Greenhouses can extend your garden's growing season by weeks in the spring and the fall. In colder climates, a greenhouse can provide you with a way to grow more productive warmer weather crops. Sowing seeds and transplants in a greenhouse is a similar process to sowing them outdoors. However, successful greenhouse gardening requires special attention be paid to soil management, watering and ventilation.
Greenhouse soils almost always need to be amended. Be sure to incorporate compost and mulch into planting beds before planting your first crop. It is also a good idea to add compost each time you remove one crop and add another. Greenhouses tend to have fewer worms and other organisms to amend the soil naturally.
In the late winter, you can start seeds indoors. All seeds to germinate and grow for four to six weeks. Then transplant them to the greenhouse. Start with planting cool weather crops such as lettuces, mustard mixes, peas and arugula. The heat generated by the greenhouse will provide these spring crops with enough warmth to mature a few weeks earlier than if they were planted outside. In the summer, plant warm weather crops in the greenhouse to increase production. Tomatoes, cucumbers, peppers and basil are all warm weather crops that will benefit from the extra heat in a greenhouse. In the fall you can once again plant cool weather crops. Sow seeds directly into the greenhouse soil in the fall. They will have time to establish themselves before the weather grows too cold.
Plant crop rows from north to south whenever possible in a greenhouse so they receive equal amounts of sunlight as the sun rises and sets. Perhaps the most important factor in growing in a greenhouse is proper ventilation. Plant crops in double rows in individual beds. Between the beds leave a pathway. This allows you to easily access the plants for harvesting and weeding, but also increases airflow. The greenhouse structure should also allow for ventilation either on the sides, via the roof or both. It is also best to have doors on both ends of a greenhouse which can be opened for extra ventilation.
Drip irrigation in a greenhouse is best. Overhead watering will leave water on the plants that may not dry depending on the heat and humidity levels in the greenhouse. Wet plants are at an increased risk for disease. Since most greenhouse designs do not allow access to rainfall, it is important to check soil moisture levels often to prevent plant drought.
Due to the extra heat and humidity, weeds can be more abundant in greenhouses. Weed often so airflow around the base of plants is not impeded. Managing weeds also creates a more sanitary environment, helping to reduce disease. Weed carefully so as not to disturb crop roots.