In the November 2010 issue, I discussed the fundamentals of how temperature influences the development and timing of greenhouse crops. Conceptually, floriculture crops respond to temperature similarly: they develop in response to the average daily temperature, and they have minimum, optimum and maximum temperatures. However, the values of these minimum, optimum and maximum temperatures vary from one crop to another.
The base temperature (or minimum temperature) is the cool temperature at which a plant does not develop. At or below the base temperature, plants no longer develop leaves and progress towards flowering ceases. As temperature increases above the base temperature, plants develop progressively faster. The base temperature value of a crop can be experimentally estimated by growing plants at a range of temperatures, recording time to flowering (or some other developmental event), then extrapolating from the data the temperature at which time to flower is infinity.
In the past several years, we have performed experiments to determine the base temperature for a range of bedding plant crops (Table 1). Using the base temperature, we can subjectively place crops into different temperature response categories:
Cold-tolerant crops are typically those that perform best at relatively cool temperatures, and their flowering time is less influenced by lowering the temperature (on an absolute basis) than other crops. In contrast, flowering of cold-sensitive crops is substantially delayed when temperature is lowered and thus, plants should generally be grown at warm temperatures. The temperature response categories can help growers determine which crops should be grown together, and which ones should be grown separately at different temperatures. For example, a grower in the North could have three greenhouse sections with different spring temperature settings, one for each of these categories: 60-65° F for cold-tolerant crops, 65-70° F for cold-temperate crops and 70-75° F for cold-sensitive crops. Perhaps more importantly, growers should avoid growing cold-tolerant and cold-sensitive crops together because crop timing, crop quality and heating costs cannot be optimized.
These base temperature values (click on PDF version to the right to view table with estimated base temperature values and temperature response categories) should be used with other factors and considerations to determine desirable growing temperatures. Selection of a desirable growing temperature should also be based on the time of year (including outdoor temperature and daily light integral), heating cost, finish date, starting plant size and desired finish quality. In other words, an “optimum” growing temperature is situational and involves considering multiple factors at the same time.