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| Temperature effects on plant production | |
| In the greenhouse during the winter, plants will develop leaves and flowers progressively faster as the temperature increases. Therefore by lowering day or night temperature will lengthen the production period. This implies that at the lower temperature the production of a given crop must be started earlier. But plants respond differently to temperature. There is a temperature at which a given plant species will cease to develop. This temperature is called the base temperature. For seed raised Petunia this is about 4°C while for seed raised Vinca (Catharanthus) it is around 10°C. As temperature increases aboove the base temperature, plants grower faster and faster and only a small temperature rise can make a big diffrence on the time it takes to flower. As we get to warmer temperatures, the same increase in temperature (number of °C) will have a proportionately smaller effect on accelerating flowering. A reduction from 18° to 16°C will lengthen production time in Petunia by about 13 days and in Vinca by about 30 days. The reason for how temperature influences production timing is related to the base temperature of a given plant species. Those with a low base temperature are called ‘cold-tolerant’ while those with a high base temperature are called ‘cold-sensitive’. Plants such as Vinca and Celosia will still grow very slowly at 16°C. Some species fall between these two categories (base temperature 4°-8°C) such as Salvia. All plants respond to temperature at all stages of development at both the level of root and shoot growth. At the same stage of development and light intensity, the overall quality of the finished plants can be improved by lowering the temperature. At these lower temperatures, plants often have thicker stems, greater branching, more roots and more larger flowers. Exceptions to this are plants that grow best at warm temperatures such as Hibiscus rosa-sinensis. The length of the crop cycle is a key factor; energy consumption per crop grown in the spring can still be higher when crops are grown cooler because the productiuon period is that much longer. The production period can be shortened by providing optimum light intensities and the correct photoperiod to induce flowering. Many bedding plant crops are long-day plants. By providing supplementary light (day extension or night interruption) the production period can be shortened by several weeks. The Petunia ‘Wave’ series is particularly responsive in this regard. Conversely, for some short-day plants such as Zinnia and Cosmos, it might be useful to provide night interruption light during the first few weeks of growth to speed up flowering and hence shorten the crop cycle. Cooler air temperatures can mean cool substrate temperatures. The temperature of the substrate determines the uptake of water and nutrients and is therefore very important to plant growth. The effect is most noticeable and important when growing young plants. Substrate temperature can be as much as 4-5°C lower than air temperature when using overhead heating systems when plants are grown on benches. The difference in temperature is even greater if the plants are grown at ground level. The biggest reason for cool substrate temperatures is caused by watering with cold water. When plants are young, water is the greatest component of the substrate. Water will absorb a lot of heat without changing temperature and the movement of energy (heat) from the air to water is not very efficient. The best way to increase substrate temperatures is to water with water at ambient temperature or ‘tempered’. At cool temperatures, plant roots will not be active and they will be unable to take up water and nutrients. These plants can even wilt despite the fact that the substrate has adequate water. Eric Runkle, Ryan Warner and Dean M. Krauskopf, Michegan State University Advisory Team www.ipm.msu/greenhouse |
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