IDEAL TEMPERATURE AND LIGHT FOR PLANTS
Plants grow well in moderate temperatures between 21°-29° C. Both higher and lower temperatures slow the plants rate of metabolism and growth. Plants grow fastest when the temperature during the lit period is kept between 22°-26°C. When CO2 is being used, the plant prefers to be a few degrees warmer, between 26°-29° C. Plant varieties differ in their temperature preferences by a few degrees, so some experimentation is required to find the ideal temperatures for the type you are growing.
The relationship of light and temperature on plants
Ideal temperature is tied to light conditions. As more light is available, the ideal temperature for plant growth increases. Strong light and low temperatures slow growth and decrease stem stretching. Conversely, when plants are given high temperatures and only moderate light, the stems stretch.
Plants growing under lower intensity lamps (40-50 watts per sq ft, 430-540 watt per sq m) should be kept on the cooler side of the recommended temperature range. Plants growing under higher intensity lamps (60 watts per sq ft, 650 watts per sq m and higher) should be kept on the warmer end of the scale.
Night time temperature
During dark periods, the temperature can be kept as much as 12? C cooler than the lit period without any negative effects. Wider temperature differences cause slower growth, stretching and delayed ripening. Plants that are kept at a constant temperature are more probable to grow thicker, stronger stems and have denser growth. Plants that experience a large differential between day and night temperatures suffer from stretching and slowed growth rates.
Low temperatures are not cool
At temperatures below 15° C, photosynthesis and plant metabolism slow, stopping growth as it waits for better conditions. As soon as the temperature rises, the plant resumes full functioning. When the temperature falls below 4° C, most plants experience tissue damage and require about 24 hours of warmer conditions to resume growth. Young plants are somewhat tolerant of lower temperatures; when outdoors, seedlings have been known to pierce snow cover without ill effect. But low temperatures during ripening, even just overnight, delay or prevent maturation. Some equatorial varieties stop growth after a few nights with temperatures below 4° C.
Planning your garden for South East Queensland
Around Brisbane, try planting these in colder months: broad beans; peas; lettuce; English spinach; silverbeet and beetroot; carrots; onions and garlic; cabbages; broccoli; kohlrabi and turnip. In frost free areas you can also grow regular beans; tomatoes; capsicums; eggplants; zucchini; squash; and cucumber. As the weather warms, a wide variety of summer vegetables can be planted. Try tomatoes, capsicum and eggplants, beans, cucumber and pumpkin, beetroot and silverbeet, cabbage, carrots, lettuce, radish. In the hotter months, try planting tomatoes, capsicums and eggplants; lettuce (heat-tolerant varieties); sweetcorn; radish; silverbeet; beans; cucurbits; sweet potato.