Is Your Greenhouse Too Cold? Here’s What To Do


Many gardeners choose to have a greenhouse to allow them to grow exotic plants or grow fruit and vegetables all through the year. In certain places, the winter can be icy, causing the temperature in the greenhouse to drop significantly. When is a greenhouse too cold, and what can be done about it?

Greenhouses become too cold when the outside temperature drops and there is insufficient sunlight to warm the greenhouse. The minimum temperature varies according to the plants, but a minimum temperature of 55° F is generally acceptable. Heat the greenhouse using electrical, gas, or wood heaters. Use compost and water barrels to add heat.

Adding a greenhouse to your garden can be very rewarding and allow you to spread your gardening wings. Some greenhouses can even be used to extend a family’s living space and include areas for recreation. Unfortunately, sometimes winters can be harsh and lower the temperature to unacceptable levels in the greenhouse. There are several ways to combat this.  

When Is A Greenhouse Too Cold?

The required temperature in a greenhouse is dependent on what plants you are growing in your greenhouse. Generally, greenhouse temperatures should be at approximately 80° F to 85° F.

  • Tropical plants can deal with a minimum temperature of 55° F.
  • You should check the minimum temperatures the plants can deal with, and this will give you a guideline on whether your greenhouse is too cold.
  • It is useful to note that plants require different temperatures at different stages of their life cycle.
  • Flowering and fruiting often need higher temperatures than other stages in the plant’s life.
  • Germinating seeds also need specific heat requirements.

Cold Spots In The Greenhouse.

It is quite common for greenhouses to have areas that are cooler, and this should be closely monitored. Installing multiple thermometers or using a moveable thermometer will allow you to check all areas of the greenhouse. 

Why Would A Greenhouse Be Too Cold?

Greenhouses can become too cold in freezing climates, especially in far northern areas. The winter temperatures in these areas fall extremely low. There is a limited amount of sunshine or daylight hours to warm the greenhouse during the day.

In some climates, winter is accompanied by cloudy overcast skies, which means that the greenhouse cannot warm up from the sun’s rays. There are times when the temperatures in greenhouses can drop so low that the greenhouse plants die from frost damage, defeating the purpose of having a greenhouse.

Here’s a great video with some tips to keep a greenhouse warm, with more info below:

What Should Be Done About A Cold Greenhouse?

There are two approaches to warming a greenhouse. The first is to ensure there is adequate insulation to prevent heat loss through the outer covering of the greenhouse. The second is to provide a heating source to raise the temperature.

Insulating A Greenhouse.

Greenhouses come with a variety of covering materials. They may be made of glass or polycarbonate sheeting. These two materials preserve the heat much better but may still need extra insulation in frigid climates.

  • Other greenhouses have some form of plastic sheeting, and these greenhouses are prone to losing heat quickly.
  • One of the easiest ways to improve insulation is to layer the inside of the greenhouse with horticultural bubble wrap.
  • The bubbles entrap warm air during the day and release it at night when the temperature drops. 

Frost Fabric (link to Amazon) or cloth is another material that may be used to insulate a greenhouse. Frost fabric is also known as row cover or reemay and is made from polyester or polypropylene.

  • Frost fabric is usually used to cover a row of plants or individual plants.
  • Sometimes gardeners use it to line the inner surface of a greenhouse to improve insulation.

How To Heat A Greenhouse.

Greenhouse heating can vary widely depending on the budget and how eco-friendly the gardener wishes to be. Some professional greenhouses have large electric space heaters governed by automatic thermostats.

You can find decent Electric Space Heaters Here (link to Amazon) for under 50 dollars that will help keep a greenhouse warm.

The temperature is consistently monitored by computer systems which switch the heating on and off as needed. These systems can be adapted to fit smaller backyard greenhouses, but they come with a hefty price tag and electricity costs.

  • Some gardeners dispense with the automated computer system and switch the electric space heater on themselves.
  • This can work well but must be monitored carefully to ensure plants do not overheat. 

Portable gas heaters and heat lamps are often used to warm the greenhouse. They can be positioned in cold spots in the greenhouse to raise the temperature.

I’ve seen people have success with propane fueled Patio Heaters (link to Amazon) but you have to keep an eye on the propane tanks and switch them out when needed.

Wood stoves can be used as a cheap alternative if you have access to wood for fuel. These stoves can also burn compressed biomass pellets.

Here’s a quick video showing a basic heating setup for a greenhouse:

It is essential to monitor carbon dioxide and carbon monoxide levels in the greenhouse and allow for ventilation if you are burning wood. These gases can quickly build up to toxic levels, which can be hazardous to people and detrimental to the plants.

For some gardeners, greenhouses are used to extend their living space and allow them to enjoy nature during the cold winter months. They may install a hot tub that serves the purpose of heating the greenhouse while providing a place for recreation for the family.

Check out our in detail article about Hot Tubs In Greenhouses for more info on that.

Some gardeners are convinced that heating the greenhouse space is a wasted effort and it is best to heat the soil, or some prefer to do both. The ground can be heated using soil heating cables.

These water-resistant cables are laid just under the surface of the soil and are governed by a thermostat. The thermostat keeps the soil at a constant temperature. Soil heating cables are very popular for germinating seeds.

Placing a fan in a greenhouse will ensure that all areas of the greenhouse are warmed. It helps to prevent cold spots and circulates the air to ensure the health of the plants.

Some Eco-Friendly Methods To Heat A Greenhouse.

Compost generates a lot of heat as it decomposes.

  • Some gardeners will make piles of compost in the greenhouse or create a heat sink.  
  • To create a heat sink, dig a ditch in the center of the greenhouse.
  • Fill the ditch with compost to increase the temperature substantially in the greenhouse.
  • If you only need to increase the temperature a little, you can use bricks or concrete in the ditch.
  • Some people put pipes into the ditch, which lead up into the greenhouse to increase the heating ability of the heat sink.

Hotbeds are raised beds that have an underlayer of composting material.

  • The layer of compost is covered by planting soil, and the plants are planted into this top layer.
  • The decomposing compost warms the soil and plant, preventing frost damage.
  • This method has been used for many hundreds of years.
  • Popular composting material was ( and still is in many areas) straw and horse or cow manure.

Placing water barrels in the greenhouse adds thermal mass.

  • The water absorbs heat during the day.
  • Water has a high specific heat capacity and so can store a great deal of heat.
  • As the greenhouse temperature drops at night, the heat energy moves from the water into the surrounding air, heating the greenhouse.
  • To increase the heat absorption of the water, paint the barrels or containers black.

Greenhouse Design That Adds Warmth.

Greenhouses can be built to limit heat loss and reduce the need for heating. A greenhouse built underground with only the roof or a small section of the walls and roof above ground creates a well-insulated space to grow plants.

The soil at approximately four feet under the surface does not usually drop below 50° F to 60° F. This allows the greenhouse to stay warm in the winter.  The angle of the roof should be built to maximize absorption of the available winter sunlight.

Conclusion

Greenhouses can be rewarding gardening experiences allowing you to grow plants in seasons and climates that would typically prohibit it. Greenhouses can become too cold, especially in cold northern winters. They can be heated by various means depending on your budget and preferences. Designing your greenhouse correctly can limit the amount of heating required.

Robert Sampson

I'm Robert Sampson and I live in Colorado where I spend a lot of time in the backyard with my family either grilling, playing games and sports, or working on a project to make our backyard a better place to be.

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