Sometimes temperatures are used in connection with day length to manipulate the flowering of plants. Chrysanthemums will flower for a longer period of time if daylight temperatures are 50°F. The Christmas cactus forms flowers as a result of short days and low temperatures.

Temperatures alone also influence flowering. Daffodils are forced to flower by putting bulbs in cold storage in October at 35 to 40°F. The cold temperature allows the bulb to mature. The bulbs are transferred to the greenhouse in midwinter where growth begins. The flowers are then ready for cutting in 3 to 4 weeks.

Thermoperiod refers to daily temperature change. Plants produce maximum growth when exposed to a day temperature that is about 10 to 15°F higher than the night temperature. This allows the plant to photosynthesize (build up) and respire (break down) during an optimum daytime temperature, and to curtail the rate of respiration during a cooler night. High temperatures cause increased respiration, sometimes above the rate of photosynthesis. This means that the products of photosynthesis are being used more rapidly than they are being produced. For growth to occur, photosynthesis must be greater than respiration.

Low temperatures can result in poor growth. Photosynthesis is slowed down at low temperatures. Since photosynthesis is slowed, growth is slowed, and this results in lower yields. Not all plants grow best in the same temperature range. For example, snapdragons grow best when night time temperatures are 55°F, while the poinsettia grows best at 62°F. Florist cyclamen does well under very cool conditions, while many bedding plants grow best at a higher temperature.

Buds of many plants require exposure to a certain number of days below a critical temperature (chilling hours) before they will resume growth in the spring. Peaches are a prime example; most cultivars require 700 to 1,000 hours below 45°F and above 32°F before they break their rest period and begin growth. This time period varies for different plants. The flower buds of forsythia require a relatively short rest period and will grow at the first sign of warm weather. During dormancy, buds can withstand very low temperatures, but after the rest period is satisfied, buds become more susceptible to weather conditions, and can be damaged easily by cold temperatures or frost.

Oregon State University, News and Communication Services
EFFECTS OF FREEZING WEATHER ON LANDSCAPE PLANTS

Review of Temperature Effects on Plant Growth:
Photosynthesis: Increases with temperature to a point.
Respiration: Rapidly increases with temperature.
Transpiration: Increases with temperature.
Flowering: May be partially triggered by temperature.
Sugar storage: Low temperatures reduce energy use and increase sugar storage.
Dormancy:Warmth, after a period of low temperature, will break dormancy and the plant will resume active growth.

Consumer Horticulture | Weather

Prepared by: Erv Evans, Consumer Horticulturalist