We can make greenhouses from a variety of materials. Supporting frameworks range from iron, aluminum, and plastic, to wood and brickwork. Typical coverings include transparent fiberglass, acrylic materials, polyethylene, and traditional glass.
Regardless of the type of greenhouse you choose, there are many different ways you can heat it. Primarily, the size of your structure and budget will determine what you use. Greenhouse heating types include electricity, natural or bottled gas, fuel oil, hot water, and wood. Or you can design a passive-solar greenhouse that relies on energy from the sun or install solar-powered heaters.
Do greenhouses need to be heated during winter?
An unheated greenhouse is a warmer environment than growing plants outside unprotected because the heat stays trapped inside. But if you live in an area with cold winters, and don’t heat your greenhouse during the winter months, sub-zero temperatures could kill your plants.
If you live in a mild climate, there may be no need to heat your greenhouse in winter.
Ways you can heat your greenhouses in winter
There is a surprisingly wide range of options available for heating greenhouses. You will need to weigh up the cost and decide whether you want to heat throughout the year, in the growing season, or only in winter.
If you live in a cold climate you are more likely to want to invest in a system you can use throughout the year. If winters are generally mild, it may not make much sense to spend money on an expensive heating system that you will only use for a few months every year.
Passive solar heating
This is one of the most cost-effective ways to heat a winter greenhouse and grow crops all year round. In essence, what you want to do is capture heat during the day and then redistribute it during the night.
The key to successful passive solar heating greenhouses is to incorporate materials with a high thermal mass. Examples include bricks, soil, rocks, and water.
Materials with a high thermal mass retain heat long after the original heat source, in this case, the sun, is no longer part of the equation. It’s how heat stays in your greenhouse after dark!
A heat sink is another possibility for passive solar heating. It’s a fairly simple process that involves digging a hole in the middle of your greenhouse that is a little less than one-third the size of your structure.
Then, you do is to put a PVC or similar pipe in the center of the hole and backfill it with materials that have a high thermal mass. They will absorb the heat during the day and the heat will dissipate into the greenhouse via the tube at night.
You can optimize a heat sink by using a pump or, better still, a solar fan to move the air in and out of the hole.
Heaters powered by solar power
Considerably more expensive than passive solar power systems, a heating system powered by solar panels is a great green option. If you have solar panels in place for heating your home, this may be a cost-effective choice for your greenhouse
Hot water heating
This option also exploits natural heat from the sun. But you use coiled water lines in conjunction with solar panels, running these into the greenhouse so that they heat water in a large barrel.
The heat (or thermal energy) from the water radiates out into the greenhouse, heating the air. Another option is to dig the hot water lines into the plant beds to heat the soil.
You will need a water pump for this option so that the water circulates through the panels and keeps the greenhouse warm. You can also heat water with electrical devices, boilers, or wood burners. Simple solar options are generally the least expensive.
It stands to reason that if you are planning to heat your greenhouse with an electric heater you’re going to need a power supply. Practically, this works best for greenhouse heating when the greenhouse is attached to your house. It is also best when used for a small greenhouse.
The caveat is that because you are heating the air, you’ll need to keep the heater operating constantly, at least during winter. While the setup is minimal, running it can be costly.
But, if you are likely to need heating only when there are occasional cold snaps, this could be the answer. Something else to bear in mind is that thermal mass will help you capture heat you might otherwise lose.
Like the heaters we use in our homes, oil-filled heaters do the same job as electric heaters, but they are more cost-effective to run. Nevertheless, they also need a power supply to run.
An advantage of using an oil heater vs an electric heater is that the oil version doesn’t dry the air as electricity does. Plants need a level of humidity to thrive, and it’s much easier to achieve this with a gas rather than an electric heater.
If you don’t have a power source in your greenhouse, your greenhouse winter heat supply could come from a kerosene (paraffin) heater. These usually work in conjunction with a tank that you refill or replace when required.
While they cost a lot less than electric heaters to run, kerosene heating also tends to dry out the air.
Gas heaters are another option, but it’s vital to ensure that you have a good supply of natural air for your plants.
How do you keep plants warm in an unheated greenhouse?
There are ways to keep your plants warm in a greenhouse that isn’t heated, especially if it’s a small greenhouse. Insulation is a goodie because you can do it cheaply while still allowing lots of light into the greenhouse.
Inside a greenhouse, plants will be protected from icy winds and any really cold spells. For those of you who want to be proactive and heat greenhouse spaces on a tight budget, here are some ideas.
One very inexpensive but effective method is to cut the bubble wrap to size and fix it on the inside surfaces of the greenhouse.
Either way, when the weather warms up again, you can remove the polythene bubble wrap, wipe it clean, and store it until next winter.
As mentioned in the section on passive solar, thermal mass is a great way to increase heat. You can do this very simply by using mulch inside the greenhouse.
Straw is one of the cheapest mulch materials and it works very well as an insulator. Cedar bark mulch, grass clippings, and dead leaves are other options that you won’t have to pay for.
But, of course, if it’s a warm winter, you probably don’t have to bother doing anything to keep them warm.
While there are many different options for heating greenhouses in winter, some work better than others. Also, some cost a lot more than others.
Whatever type of heating you opt for, you need to be cognizant of any potential risks. For instance, if you place a kerosine heater too close to the PVC or polycarbonate walls, these could melt.
Ultimately, if you combine a couple of techniques, like passive solar and hot water heating, you’ll be able to heat your greenhouse in winter with a minimum spend.