Winter Growing in a 4-Season Greenhouse January 11, 2017 – Posted in: Blog – Tags:

Winter Growing in a 4-Season Greenhouse

Greenhouses are manmade micro-climates. They are mini ecosystems that capture the growing forces of nature and house them in an artificial bubble, mimicking the natural effects of sunlight, nutrients, water, heat, humidity and air movement. Just as in large-scale outdoor agricultural farms or even outside urban gardens, optimum plant growing conditions can occur within indoor greenhouses.

That includes greenhouses in huge commercial operations or on smaller, subsistence scales. It also includes winter growing in a greenhouse.

The very nature of a greenhouse is to modify climate for expanding the growing season. Edible plants and decorative flowers can be seeded much earlier in greenhouse conditions than in outdoor environments. This allows plants to be continually harvested in a year-round greenhouse, yielding more crops and opportunities than if left for nature to mature.

Multiple crops over a longer growing season equal more yield and more return, whether success means more profits in a commercial greenhouse business or more food for home and hobby applications.

Year-Round Greenhouse Growing

Year-round greenhouse operation is becoming increasingly popular in cooler North American climates, thanks to advancements in technology, more accessible horticultural knowledge and evolving plant species that are getting hardier and better able to withstand winter growing in a greenhouse. Today, many commercial and home gardeners are operating a year-round greenhouse that returns nearly as much produce in the cold winter months as in the hot and humid summertime.

A combination of the right winter greenhouse plants and the proper climate control is the key to successful winter growing in a greenhouse. Late fall to early spring used to be the time when green-thumbs patiently waited for ice and snow to leave, while they passed the time scanning seed catalogs and planning their spring plantings.

But now, greenhouse growing in the winter is becoming a popular way to produce more local produce options and enjoy the benefits of great-tasting, cold-hardy crops that require minimal labor in weeding, watering and pest control.

Planning crop plantings for optimal harvesting is a vital component for greenhouse gardening in winter. Cold season growing is slower to start and later to return than warm weather gardening, and this takes special knowledge of what winter greenhouse crops to plant and what conditions they need to survive.

While winter greenhouse gardening takes longer to show results, there is a tremendous trade-off in that winter greenhouse plants stay fresher far longer than vegetables raised during long, hot days and high-humidity nights.

Successful winter growing in a greenhouse requires a basic understanding of the science involved in greenhouse construction and climate control, as well as solid knowledge of the right types of winter-hardy plants that thrive in wintertime greenhouse conditions.

Winter Greenhouse Conditions

Winter vs summer greenhouse growing

Winter growing in a greenhouse has two significant differences from summer operation:

  • First is the available amount of natural sunlight — especially in the northern latitudes.
  • Second is the ambient, or existing, natural outdoor temperature.

Both of these factors are easily overcome by introducing artificial lighting in the proper ultraviolet scale that replaces sunlight, and by introducing artificial heat to bring the temperature up high enough to replicate ideal summer conditions.

But the expense associated with these processes defeats the purpose of working with nature to use existing winter conditions to make growing conditions perfect — e.g. by choosing plants better suited to shorter daylight hours. It also defeats, in the form of higher utility bills, any cost savings realized by raising food in a winter greenhouse, when taking into account the energy generated by using electric lights and some type of heating fuel source.

Working with nature, not against nature, is the philosophy behind greenhouse winter gardening. Crops chosen that respond to lower light levels and shorter days are a mainstay in planning a winter greenhouse, just as is choosing crops that withstand colder temperatures and greater fluctuations of relative humidity.

These conditions are controllable, however. Once the right plants are selected and planted within the proper seasonal rotation periods, the steps to success in winter greenhouse gardening focus on controlling the greenhouse climate to make it more suitable for cold-weather produce to prosper. It starts with understanding how cold-climate crops respond to the natural conditions that are being controlled in a greenhouse.

Understanding Plant Science to Achieve Optimal Conditions

In principle, plant science is straightforward. Plants use photosynthesis to convert sunlight into energy. They intake carbon dioxide and expel oxygen.

Plants also require hydration from water sources that supply them with nutrients. Water is drawn in through plants’ root systems and excessive water is discharged by a process called transpiration, where plants evaporate moisture from their leaves. This moisture cycle is critical for plant health, and it’s critical to manage it properly within an artificial greenhouse climate.

Too much humidity in a greenhouse is bad

Too much humidity in a greenhouse is just as harmful as conditions that are too dry. Plants that can’t transpire water will quickly go into distress and rot.

Each type of plant has their own specific requirements but, generally speaking, plants that thrive in winter conditions depend more on being exposed to the right temperature and humidity levels than lengthy hours of sunlight. It is achieving the proper balance of light, water, nutrients, temperature, ventilation, air movement and relative humidity control that presents a challenge for winter greenhouse gardening.

There’s an entire branch of science behind successful winter growing in a greenhouse, and it starts with appreciating how humidity, ventilation and temperature affect which plants that are suitable to grow in a winter greenhouse.

Winter Greenhouse Climate Science

Next to selecting plants that perform well in winter greenhouse conditions, controlling humidity through temperature and ventilation is the most important function in greenhouse management.

In most winter greenhouse situations, ambient or existing air temperature is determined by outside conditions, greenhouse design and an artificial interior heat source. Winter-hardy plants will survive cold air temperatures as long as their root systems aren’t subjected to freezing. Plant leaves will withstand short-term cool temperatures, but they are highly susceptible to standing water.

High humidity is the biggest threat to winter plant survival, and it is at cold temperatures where the air’s relative humidity is the most dangerous to plant health. High humidity in cold temperatures results in condensation forming on plants. Liquid water allows plants to freeze easily, and contributes to fungal pathogen pollutants such as spores, mold and powdery mildew.

Fortunately, relative humidity is easily controlled by reducing the amount of excessive water in the greenhouse and increasing the ventilation and airflow. They are intrinsically tied to each other.

Controlling Relative Humidity (RH)

Relative humidity is an often-misunderstood term. Relative humidity, or RH, is the ratio between the actual weight of water present in the air compared to the total water-holding capacity of the air at a specific temperature and pressure. Air temperature is the biggest factor that determines how much moisture air can hold without becoming super-saturated and giving off excessive water, usually as condensation. Pressure is a secondary factor.

Warm air holds more moisture than cold air, just as high-pressure air holds more moisture than low-pressure systems. The actual water content in the air is relative at any time to the amount of moisture the air is capable of holding at that particular air temperature and air pressure. By changing any one of the factors — available moisture, actual temperature or standing pressure — relative humidity is immediately affected.

70 degree air can hold twice as much water as 50 degree air

The RH ratio is presented as a percentage that changes depending on these factors. For example, warm air holds more water than cold air. Air at 70 degrees Fahrenheit can hold twice as much moisture as air measuring 50 degrees Fahrenheit.

This 20-degree difference describes an inverse proportion where, when the holding capacity of air doubles, the relative humidity is cut in half. Therefore, 70-degree air may have a relative humidity of 50 percent (depending on the available moisture), while the air at 50 degrees (with the same available moisture) has a 25 percent relative humidity.

This somewhat confusing formula is easy to understand when put into a winter greenhouse perspective.

Cold, wet air more quickly reaches its dew point, or condensation point, than does warmer, drier air. Winter greenhouse conditions where cold-weather, hardy plants prosper often reach close to the freezing point at night and then rise sharply from daytime solar gain. If more water is available than the cooler night air can hold, it is a scientific certainty that condensation will happen inside the greenhouse.

Just as rain is the result of outside air being at 100 percent relative humidity, the same effect happens inside a greenhouse that doesn’t have proper humidity and ventilation control. If the relative humidity level inside a greenhouse reaches maximum capacity and exceeds the dew point, water droplets begin to form on the coldest surfaces first. This is usually metal components like doorknobs, taps, pipefittings, metal shelves and glass.

Greenhouses constructed with glass windows, as opposed to ultraviolet ray-resistant plastic, can condense so quickly and completely that it is similar to a rain effect inside the greenhouse. This situation can worsen in winter greenhouses, where the air and surface temperatures are naturally lower by design. Dew droplets can fall from the upper surfaces and land on the plants, transferring water and reducing the plants’ natural ability to transpire water as part of their hydration cycle.

This can spell disaster for winter greenhouse plants that can’t naturally rectify the excess humidity and standing water by way of evaporation. Water-logged leaves and roots will go into distress and become vulnerable to disease and compounding effects from excessive wetness and cold temperatures. It can be even worse for plants that are grouped closer together and have no natural or mechanical means of exchanging the high-humidity air with drier air.

Recommended Relative Humidity and Temperature Levels

There are general relative humidity and temperature levels that are the most beneficial for plant health and disease control. “Relative” is the operating word.

These benchmarks should be used primarily to control greenhouse temperature, and secondarily to choose equipment to balance the air’s moisture content. Suggested optimum relative humidity percentage levels per degrees Fahrenheit are as follows:

Relative humidity chart by temperature

  • 32 F — 55% RH
  • 40 F — 68% RH
  • 50 F — 83% RH
  • 60 F — 88% RH
  • 70 F — 91% RH
  • 80 F — 93% RH
  • 90 F — 95% RH

Controlling humidity in winter greenhouses is a primary part of climate management. There are two common ways to achieve proper humidity control in greenhouses.

One is to increase ventilation, and the other is to increase air movement. The best solution is a combination of dehumidifying and ventilation.

Winter Greenhouse Dehumidifying and Ventilation

Controlling the relative humidity inside a winter greenhouse is the primary focus of solving problems associated with wintertime water and temperature conditions. Good winter greenhouse humidity control relies on three primary climate management factors that need to work cooperatively in order to achieve the right humidity level for productive growing conditions:

  • Temperature is the baseline for controlling humidity, and has the biggest influence on RH capacity. The nature of winter greenhouse gardening is to rely on low natural temperatures, so any attempt to artificially raise the greenhouse’s internal temperature is counterproductive and unnecessarily expensive. Keeping internal ambient temperatures low is where humidity control starts.
  • Ventilation is all about keeping airflow moving. Ventilation can be achieved through passively opening vents in the greenhouse walls, using the principle that warmer, wetter air rises and cooler, drier air lays low. Passive exhaust vents should be located at the ridge of the greenhouse ceiling, while intake vents should be located near the floor.

    This cross-flow of air is extremely important for achieving a transfer of wet interior air and replacement by drier air. Ventilation can also be achieved by using mechanical fans or air movers to push the air from bottom to top and to circulate wet, stagnant air that is trapped around plant leaves.

  • Water Management is the third and equally important component in controlling winter greenhouse humidity. Water management can easily be regulated by controlling the times and volumes of plant watering, properly spacing plants to allow drying airflow and ensuring the greenhouse floor is well-drained to prevent puddling.

Achieve Ideal Winter Greenhouse Conditions With Air Movers and Dehumidifiers

The best winter greenhouse climate management results from a systematic approach:

  • It starts with choosing the plants that perform best in naturally cool climates, where artificial heat will only be needed occasionally.
  • Proper water management is the next vital component, and includes ventilating the greenhouse to prevent unacceptable humidity levels.
  • Moving air is the simplest solution for keeping relative humidity levels safe, but sometimes simply moving air by passive or mechanical means is not enough for effective winter greenhouse gardening.

This is why an industrial dehumidifier may be the perfect answer for removing moisture from your greenhouse’s air and exchanging it with exterior air with the assistance from mechanical air movers. When passive air ventilation is simply not enough to control winter greenhouse humidity, dehumidifiers working in conjunction with air movers are the best devices to get the job done fast, effectively and affordably.

Aer Industries is proud to offer an extensive inventory of commercial greenhouse dehumidifiers from leading industry brands like B-Air© and BlueDri©. Conventional refrigerant dehumidifiers are suitable for smaller winter greenhouse applications, while larger operations are better served with low-grain refrigerant (LGR) dehumidifiers.

The best results in controlling humidity in winter greenhouses can be achieved when commercial dehumidifiers are paired with B-Air©, BlueDri© and Soleaire© air movers. Aer Industries has a large selection of professional, commercial-grade dehumidifiers and air movers to solve nearly any winter greenhouse humidity problem.

Our expert team members are available to provide guidance on dehumidifier capacities, specifications and price points for all of your climate control needs. Browse through Aer Industries products for winter greenhouse humidity control to find the right solution for successful year-round greenhouse growing.