By Guest Author Mike Duus, Solar Innovations.
The ability to overwinter perennials from your garden in a greenhouse provides a self-sufficient alternative to repurchasing plants each year. But it is important to understand the quality and quantity of sunlight that your greenhouse location receives during winter months to achieve optimal results for your plants.
Winter light requirements for greenhouse plants.
Greenhouses located in the northern hemisphere at middle latitudes or higher must adjust to the decreased photoperiod (duration of sunlight) and light intensity (amount of sunlight per area) in the winter. During the winter, the sun’s path across the southern horizon becomes much lower and shorter, reaching its minimum on December 21. To correspond to the light shortage, the greenhouse’s site should be oriented to have as much solar exposure as possible. The arrangement of the plants inside a greenhouse should also be adjusted to maximize their exposure to sunlight.
When evaluating your winter greenhouse, the first step is to identify the light requirements of the plants you will be growing. Plants have varying light requirements from full shade to full sun. Understanding these requirements and how the sun influences your greenhouse will help you devise a proper plan for the arrangement of your grow space.
Full sun plants should be located along a southern glazed wall, allowing them to receive as much direct sunlight as possible. These plants will shade the plants located behind them, which should benefit any partial shade plants. Plants designated as full shade should be located near the north wall of the greenhouse or below benches to provide an ideal environment. The angle of the sun and how it casts shadows should be observed during winter months. Because the winter sun is at a lower angle, more space may be needed between full sun plants to compensate.
Supplement winter sun with grow lights.
If you wish to produce plants at the same rate during the winter as you did in the summer, grow lights will be required. By using grow lights, gardeners can extend the shorter winter days, allowing plants to receive appropriate amounts of light and optimizing productivity. When growing vegetables or fruit, grow lights can be adjusted to provide plant-specific lighting to enhance the growth process.
The most commonly used grow lights are high-intensity discharge (HID) lamps that feature high-pressure sodium (HPS) bulbs and metal halide (MH) bulbs. HPS is used to enhance fruiting, while MH is used to enhance vegetative growth. High-output fluorescent and LED grow lights are increasing in popularity. The grow lights use a fraction of the energy and put off less heat, allowing them to be positioned closer to the plants.
Through proper planning and light study, you can successfully overwinter your perennials in a greenhouse during the winter months, even in the northeastern United States.
Mike Duus works for a greenhouse manufacturer, Solar Innovations, Inc. He has spent twelve years researching and designing greenhouses, with a concentration on passive solar design and energy efficient strategies.