Growing annuals in a greenhouse is a way to produce annual flower and vegetable plants in a regulated environment away from pests and extreme temperatures. When growing annuals in a greenhouse you will need to be able to regulate the temperatures so the plants will not suffer from extreme heat or cold. Also, you will need to provide moisture. Annual plants are sensitive to the amount of light they receive in order to produce flowers and vegetables. This is called daylight sensitivity, and if you are growing annuals in a greenhouse in the fall and winter when light levels are low or decreasing, you will need to increase light levels as needed for optimal production.
How to Grow Annuals in a Greenhouse
Start growing annuals in the greenhouse after Dec. 21 when the amount of daylight available begins to increase if you are unable to supply increasing levels of light.
Heat or cool the greenhouse as needed to keep temperatures in a 55 to 85 degree Fahrenheit range. Most annuals sprout in warmer temperatures but prefer to grow in slightly cooler temperatures. Tomato plants, for example, have trouble producing fruit in temperatures about 85 F or below 55 F but sprout well in soil between 85 and 90 F.
Keep moisture levels constant but avoid overwatering. Building a watering system that includes a timer is convenient but don't allow the plants to become water-logged. Keep in mind that different annual plants have different moisture requirements.
Monitor pest levels in the greenhouse by looking at leaves or using yellow sticky cards, which are cards covered with a substance that attracts pests. Insect pests are attracted to and will become stuck to the cards so you can see what pests have entered the greenhouse production area. Yellow sticky cards are widely available at nursery supply centers.
Plant your greenhouse plants in clean pots using clean potting soil from sealed bags. Potting soil stored outside or in an open bag will contain pests that feed on the organic material in the potting soil. You will bring these pests into the greenhouse by using the old potting soil and they are difficult to control once they are established.