Greenhouse Functions And Controls 8265r

Greenhouse Functions and Controls

Horticulture II

Greenhouse Basics Plants grow naturally outside, so why do we need a structure to grow them in?  What is the purpose/function of a greenhouse? 

– The basic function is to provide a protective environment for crop(plant) production.

Greenhouse Environment 

What things make up the greenhouse environment? – Temperature – Moisture – Pest Control – Nutrition

Temperature 

Different plants have different temperature “preferences” for optimum growth. – Some plants prefer cool or even cold temps – Some plants prefer warm or hot temps

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The trick is to provide a temperature range that is conducive to plant growth. – Grow plants together that prefer the same temperature range.

Moisture 

Just as temperature matters, so does moisture.  Some plants need dry environments, while others need very wet environments.  How are moisture levels controlled? – Watering – Humidity • Level of water vapor in the air.

Pest Control One of the biggest problems growers face is pest control.  What types of pests? 

– Insects (aphids, whiteflies, etc) – Diseases (fungus, bacteria, viral) – Weeds (oxalis, henbit, etc) – Rodents (mice, rats)

Nutrition 

Plants, like animals, need nutrients to survive.  Growers provide plants with the nutrients they need by supplementing either the water or soil with added nutrients.  Growers also have to ensure adequate ventilation. – Carbon dioxide

Greenhouse Operating Expenses 

In addition to the cost of plants, pots, soil, fertilizer, etc, growers also have to deal with two other MAJOR expenses. – Labor – Heat

Labor is usually the greatest expense.  Heat is usually the second highest expense. 

What is the greenhouse effect? Lets try a demonstration to find out.  You will need: 

– Scissors – Plastic wrap or bag – Popsicle sticks – Glue – Tape

Building your “Greenhouse” Attach your Popsicle sticks together to build a small house or box.  Cover the entire structure with plastic, leaving the bottom uncovered.  Tape the plastic to the “greenhouse” 

The Greenhouse Effect Place your completed greenhouse on the window sill.  Note the temperature on the thermometer by the window.  At the end of the period we will put the thermometer in your greenhouses to see what happens. 

Environmental Control In order for a greenhouse to be effective, the greenhouse environment has to be closely controlled.  What happens if the environment is not controlled?  How do we ensure that the environment is kept constant? 

Environmental Controls 

In a greenhouse we use environmental controls to manipulate and control the environment. – Thermostats • Temperature

– Humidistats • Humidity

– Photocells • Light

The Greenhouse Effect Summarized The greenhouse effect is the way greenhouses collect and store heat from the sun.  Radiant energy (the heat you feel when near something hot) warms the greenhouses (and whatever is inside of it). 

Heat Loss Greenhouses, while efficient collectors of heat, are not good at storing it.  Heat is lost through the greenhouse covering (called glazing) and additional heat may have to be provided. 

– Especially in winter.

Heat Control Heat, or more specifically temperature, has to be closely monitored and continually adjusted in a greenhouse.  Temperature in a greenhouse is monitored by a thermostat, which controls sensors to open vents, operate exhaust fans or intake fans which pull air through a pad cooling system. 

Thermostats 

Where should a thermostat be located? – Ideally they should be located at plant level for the best and most accurate control.

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Thermostat types – Aspirated – Thermistor

Aspirated Thermostats An aspirated thermostat utilizes a small fan which blows air continuously over the temperature sensing unit.  This helps to maintain a more uniform temperature. 

Thermistors 

A thermistor is a relatively new control that utilizes solid-state electronics.  These devices allow the grower to use computers to monitor and control the greenhouse environment.  Growers can set defined temperature ranges for certain time periods and the computers turn heat and cooling devices on and off as needed to maintain the desired temperatures.

Light Control 

Light levels have to be monitored in greenhouses to ensure that plants are receiving the correct amount.  Photocells measure light intensities and may relay that information to computers that turns on additional lights or activates a shade cloth to reduce light.  Timers may also be used to lights on or off at certain times.

Moisture Control 

Moisture in the air (humidity) should also be closely monitored for optimum growing conditions.  A Humidistat measures the relative humidity in the air and controls fogging or misting systems to regulate the humidity.  Humidity changes as heat dries the moisture from the air.  Moisture must be added by both watering and fogging or misting for most plants.

Summary The greenhouse environment has to be closely monitored so that plants grow in the best possible conditions. A plant that grows in an environment where temperature, moisture, light, nutrition, and pest levels are ideal and continuous will be as strong and healthy as it can possibly be.