Is it possible to grow anything in a greenhouse? It is often asked in garden talks, on allotment forums, and in the comments section of greenhouse buying guides. There is no doubt that the concept is appealing. Greenhouses suspend the usual rules of climate and seasons, allowing the gardener to decide what grows rather than relying on the weather. It seems like a workaround. It’s a reasonable assumption. In addition, it is only partially accurate, and when it is inaccurate, new greenhouse owners often question what they have purchased.
Horticulturists with significant experience will tell you straight out that greenhouses have their advantages and disadvantages. There are still ecological limitations. Often, plants that fail to thrive in greenhouses do so because their selection was made without knowing which constraints were altered and which were not.
Plants need to be matched to their surroundings, and the greenhouse is the equipment.
Temperature is the most obvious limitation, which can be applied in both directions. Early in the morning in a closed greenhouse, a bright April morning can reach 38°C or higher, which is hot enough to kill seedlings that were flourishing the previous day. Overheating dominates most first-season learning curves. It is also important to consider the opposite direction. Due to the structure’s retained heat, an unheated greenhouse is about the same temperature as the outside air in January. It is necessary to heat plants that require true cold protection throughout the winter, and the cost of heating increases with ambition. According to the RHS, heated greenhouses allow hobby growers to use them year-round, but they are seldom cost-effective. Before making a decision, you should read this honest assessment.
Humidity causes a second type of mismatch, which many growers are unprepared for. Greenhouses trap moisture. It benefits cucumbers, tomatoes, and other delicate crops, but harms plants that evolved in arid environments. Cacti and succulents are some of the least suitable plants for a greenhouse even though they are widespread, easy to maintain, and readily available. The humidity around a plant’s root zone causes rot to develop invisibly until the plant collapses. Ornamental grasses lose their upright structure when they are exposed to static humid air. Failures like these are not marginal. It is predictable that these results will occur when the incorrect plant is placed in an inappropriate microclimate.
Pollination is arguably the least discussed restriction among greenhouse owners, and it can have some of the most frustrating effects. Closed or semi-closed structures are not suitable for bees. There are no butterflies to be seen. In an outdoor garden, there is no natural pollination service. Self-pollinating crops, such as tomatoes, can release pollen by gently shaking flowering trusses every day. It is even possible to simulate a bumblebee’s vibration by using an electric toothbrush against the stem. In crops such as squash, melons, and other fruiting plants that rely on insect vectors to transfer pollen between flowers, hand-pollination results in failure to fruit. It appears that the plant is doing well. The flowers open at the right time. There is nothing set in stone. Knowing this before the season begins rather than halfway through saves a lot of confusion.
Another issue is scale. Vining plants that require 20 to 30 feet of running space include pumpkins, squashes, and some types of melon. It will overwhelm a small greenhouse before the season properly begins, shading everything else and obstructing airflow. The growth rate of sunflowers exceeds the capacity of most greenhouse structures. The root systems of big trees and shrubs do not work well in containers, which eventually leads to structural problems. There is no subtlety to these incompatibilities. Since they are size mismatches that quickly become apparent, it’s a little puzzling that they occur so frequently.
A greenhouse can extend the growing season for crops that require more heat than a temperate climate can consistently supply. The yields of greenhouse crops, such as tomatoes, peppers, chillies, aubergines, and cucumbers, are rarely comparable to those of outdoor cultivation in Britain or northern Europe. It is best to start hardy vegetables weeks before outdoor sowing is permitted. When light is returning but frost is still a possibility, seedlings should be protected. When outdoor cultivation ends in autumn, late salad crops can be cultivated. These genuine, significant benefits justify the investment in a structure. They don’t apply to everything, nor do they take into account the remaining limitations. They only apply to specific plants that grow in particular ways.
It appears that seasoned greenhouse growers’ confidence comes more from an understanding of the structure’s limitations than from the structure itself when watching them navigate their area. Rather than working against the greenhouse, a gardener should work with it. Crops that are suitable for enclosed pollination, warmth, and humidity must be selected. This requires acknowledging that some plants thrive outside, regardless of the weather, and that bringing them indoors leads to subpar results. Typically, failure was caused by growing the wrong thing in the wrong place.