Plastic Greenhouses: Warmer Climates, Better Yields

Greenhouses are invaluable for gardeners who want to extend the growing season. They can be significantly warmer than the outside environment, often by up to 30˚F. A small, plastic-covered greenhouse might see temperatures spike more rapidly on a sunny day compared to a larger, glass-structured greenhouse. The temperature difference also depends on the number of layers of covering. A single-layer greenhouse might show a temperature difference of 5 to 10˚F, while a double layer can be around 9 to 14˚F warmer. The materials of the greenhouse, such as the floors, also play a role in temperature control. During the winter, it is important to keep an eye on the forecast and provide extra protection to plants, such as covering them with a blanket or using heaters.

Greenhouse temperature control

The temperature of a greenhouse can be significantly warmer than the outside environment, often by up to 30˚F. A single-layer greenhouse might show a temperature difference of 5 to 10˚F, while a double layer can be around 9 to 14˚F warmer. The maximum temperature a greenhouse can reach is influenced by factors such as the type of greenhouse, its location, the materials used, and the prevailing external weather conditions. For example, a small, plastic-covered greenhouse might see temperatures spike more rapidly on a sunny day compared to a larger, glass-structured greenhouse.

To control the temperature in a greenhouse, it is important to understand how it traps heat. Sunlight enters the greenhouse and is absorbed by the objects and plants within, converting it into heat. This heat, in the form of infrared radiation, tries to escape but is trapped by the properties of the greenhouse materials, creating a warmer environment. This effect can be enhanced by using materials that have good thermal inertia, such as water containers and certain flooring materials, which store heat during the day and slowly release it at night, helping to maintain a warmer temperature inside.

To regulate the temperature, there are several methods that can be employed:

• Ventilation: External ventilation is crucial to prevent trapped air from losing energy and dropping in temperature. Rolling up plastic panels or opening screen windows during warmer nights can help maintain optimal temperatures.
• Insulation: Bubble wrap polythene, cut to size and fixed to the inside of the greenhouse, can reduce heat loss during colder months. Partial insulation is also an option, where only part of the structure is insulated to protect specific plants.
• Supplemental heating: Electric heaters, such as a 2.5-kilowatt electric fan heater, can be set to turn on when temperatures fall dangerously low. A propagator with electric bottom heat and thermostatic settings can also help maintain temperatures without providing an additional heat source.
• Soil warming: Simply warming the soil in a bed or propagation bench can be one of the most economical heating systems.
• Lighting: Supplemental lighting can extend the growing season, especially in regions with fewer sunny days.

It is important to note that during extremely cold temperatures, a greenhouse might not be able to provide sufficient protection from freezing. In such cases, extra protection for plants is necessary, such as covering them with a blanket or frost cloth, or bringing young seedlings indoors. Additionally, keeping potted plants drier can protect the roots and enhance their hardiness during colder periods.

Plastic vs glass greenhouses

When it comes to choosing between a plastic or glass greenhouse, there are several factors to consider. Both materials offer effective light transmittance, but they have distinct advantages and disadvantages that cater to different gardening needs and preferences.

Plastic greenhouses, often made from polyethylene or polycarbonate, are more affordable, easier to assemble, and more energy-efficient than glass alternatives. They are also more portable, making them ideal for renters or those who may move in the future. Polycarbonate greenhouses are also low maintenance and block harmful UV rays, creating a diffused light environment that promotes consistent light conditions. However, plastic greenhouses may not provide the full spectrum of light that glass does, potentially requiring additional accessories like grow lights. They may also not offer the same level of insulation as glass, especially those made from materials like PVC or polyethylene.

On the other hand, glass greenhouses are known for their durability, sustainability, and exceptional light transmittance. They offer a broader spectrum of light penetration, promoting better photosynthesis and healthier plant growth. The clarity and transparency of glass minimise light distortion, creating an optimal environment for plants to thrive. Glass is also flame-resistant, chemical-free, and will not discolour or fade over time. In terms of heat retention, glass greenhouses excel, effectively trapping heat while maintaining a stable and consistent internal temperature, making them ideal for colder regions.

While glass greenhouses are more expensive upfront due to material and installation costs, they offer long-term value with fewer replacements and lower maintenance costs. The decision between a glass or plastic greenhouse ultimately depends on specific needs, preferences, and budgetary considerations.

Greenhouse insulation

A greenhouse can be significantly warmer than the outside environment, often by up to 30˚F. A single-layer greenhouse might show a temperature difference of 5 to 10˚F, while a double layer can be around 9 to 14˚F warmer. The warmth of a greenhouse is a result of a balance of science and design. When sunlight enters the greenhouse, it is primarily in the form of visible light, which easily passes through the transparent materials, like glass or polycarbonate. Once inside, the objects and plants within the greenhouse absorb this light, converting it into heat. This heat, which is infrared radiation, tries to escape back but due to the properties of greenhouse materials, it finds it difficult to pass through. Thus, much of this heat gets trapped inside, creating a warmer environment.

To further improve the insulation of a greenhouse, one can employ a variety of methods. Firstly, the choice of cladding is important. Double-layered cladding, or twin-wall polycarbonate, offers one of the most prominent solutions. The thicker material improves insulation while still providing light diffusion and UV protection. The spaces between the two layers of polycarbonate trap air, producing air pockets that help prevent heat from escaping. The thickness of this material can vary between 4mm, 6mm, and 8mm, with the latter offering the highest degree of greenhouse insulation with an R-value of 1.72.

Additionally, growers can employ other insulating materials to improve heat retention. Horticultural bubble wrap has become a popular option, with clear material that can be used to line the inside of the greenhouse, increasing heat retention without impacting light transmission. Other materials such as Insulfoam, foam boards, and cardboard boxes lined with styrofoam can also be used to insulate the walls and cover plants at night.

Furthermore, "thermal batteries" like barrels of water can help mediate temperature, providing warmth in the spring and cooler temperatures in the summer. Proper ventilation is also key, as trapped air can condense and drop the temperature. Finally, external factors such as the location, size, and materials used in the greenhouse construction will impact its ability to retain heat.

Greenhouse heating methods

A greenhouse can be significantly warmer than the outside environment, often by up to 30˚F. The warmth of a greenhouse is a result of a balance of science and design. When sunlight enters the greenhouse, it is primarily in the form of visible light, which passes through transparent materials like glass or polycarbonate. The objects and plants within the greenhouse then absorb this light and convert it into heat. This heat, now infrared radiation, is trapped inside the greenhouse due to the properties of the structure, creating a warmer environment.

However, as the daylight hours are shorter in the winter, you will need to consider other methods of heating your greenhouse. Here are some innovative ways to heat your greenhouse during the winter:

• Hotbeds: Traditionally, a hotbed is filled with horse manure and straw, but you can use any compostable materials to create the same effect and generate heat. Hotbeds provide heat from below as the materials break down. After adding your compostable materials, top your hotbed with a mix of soil and compost. The ratio of heat-producing material to the growing medium should be 3:1, helping to achieve an ideal temperature of around 75 degrees F.
• Passive solar heating: This method involves collecting heat from the sun during the day and releasing it into the greenhouse at night. While this carries no energy cost, it may not be sufficient in extreme winter temperatures or snowy climates.
• Thermal mass: The most common thermal mass object used in greenhouse heating is water. Drums can be painted black, placed in direct sunlight areas, and filled with water. This water thermal mass method is also known as a heat sink. The water in the containers holds heat longer into the night, benefiting nearby plants. Another way to store heat is by using bricked pathways or adding bricks or stones to your greenhouse as they hold heat and can help heat up your greenhouse gently during the night.
• Insulation: Painting the south interior wall with white or reflective paint provides more sun to plants, while painting the other walls black will absorb more warmth. Adding a second layer of polycarbonate can also help hold warmth inside.
• Geothermal heating: This method involves capturing heat from the ground and carrying it to a heat exchanger. It requires burying piping at least 6 to 12 feet below the soil, through which either air or liquid travels.
• Bio-heating: Small animals like chickens and rabbits create body heat and manure that can be composted to warm the air in the greenhouse. They also produce carbon dioxide, essential for plant growth.
• Electric heating: Using Christmas lights or an electric thermostat plug can help heat your greenhouse.

Greenhouse ventilation

A greenhouse can be significantly warmer than the outside environment, often by up to 30˚F. A single-layer greenhouse might show a temperature difference of 5 to 110˚F, while a double layer can be around 9 to 14˚F warmer. A small, plastic-covered greenhouse might see temperatures spike more rapidly on a sunny day compared to a larger, glass-structured greenhouse.

Proper ventilation is a crucial aspect of maintaining a healthy and productive growing environment. It helps to regulate temperature, humidity, and carbon dioxide to oxygen ratios, all essential for plant growth. Excess heat is a major plant killer, and many plants are quite heat-sensitive.

There are two primary types of greenhouse ventilation: passive and active. Passive ventilation relies on airflow through vents, windows, and doors to regulate temperature and humidity. This method is energy-efficient and cost-effective, as it doesn't require the use of fans or other electrical equipment. Natural ventilation can be achieved through roof vents, roll-up sidewalls, and opening windows and doors.

Active ventilation systems, on the other hand, use fans and other equipment to actively circulate air throughout the greenhouse. This method offers more precise control over airflow, temperature, and humidity but can be more expensive to install and operate. Mechanical ventilation options include exhaust fans, circulation fans, and air exchange systems.

When selecting a ventilation strategy, consider the greenhouse's size and design, the local climate, and the specific needs of the plants. Larger greenhouses may require more powerful fans or additional vents to maintain proper air circulation. Greenhouses in hot or humid regions may require more aggressive mechanical ventilation methods, while those in cooler or drier climates may be better suited for natural ventilation.

Frequently asked questions