The controlled environment of a greenhouse is ideal for growing orchids. Most of them bloom best with a consistent temperature change each day from about 78 degrees Fahrenheit in the day to 60 degrees at night and with a high relative humidity. Apply light, water and food at appropriate levels, and you'll mimic the natural environment for your orchids and reduce the risk that they will wilt or become diseased.
Fill a pot halfway with a potting medium for each orchid. Do not use potting soil with orchids because it will damage or kill the plants. Orchids require a potting medium only for the purpose of holding up the plant. Choose from among New Zealand sphagnum moss, peat moss, bark chips, coconut fiber, sifted perlite, granular charcoal, cork and rockwool. Place the orchid in its new pot and pour the potting medium around the plant. Pack the potting medium firmly around the orchid.
Set the humidifier in your greenhouse to 60 percent to 80 percent. Most orchids grow best in this environment. If the humidifier does not have a way to set the humidity this way, buy a hygrometer from a home improvement store to measure the humidity and adjust the humidifier until you get the correct percentage.
Set the thermostat at 60 degrees Fahrenheit during the night and 78 degrees during the day. Turn up the thermostat first thing in the morning and turn down the thermostat around sunset.
Mount small fans and run them throughout the day and night to expose the orchids to moving air. In their natural environment, orchids grow in breezes.
Watch the orchid leaves to tell whether the plant needs more or less sun. Most orchids' leaves should be light green, according to Orchids.org. If the leaves become yellow, or have bleached or brown spots or orange patches, move the plant to a shadier side of the greenhouse or a lower covered shelf. If the orchid leaves become dark emerald green, move the plant to a brighter spot such as a sunnier side of the greenhouse or a top shelf. During darker, cloudy months, supplement light by using grow lights. Suspend the grow lights 6 inches above the plants. Keep the lights on for 12 to 16 hours per day, turning them off at night.
Water orchids when the soil appears dry, if leaves start to wrinkle or bulbs start to shrink. Orchids endure being dry better than being saturated. Check the plants daily. Let the water run through the pot's drainage holes to wash away salt and mineral buildup.
Apply a water-soluble fertilizer to the orchids every couple of weeks. Purchase a fertilizer formulated for orchids from a gardening center and follow the package's mixing and application directions.