Chapter 10

Temperature Requirements for Greenhouse Crops

Cold-Tolerant, Cold-Temperate, and Cold-Sensitive Crops

The base temperature (or minimum temperature) is the cool temperature at which a plant does not develop. Using the base temperature, greenhouse crops can be broken down into three basic categories:

Growing Cold-Tolerant, Cold-Temperate, Cold-Sensitive Crops

Cold-tolerant crops are those crops for which development is less influenced by lowering the temperature set point. The quality of these crops is typically much higher when they are grown cooler (<65ºF), especially when the daily light integral (DLI) is low (< 10 mol/m−2/d−1). Although the base temperature of cold-tolerant crops is below 39 degrees F, growers find they can save on energy, achieve their market dates without delays and produce high-quality crops when they are grown at temperatures of 60 to 65 degrees F (16 to 18°C). The grower might also consider growing cold tolerant crops in a minimally heated high tunnel or with root-zone heating when the air temperature set point is reduced to save on energy costs.

Temperature Requirements for Crop Growth Stages

When contemplating greenhouse temperature management, it is important to take into consideration the influence of temperature on plant growth and development for the various stages of growth—seed germination, vegetative phase, reproductive phase, and ripening phase. A plant species optimum temperature changes as a plant grows from a seedling to a mature plant. Generally, seed germination and early seedling growth occurs most rapidly at warmer temperatures. Warmer temperatures are generally favored by younger plants. Even though warmer temperatures cause increased rates of both photosynthesis and respiration, warmer temperatures favor photosynthesis and net growth in younger plants since there is less stem and root tissue compared to older plants.

Temperature Influence of Plant Quality

There is often a trade-off between high quality crops and crop timing. Cooler temperatures typically produce higher quality plants (especially cold-tolerant crops), but they take longer to reach maturity, and energy consumption per crop can be higher. Whereas crops grown at warm temperatures develop faster and thus have shorter crops times and require less energy for heating, but the quality of plants is often not as high. For example, transplants and plugs grown at cool temperatures they often have thicker stems, better rooting, and greater branching.

Plant Temperaure

It is important to consider the actual plant temperature during production and not just the air temperature. During the day, air temperature, shortwave radiation, and transpiration have the largest effect on plant temperature. Heat can move from the air to the plant, or vice versa, through a process called convection. Shortwave radiation that is not absorbed by leaves for photosynthesis is either reflected or transmitted through the leaf and can increase plant temperature. As radiation from the sun increases, plant temperature increases unless leaves can dissipate heat through transpiration or convection. Plant temperature can also increase above the air temperature if light is delivered from high-intensity lighting, such as high-pressure sodium or metal halide lamps.

Root Zone Temperature

Measurement of the substrate temperature is particularly important when propagating plants for rapid callus formation of cuttings and rooting of young plants. It also controls emergence and initial development of potted bulbs, rhizomes, and tubers. In the absence of crop-specific information, a root-zone temperature of 73 to 77 degrees F (23–25°C) and a slightly cooler air temperature is recommended. However, there are some crops that root faster at a higher air temperature, particularly those that are cold sensitive.

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