Optimum Greenhouse Temperatures

Written by nicole faires
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Optimum Greenhouse Temperatures
A greenhouse is a valuable gardener's tool. (Metal greenhouse in summer with open door image by Scott Latham from Fotolia.com)

A greenhouse is a valuable tool that extends the growing season and expands the possibilities available to a gardener. The way a greenhouse does this is by trapping infrared rays from the sun, creating a warm environment perfect for plants even when it's cold outside. Ideally a greenhouse should stay below 29.4 degrees C, but the optimum temperature varies according to the plants being grown.


During the early spring a greenhouse generally stays an ideal temperature for young plants, somewhere between 15.5 to 23.8 degrees C. As it warms up, however, the greenhouse can reach temperatures over 37.8 degrees C very quickly. In the mornings it is usually still cool enough to keep the greenhouse door shut, but by afternoon some kind of ventilation is needed. Each plant has an ideal growing temperature. By monitoring greenhouse temperatures with a thermometer, it is possible to keep an optimum temperature by ventilating accordingly. Many people install a fan with a timer to create enough air circulation to keep temperatures down.


During the winter a greenhouse will stay a bit warmer than the outside temperature and will stay sheltered from the elements, but usually only the hardiest cold-weather varieties will be able to grow even in a greenhouse. It is possible to heat a greenhouse using a simple space heater, or hot water pipes that radiate heat and even a large compost pile can warm a greenhouse. These heating methods can warm the greenhouse enough to continuously grow many plants all year, if the temperature stays above 4.44 degrees C.


According to garden expert Linden Staciokas, writing for the website Northern Gardening, humidity over 80 per cent can permanently damage plants if sustained for too long. Keeping the greenhouse ventilated has the additional benefit of reducing humidity, which in turn can reduce the risk of disease. Diseases thrive in warm, humid conditions.


Stagnant air can cause plants to overheat. Keeping plants spread out for adequate air circulation and keeping the doors open in the afternoon can help prevent overheating. A fan is a very effective way of circulating air, and with a timer it is very convenient.


Pests and disease thrive in a warm, moist climate. Without good ventilation and air circulation, plants can bake, attract bugs and get destroyed by fungus. Checking your greenhouse every afternoon to ensure temperatures are under 29.4 degrees C and that humidity is under 80 per cent can prevent these issues from arising.

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