foolproof. One of its major limitations is lack of information about occurrence of temperature fluctuations.
A large percentage of losses occur after a warm winter followed by a cold spring spell. Plants loose their hardiness and freeze. Obviously, zone mapping is a general guideline, and each grower has to acquire the knowledge of their specific microclimate conditions.
Factors that will change climate:
For annual crops and others that are treated as such in your area, minimum winter temperatures are not a major concern.
It is advisable to analyze your climate for a frost- free growing season. Data can be obtained that will indicate dates of last occurring frost and the first frost in the fall. The best source of such information is your local Extension Service Office.