A greenhouse can be placed in a shaded area, but this fundamentally alters its purpose. While a sun-drenched greenhouse maximizes solar heat and light for sun-loving crops, a shaded structure functions as a controlled environment for specialized plants. It must be viewed as a modified cold frame or a structure requiring technological intervention. The feasibility of a shaded greenhouse depends on managing the low light and unique climate conditions, which guides the selection of appropriate plants and necessary supplemental modifications.
Understanding Shade and Greenhouse Placement
Shade is not a single condition; it exists on a spectrum that significantly impacts a greenhouse’s potential. Deep shade, often found on the north side of a solid structure or under a dense evergreen canopy, means the location receives less than two hours of direct sunlight daily. Partial shade is a more forgiving environment, offering two to four hours of direct sun, usually in the morning or late afternoon. Dappled shade, which is filtered light passing through a light tree canopy, is generally the most desirable type of shade for many plants because it mimics a woodland floor environment.
The source of the shade also determines the seasonal light availability. Shade from a building or a dense conifer tree is constant throughout the year, limiting winter light just as much as summer light. In contrast, a deciduous tree provides a beneficial seasonal variation, blocking intense summer sun but allowing up to 600 watts per square meter of solar radiation through its bare branches in winter. Assessing the specific location by observing sun patterns throughout the day and across seasons is an important first step. This evaluation determines whether light augmentation will be a minor supplement or the primary light source.
Low-Light Tolerant Plant Selections
Plants that thrive in lower light conditions generally prioritize leaf production over fruit or flower development. These species have adapted to utilize photons more efficiently at lower light intensities. Leafy greens are excellent choices, including spinach, which thrives in cooler, shadier spots, and various lettuces such as loose-leaf and butterhead varieties. These crops often produce higher-quality, less bitter leaves when protected from intense midday sun.
Many herbs are also well-suited for a shaded environment, particularly mint, parsley, chives, and cilantro. These plants evolved as understory species and have lower daily light integral (DLI) requirements compared to sun-loving herbs like basil or rosemary. Ornamental plants, such as ferns, philodendrons, and peace lilies, which are naturally adapted to the humid, filtered light of a tropical understory, can also be grown. Selecting plants based on their low DLI needs ensures the limited available light is sufficient for their growth cycle.
Maximizing Available Light
Compensating for a lack of natural light requires a multi-faceted approach, with supplemental lighting being the most effective tool.
Supplemental Lighting
Modern LED grow lights are efficient because they can be tuned to emit specific wavelengths, such as blue light for vegetative growth and red light to encourage flowering, while producing minimal heat. The placement of these lights is carefully considered, often using adjustable hangers to keep them close to the canopy without causing heat damage. For deep shade locations, supplemental lighting must often run for 12 to 16 hours daily, effectively becoming the primary light source.
Reflective Materials
Reflective materials are a passive way to increase light efficiency by redirecting ambient light toward the plants. Interior walls, floors, and benches can be painted with a highly reflective white paint or lined with specialized materials like Mylar sheeting. These surfaces bounce stray light back into the plant canopy, ensuring that every available photon is utilized for photosynthesis. Avoiding tinted or heavily diffused glazing materials is necessary, as these would further diminish the already limited incoming light.
Advanced systems can integrate photosensors that automatically adjust the LED output based on the natural light levels, ensuring a consistent daily light dose without wasting energy on sunny days. This automation maintains the necessary Photosynthetic Photon Flux Density (PPFD) required for consistent growth regardless of external weather conditions.
Managing Internal Climate Conditions
A shaded greenhouse presents unique challenges regarding temperature and moisture control due to the absence of intense solar heating.
Temperature Management
The structure will experience significantly reduced solar heat gain, meaning it will run cooler than a sun-exposed greenhouse. While this can be beneficial in summer, it requires greater reliance on artificial heating in cooler months. The overall ambient temperature will be lower, potentially necessitating an electric or propane heating system if warm-weather crops are attempted.
Humidity and Disease Control
The greatest operational challenge in a shaded environment is managing humidity and mitigating the risk of fungal disease. Shaded areas retain moisture longer, and the lack of intense UV light eliminates a natural sterilizing effect, creating ideal conditions for mold, mildew, and pathogens like Botrytis.
Constant air circulation is a necessity, achieved through the strategic placement of horizontal airflow (HAF) fans that run continuously to prevent stagnant air pockets and reduce moisture condensation on leaf surfaces. Proper ventilation, involving both intake and exhaust fans, is crucial to exchange humid interior air with drier exterior air, reducing the relative humidity to below 70% to discourage spore germination.