7.0 Greenhouse climate / Environmental control
Climate plays a major role in how well plants thrive year-round within a greenhouse. Climatic considerations influencing the function of a greenhouse include light, temperature and humidity. Being a greenhouse gardener requires careful monitoring of these climatic factors on a day-by-day and season-by-season basis.
Plant growth is dependent on photosynthesis, converting light energy into chemical energy, which means the most important consideration in designing and operating a greenhouse is light! As covered in the greenhouse location section, to maximize sunlight, greenhouses should be constructed with long-sides facing south-east to south-west. A good rule of thumb is that too much light is better than too little since it is far easier to block light and less expensive than adding lighting. Plants suffering from too little light will require the addition of a grow light or removal of objects that are blocking light from the greenhouse. Another strategy for boosting light intensity in a greenhouse is to paint objects white so they reflect as much light as possible.
The most common purpose of a greenhouse, and the second largest factor affecting its function, is to control temperature. The greenhouse is used to maintain optimal temperatures during the spring and the fall (avoiding frost) thereby extending the growing season and enhancing harvest yields. Keeping the cold out is clearly one of key purposes of the greenhouse, but during the day the greenhouse structure can interfere with temperature control by trapping heat.
7.2.1 Plant hardiness zones
The USDA puts out a very useful index map that identifies plant hardiness zones. Plant Hardiness Zones divide the United States and Canada into 11 areas based on a 10 degree Fahrenheit difference in the average annual minimum temperature. The United States falls within Zones 2 through 10. Suitable hardiness means a plant can be expected to grow in the zone’s temperature extremes, as determined by the lowest average annual temperature. It is used by gardeners to determine which plants they can grow in their particular area. For example, USDA zone 10 has mild winters while USDA zone 6 has freezing cold winters. Using a greenhouse you can increase the plant hardiness of your planting area, so those living in zone 5 can grow plants that would thrive in zone 8.
7.2.2 Maximizing the growing season
Plants grow over a wide range of temperatures. This range may be defined at three basic levels:
1) a minimum temperature below which no growth occurs
2) an optimum temperature at which the greatest growth occurs
3) a maximum temperature above which no growth occurs
Growth rate increases above the minimum temperature until an optimum is reached, then declines until the maximum temperature is reached. The minimum, optimum, and maximum temperatures varies widely among plant species.
As a general rule, greenhouse crops are grown at day temperature 5-10° F higher than night temperatures on cloudy days and 15° F higher on clear days. Therefore, optimum day temperature for growth generally decreases as solar radiant energy decreases. This should be kept in mind as the seasons change. Night temperatures may be 50-70° F; 60-65° F is a good starting point for warm season crops and 50-55° F for cool season crops. Plants generally grow better if the day temperature is warmer than the night.
During warm times of the year, dropping the day temperature close to the night temperature may not be possible throughout the day. Recent work has shown that a temperature drop or rise for 2-3 hours at the beginning or end of the light period has a strong effect on plant growth and length. In the greenhouse a temperature drop is usually applied by turning on the fans and/or opening vents 20 to 30 minutes before dawn then returning to a normal venting pattern 2 to 3 hours later.
7.2.3 Temperature regulation
Regulation of temperature is done through one or both of the following ventilation systems:
1. natural (roof vents, windows)
2. mechanical (fans).
Additionally, in colder regions greenhouses require additional heating during the winter months. Ventilation allows the exchange of the moist greenhouse air with the drier air from outside. Greenhouses are vulnerable to overheating from the spring until the autumn; overheating is one of the most common mistakes of a novice greenhouse gardener.
There are typically three places air leakage occurs in a greenhouse:
1. The door
2. The roof vents
3. The side vents
Side ventilation is less effective than roof ventilation. A rule of thumb is to have 1 sq m of ridge ventilation (roof ventilation) for every 5 sq m of floor area. This will provide the ventilation capacity of one complete change of air within the greenhouse every two minutes.
For effective ventilation monitor the greenhouse temperature with a maximum-minimum thermometer. If temperatures are allowed to exceed 27C plant damage is likely to occur. Automatic roof ventilation openers are an excellent way to regulate the greenhouse temperature if you are not at home. During hot days it is best to provide maximum side ventilation in case the roof ventilation opener does not start.
Shading is another way to reduce the overall heat within a greenhouse, unfortunately shading limits the light the plants receive, thereby reducing overall growth. Limiting the amount of shading when temperatures are below 27C is the best protocol. The types of shading include:
1. External blinds – they provide maximum cooling as they prevent the sun’s rays from entering the greenhouse
2. Internal blinds – not as efficient as external blinds since the light has already passed through the glass and generates heat, however they are much more easily automated and provide less interference with roof vents
3. Netting – mesh simply clips onto the greenhouse structure—the cheapest option
Humidity can be the most difficult environmental factor to control in a greenhouse. Relative humidity (RH) is the measure of how much water is dissolved in the air at a particular temperature expressed as a percentage. For every 20F rise in temperature, the water-holding capacity of the air doubles and the RH is reduced by one-half. For example, air at 70F will hold twice as much moisture as air at 50F. Plants accept a relatively wide range of humidity levels, with an optimal range between 45% and 85%. With RH levels below 45%, growth and water consumption issues are common; and at RH levels above 85% fungal problems are typical.
7.3.1 Maintaining optimum humidity
Good watering practices, adequate plant spacing, well-drained floors and good ventilation are the best ways to reduce humidity within the greenhouse. Water just enough to prevent excess water on the floor and water earlier in the day to allow plant surfaces to dry before evening (when condensation occurs).
Ventilation allows the exchange of moist greenhouse air with drier air from outdoors; heating brings the outdoor air temperature up to an optimum growing temperature and increases the capacity of the air to carry moisture (avoiding condensation). A combination of heating and ventilation is the optimal solution—a greenhouse with slight ventilation will allow heated air with high RH to escape from the greenhouse and be replaced with outside air of lower RH.
7.3.2 Determining relative humidity
The sling psychrometer provides one of the most accurate methods for determining relative humidity. Humidity pocket meters are also a good alternative.
The desirable level of humidity varies with temperature, the following table provides RH set points for disease prevention at various temperature intervals.