Dial in degrees

Temperature primarily affects plant development. Just like light, temperature has several points to consider to understand how it affects growth, including average daily temperature (ADT), difference between day and night temperatures and extreme temperatures.

The ADT is an average of the temperature over a 24-hour period — an integrated measurement of temperature throughout the day. It can be roughly calculated by multiplying the greenhouse day set temperature by the number of day hours and the night set temperature by the number of night hours, then dividing by 24.

A more accurate measurement is the hourly averages throughout the day divided by 24, since temperature ramps up and down between day and night temperature set points.

The biggest influence ADT has on plants is controlling the rate of plant development, such as the leaf unfolding rate or leaf expansion rate. The way ADT affects plants varies among herb species, but there are cardinal temperatures that define common responses across herb species.

The base temperature (T_b) is the temperature at or below which development ceases (but plants aren’t damaged). As the temperature increases above the T_b, the development rate increases until it reaches its maximum rate at the optimal temperature (Topt).

Between the T_b and Topt, development increases linearly with temperature — known as the linear range. As temperature increases above Topt, development starts to slow down from stress until it ceases once the maximum temperature (Tmax) is reached.

These cardinal temperatures, as well as the linear range, are useful for managing temperatures for culinary herb production. By using the T_b and Topt, we can group herb crops by common responses.

There are cool-growing or cold-tolerant crops such as lovage; moderate-growing or cold-temperate crops like chamomile, cilantro, dill and parsley; and warm-growing or cold-sensitive crops such as basil and stevia.

By grouping crops by their response to ADT, development and growth rates are optimized across the various temperature responses.

While the temperature is integrated across the day for ADT, the difference between daytime and nighttime air temperatures is labeled as DIF (day temperature minus night temperature equals DIF).

As DIF increases, or becomes more positive, it promotes cellular elongation. As DIF decreases, becoming less positive or even negative, cellular elongation is inhibited. We can use a positive DIF to enhance the fresh weight of cut herb crops, as the cell elongation promoted by the more positive DIF increases yields.

When increasing DIF, be mindful that daytime highs don’t exceed the optimal temperature and nighttime lows don’t fall below the base temperature to avoid delaying or damaging crops.

Times of high light intensity, combined with the warmer outdoor air temperatures, make it challenging to adequately control the temperature during the day. This heat can be supra-optimal and slow down growth, cause crops to bolt and result in bitter flavors.

There is no real solution to the warm temperatures. However, you can increase the number of seeds sown in each rockwool or phenolic foam seedling and reduce the time to harvest, avoiding the development of flowers or off flavors.

When plants are too close to leaky vents in winter, the outdoor temperatures may cause chilling injury to sensitive crops. Freezing damage can also occur if temperatures are cold enough (or a heater goes out). Temperatures 50°F or lower can damage basil.

Managing temperatures is essential for the predictability of fresh-cut culinary herbs. By controlling ADT, DIF and extreme temperatures, crops can be grown on schedule with increased yields.

Christopher J. Currey is an associate professor in the Department of Horticulture at Iowa State University. ccurrey@iastate.edu