Humidity: Greenhouses that feel like rainforests don’t produce sturdy, healthy plants. Relative humidity should be close to 70 to 85 percent during high-growth periods. At levels of 90 to 95 percent, plant growth is weak, early bolting occurs, and fungal diseases become a real problem. Decrease humidity levels by venting or exhausting humid air and watering only when necessary. Growers in arid climates can increase humidity levels in the greenhouse by spraying water on the floor.
Light: Light levels in a greenhouse are partially determined by the design. When planning a greenhouse, check shade patterns from roof overhangs at the summer solstice in June and modify the plan if the shade is too great.
Fluorescent lights are very useful when you’re growing spring seedlings, particularly in cloudy regions. They can also give a boost to midwinter greens and the last of the fall-fruiting crops. Ornamentals will also be healthier and more attractive with supplemental lighting. Shop-light setups suspended over raised benches are easy to install. You can choose fluorescent bulbs designed for plant growth, bulbs that mimic sunlight, or simply pair cool and warm bulbs in your fixtures. For intense light in a smaller area, another option is a compact fluorescent setup.
For plants like sun-loving orchids and tomatoes that require the equivalent of direct sunlight, you can also set up HID (high-intensity discharge) lighting systems with special bulbs. These systems are costly, though, and tend to give off a great deal of heat, so they’re typically used by specialists and professionals rather than people with home greenhouses. Most plants will do fine with typical greenhouse light and supplemental fluorescent lighting.
Whatever system you choose, adding a timer will give you control over the amount of light your plants receive without having to worry about turning the lights on and off manually. Timers can also control automated watering and mist systems, fans, heaters, and other equipment.
Don’t forget that plants can receive too much light as well as too little, especially in summer. Special greenhouse shade fabric panels are available in many sizes, as well as lengths you can order or cut to fit. You can choose from screening fabrics that will provide light shade to heavy shade, or protect plants from both too much light and heat buildup. Typically, you attach them over the greenhouse roof, though in areas of intense light you can choose a size that will cover the upper part of the walls.
Another option is to use shading paint developed for greenhouses. You can find paint that dries white, but becomes transparent in rainy weather to let in more light. Make sure you choose a paint specially developed for greenhouses so you can wash it off before winter.
Soils and fertility: Commercial growers sometimes amend the soils under their greenhouses and plant right in them. Home greenhouse growers usually find it easier to use benches with individual pots set on them or growing beds filled with a soil mix.
Soil mixes for containers, benches, and beds should be lighter and more fertile than most garden soils. Good soil mixes drain fast, hold moisture well, contain balanced and slow-release organic nutrients, and have a slightly acid pH.
If you buy a potting mix, make sure you choose one that’s organic. There are many options available in garden centers and from garden supply catalogs and websites. If you choose to make your own potting soil, a basic recipe is 2 parts soil, 2 parts finished compost, 1 part peat moss, and 1 part vermiculite or perlite. If the soil is clayey, add sand; if it’s too sandy, use vermiculite instead of perlite and increase the proportion of peat moss. Test the pH and adjust it if necessary.
Add compost and other amendments such as vermiculite each spring and fall. Good mid-season fertilizers include compost tea and side dressings, earthworm castings, liquid fish emulsion, and seaweed. Foliar feed plants by spraying leaves with dilute compost tea, nettle tea, or liquid seaweed for extra nutrients and some disease resistance. Fertilize less in winter, when cool soil temperatures inhibit microbial activity.