If you want to grow tropical plants, you’ll need a greenhouse capable of maintaining steady temperatures of 65 to 70 F. (18-21 C.) or higher. Most flowers, vegetables and herbs need temps of around 55 F. (13 C.), while a “cool” greenhouse is suitable for germinating seeds and raising starter plants in spring. A greenhouse with clear panels provides warmth and light for sprouting seeds, while a diffused or glazed covering provides balanced levels of light heat for growing plants to maturity. Additionally, build a greenhouse that is adequately ventilated and insulated to prevent plants from excess heat and cold.
You can grow nearly any plant you want in a greenhouse, but it’s all about determining the needs of particular plants. For instance, you can germinate seeds and grow starter plants in an unheated greenhouse, while tropical plants need consistently warm temperatures year round. You can easily start most flower seeds and cool season vegetables such as peas, broccoli and lettuce in an unheated greenhouse, but tomatoes and peppers need plenty of sunlight and warm temperatures between 55 and 85 F. (12-29 C.). Some flowers, such as impatiens and ferns, thrive in shade, while geraniums and daisies need bright sunlight.
In a cool northern climate like Montana, it will be difficult to grow herbs in a greenhouse unless you supplement heat. Most herbs will struggle when temps drop below 40 to 50 F. (4.5-10 C.), while many prefer consistently warm temperatures of 65 to 70 F. (18-21 C.) during the day and 55 to 60 F. (13-16 F.) at night. You can, however, get a jump on spring growing by starting many herb seeds in an unheated greenhouse, but you may need a heat mat for germination. Herbs that tend to be cold hardy include lavender, garlic, rosemary, winter savory, catnip and oregano.
Most plants prefer humidity between 50 and 80 percent. Foliage loses water and wilts rapidly when levels are below 50 percent, but high humidity can cause fungal diseases such as damping off, which causes tiny stems to rot at the base. If high humidity is a problem, you can provide a drier environment by watering plants from the bottom, and by providing adequate heat and ventilation. If humidity is too low, improve conditions by misting plants, with cool mist vaporizers or small humidifiers, or by placing pots on trays of wet pebbles.