A greenhouse is a specialized structure designed to create a controlled environment, often used to extend the growing season or cultivate plant species that require specific, non-native climates. This enclosed space allows growers to manage temperature, humidity, and light levels, offering protection from external weather fluctuations. While versatile, the stable, enclosed environment introduces limitations. Not every plant species thrives under these conditions; some have inherent biological needs that conflict with the typical warmth and consistency of the structure.
Species That Require Dormancy or Conflicting Climates
The primary conflict arises when a plant requires natural environmental triggers that are intentionally suppressed within the greenhouse setting. Temperate plants, particularly many deciduous fruit trees such as apples, cherries, and peaches, have a strict biological requirement for a cold period known as chilling hours. These hours, typically defined as temperatures between 32°F and 45°F, must accumulate over the winter months for the plant to break bud and flower successfully in the spring.
Maintaining the elevated, stable temperatures beneficial for tropical or subtropical crops directly prevents this necessary accumulation of cold units. Without sufficient chilling, these trees often exhibit delayed or uneven bud break, poor flowering, and reduced fruit set. Forcing a temperate tree into a constant, warm growing cycle disrupts its innate physiological clock, making it an impractical choice for a standard heated greenhouse environment.
Similar dormancy requirements affect many spring-flowering bulbs, including tulips and daffodils, which need cold stratification to initiate the biochemical processes leading to bloom. Attempting to force these bulbs without the requisite cold exposure often results in stunted growth or the production of only foliage without flowers. The stable warmth of the enclosure essentially bypasses the natural signal these plants use to transition from vegetative growth to reproductive cycles.
A different set of challenges arises when plants with opposing humidity needs are placed together within the confined space. Many popular greenhouse crops, like tomatoes, cucumbers, and tropical ornamentals, thrive in the high relative humidity often maintained to reduce transpiration stress. This moist air, however, is detrimental to species adapted to arid or semi-arid environments, such as desert succulents and cacti.
These xeric plants require well-draining soil and very low atmospheric moisture to prevent fungal infections and root rot. Placing them in a high-humidity environment alongside tropical species almost guarantees chronic moisture-related problems. The constant dampness inhibits the natural protective mechanisms of these desert plants, making them highly susceptible to Phytophthora and other water mold pathogens.
Plants with Unmanageable Size or Aggressive Growth
Physical space limitations make certain large-growing species unsuitable for non-commercial greenhouse structures. Plants like mature citrus trees, bananas, or large Ficus species can rapidly exceed the standard ceiling height of most residential or hobbyist greenhouses, which often range from seven to ten feet. Once a plant contacts the glazing, it becomes susceptible to scorching, and its growth is physically constrained, leading to poor form and reduced vigor.
Furthermore, large, woody specimens require containers substantial enough to support their mature weight and root mass, potentially exceeding the structural load limits of benches or flooring. The volume of soil required for a large, healthy root system also dedicates a disproportionate amount of limited space, displacing many smaller, higher-yield crops. Attempting to prune these species aggressively often stresses the plant and does not fully compensate for the lack of vertical space.
Plants that exhibit aggressive, spreading growth habits pose a threat to the entire enclosed soil system. Certain varieties of mint (Mentha species) or perennial vines, if planted directly into the ground or in large beds, spread rapidly via rhizomes or runners. These aggressive root systems quickly colonize the available substrate, outcompeting neighboring plants for water and nutrients.
The confined nature of the greenhouse soil makes it difficult to isolate and remove these invasive roots without physically disturbing the entire growing bed. Once established, aggressive spreaders can necessitate the complete removal and replacement of the soil medium, which is a major undertaking in a permanent structure. This aggressive growth habit is incompatible with the intensive, space-efficient cultivation required in a small greenhouse.
Cultivars Highly Susceptible to Greenhouse Pests and Diseases
The closed, stable environment that protects plants from the outside elements also creates a breeding ground for specialized greenhouse pests. Once introduced, these insects can multiply exponentially without the natural predatory pressures or climate fluctuations that typically keep populations in check outdoors. Certain cultivars act as reservoirs, significantly elevating the risk for every other plant in the structure.
Plants such as fuchsias, specific ornamental hibiscus varieties, and certain herbs are attractive to common pests like spider mites, whiteflies, and thrips. These pests thrive under the warm, protected conditions and can easily migrate from the susceptible host plant to adjacent vegetable crops or ornamentals. A single infected plant can quickly trigger a systemic infestation across the entire enclosure.
The high relative humidity often maintained to benefit tropical species can simultaneously promote the proliferation of fungal and bacterial diseases. Cultivars highly susceptible to diseases like powdery mildew or Botrytis blight become vectors under these conditions. Botrytis, often called gray mold, spreads rapidly in cool, humid air and can cause significant damage to flowers, leaves, and soft stems.
Overly crowded seedlings or certain low-light ornamentals, which are prone to damping off caused by Pythium or Rhizoctonia, pose a similar contagious risk. Because air circulation is often limited compared to the outdoors, fungal spores are easily trapped and distributed within the structure. Introducing a highly disease-prone plant creates a weak link, risking the biosecurity of the entire cultivated area due to the difficulty of localized treatment in an enclosed space.