10 Ways to Manage Greenhouse Temperatures

Estimated reading time: 12 minutes

The first thing anyone notices in a new greenhouse is how surprisingly warm it gets even on the coldest days. All it takes is a steady stream of sunlight entering through the panels or plastic sheeting. It’s the result of passive solar heating and it’s a pleasant surprise when it first happens.

But inevitably, the nights get colder particularly in early and late season and the temperatures in the greenhouse plunge, sometimes below freezing. Without the sun a greenhouse is at the mercy of the outside temperature. The result is a wild fluctuation of high temperatures during the day and very low temperatures at night.

It can lead to a mild panic if plants are growing in the greenhouse in early spring and a cold spell moves in. That’s a bad time to have to figure out temperature control for the first time.

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There’s also the challenge of managing summer heat. Inevitably we all walk into our greenhouses and wonder how the temperature shot up to 100 degrees and what to do about it when opening doors and windows don’t seem to help too much.

We’re going to cover a range of possible solutions to managing greenhouse temperatures. Some are inexpensive and off-grid. Others are more robust and require some investment and time. Which approach you take depends a lot on the size of your greenhouse; the local weather patterns, how ambitiously you use your greenhouse and the end result.

Obvious Solutions

The obvious solution in cold weather is to provide some heat and some people pull out an extension cord and power up a small 1500 watt space heater. That works and if it has a fan it will do a good enough job to keep temperatures above freezing.

The problem with that solution is that it will become very expensive in a short period of time, and some of us have remote greenhouses that are out of reach of most extension cords.

Many greenhouse owners have faced this dilemma and a variety of solutions have emerged to keep or create the heat needed to protect plants from freezing in a greenhouse. Just as important, other off-grid solutions have emerged to keep a greenhouse cool during a long, hot summer.

12 Off-Grid Solutions to Greenhouse Heating and Cooling

1. Black rain collection barrels

If there’s one design element that makes nothing but sense in a greenhouse it’s the addition of a rain water collection system with a black barrel inside.

The roof of the greenhouse can be used to collect the rain and small gutters and some pipes can channel the water into the barrel. This will give you a ready and hopefully steady supply of water for your plants, and rain water is actually some of the best water for gardening.

But there’s a big advantage that goes beyond watering. A large, black rain barrel will absorb the sun’s rays and heat up. The water in the barrel will retain the temperature and as night falls and temperatures drop, the heat from the barrel will radiate into the greenhouse.

A lot depends on the weather, but on average a single 50-gallon rain barrel can help maintain an additional 5 to 10 degrees Fahrenheit in a greenhouse overnight.

If you have the space, you can add additional barrels to keep things warm when temperatures drop.

Here are some links with various approaches to designing and installing a rain water collection system:

2. Additional water containers

If you don’t have room for a rain barrel or additional barrels you can spray paint small water containers black and store them in your greenhouse. The black color absorbs the most heat from the sun and the water will hold the heat.

Smaller containers alone won’t make a huge difference but if you have a few of them, they’ll certainly help. Better yet, paint some 5-gallon buckets and the lid black and find a place for it in greenhouse.

3. Stone and/or brick flooring

This is the foundation of radiant heating used in some home. A floor in your greenhouse made from stone, brick or other rocks will absorb heat during the day and continue to radiate it into the greenhouse at night. It’s a passive solar system but most of the these ideas fall in the category of passive solar and they all help to maintain and hold temperatures into the night.

There are a variety of ways to use stone, bricks and even boulders to hold heat in a greenhouse shared in these links:

4. Proper insulation

Make sure there are no gaps or tears in your greenhouse. This is especially true towards the foundation, corners and around windows or vents. You’ll need ventilation as the season warms up, but you want to manage the ventilation.

One solution to proper insulation is the use of polycarbonate panels for the walls and roof. They’re double layered polycarbonate plastic and the gap in the layer adds a level of insulation. You could also add a second sheet of plastic both outside and inside the framing.

Here are some links with additional thoughts and ideas about how to properly insulate a greenhouse:

5. Terra Cotta Candle Heater

This is a classic heating solution for emergencies and in a greenhouse it can become an everyday alternative for off-grid heat.

The candle heater uses two clay or terra cotta pots with tea candles underneath to generate heat. It’s surprisingly effective and in a pinch can get you through a night of below freezing temperatures.

The tea candles are cheap and after the cold season is over you can use the terra cotta pots for plants in the greenhouse.

Here are some links with more information about how to design and construct improvised, rustic heaters.

6. Vented compost heap

Every compost heap creates heat as organic matter decomposes. It would seem like an obvious solution is to put a compost heap inside the greenhouse, but the smell can be overpowering.

A good solution is to have a compost heap setup next to the greenhouse and to insert some hollow pipes into the bed of compost. The pipes then are directed into the greenhouse and a constant flow of warm air will emerge from the open ends of the pipe in the greenhouse. The end of the pipe in the compost heap should be sealed so you don’t funnel odors into the greenhouse with any heat.

Another advantage of having the compost heap outside but nearby is it not only gives you easy access to compost, but saves space inside.

Here’s a lot more information about composting as a heat source:

7. Small wood-burning stove or vented propane heat

Any type of heat generated by wood, propane gas or any other fuel needs to be vented through some sort of chimney. A greenhouse is a highly confined atmosphere by design and carbon monoxide and other harmful gases can quickly build up.

Heaters or stoves fueled by wood or other fuels also can become very hot so the location and placement is critical. This type of heat is probably not the best idea for a small greenhouse but if you have the space and live in a part of the country with challenging winters it’s a good option to consider. Just remember that any fuel powered heat must be vented outside.

Here are some links with additional information about heating a greenhouse with fuel-powered stoves and heaters:

8. Geo-thermal heating

This is a very ambitious heating solution that requires a good amount of digging and planning. Once it’s set up it’s self-sustaining and is used around the world to both heat and cool dwellings and greenhouses.

There are various approaches the either use the ground or water to heat or cool a space. It will not make any space toasty warm but it’s biggest advantage is that it will always keep an area above freezing.

Here are links to more information about how to design and build a geothermal system:

9.  Solar powered heater

Powering a heater with solar power is actually a bit more challenging that it sounds. The problem is that many portable electric heaters require a significant amount of wattage. Most run at 1500 watts and that would require a significant number of solar panels and fairly large solar generator or battery bank.

One solution is to use a ceramic heater which usually runs at about 500 watts. That could be more manageable for a small solar array.

Another challenge is location of the solar panels. Many solar setups recommend a rooftop location for solar panels. Putting solar panels on the roof of your greenhouse contradicts the idea of a greenhouse. You want the sunlight flowing in not blocked by something like solar panels.

A good solution is setting up solar panels on the ground next to the greenhouse. They can charge the generator or battery bank and you won’t lose any sunshine as a result.

Here’s more information about solar power setups with some greenhouse specific information:

10. Proper Ventilation

Eventually the cold nights of spring come to an end and summer is upon us. That’s when a new challenge emerges in any greenhouse –the heat.

There are variety of ways to manage high temperatures in a greenhouse and one of the most direct ways it to simply open a window. The best windows for greenhouse ventilation are installed in the roof and they can be opened to varying degrees to allow the heat to both escape and draw air through open doors or side windows to further vent excess heat.

Here’s more info on various ways to manage the heat of summer in a greenhouse.

Solar Powered Ventilation Fans

Small fans don’t require a lot of power, and small solar panels can easily keep some greenhouse fans blowing all day and into the night if they have a rechargeable battery pack.

Greenhouse fans are particularly important for large greenhouses and for locations where summer heat is excessive. These days that can be just about anywhere.

Bluetooth Thermometer and Humidity Gauge

It makes a ton of sense to have a thermometer in the greenhouse. Some very good solutions that have shown up include digital thermometers with humidity hydrometers that both display and track temperatures and humidity over time. The best are Bluetooth enabled so you can check how the greenhouse is doing from any location on your property.

Here’s more about temperature management in a greenhouse:

Combine Solutions – Don’t Depend on Just One

It’s easy to combine many of these solutions to manage temperatures in a greenhouse. It could start with a black rainwater barrel supplemented by some 5-gallon buckets of water painted black. A terra cotta candle heater could help on cold nights, stone or brick on the floor will hold some radiant heat and a vented compost heap could add to the effort.

All of those ideas are fairly easy to implement and have additional benefits beyond temperature management.

If temperatures are a sever challenge you may have to consider some level of investment and time to add options like ceramic heaters, solar panels, solar fans or even a geothermal setup. It’s all a question of seeing how your temperatures flow and how well any solution helps.

Watch and Anticipate Coming Weather Patterns

This is easy to do and most smart phones have weather apps with links to 10-day forecasts. Keep an eye on what’s coming so you can anticipate the best way to hold temperatures or manage ventilation for high heat.

Stage Greenhouse Plantings Based on Average Temperatures

Another advantage of looking at coming forecasts is managing what you put into your greenhouse and when. Some plants are more cold tolerant and others can handle a heat wave better than others.

Ideally you have a blend of plants and you moderate temperatures so all of them stay healthy and protected. It’s just a matter of knowing the different ways to keep your temperatures stable and anticipating when to implement those measures.

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