How to Keep a Greenhouse Warm in Winter

While greenhouses naturally keep warm all summer long, the winter months can see temperatures plummet. This, in turn, can slow down plant growth or even risk killing more frost-sensitive plants. Naturally, many of us think about how best to keep our greenhouse warm in winter.

Fortunately there are a host of ways to keep your greenhouse warm in winter. However before we look at the individual strategies you can employ it’s worth pointing out the benefit of investing in a decent thermometer to track and monitor temperatures in your greenhouse. 

The best greenhouse thermometers store temperature readings so they can be accessed at a later date. While traditional min/max thermometers are certainly better than nothing, for a more holistic view consider a thermometer that continually monitors the temperature, and then provides a detailed record on request. 

You’ll be able to see, for example, how temperatures vary throughout the day (and night) and how extreme weather affects your greenhouse temperature.

In doing so you can gain a far better overall view of how warm your greenhouse is at different times of day and in different weather. This can help guide the process of warming up your greenhouse, letting you make informed decisions about how much effort you’ll need to put in. 

Increasing greenhouse temperatures by a few degrees just to keep it frost-free is reasonably simple. In contrast, if you’re looking to grow more tropical plants you’ll likely want to reach far warmer internal temperatures, which may require rather more effort and equipment. 

With that said, let’s take a look at some simple and cost-effective ways to warm up your greenhouse in winter...


Clean Your Greenhouse

Let’s start with the basics. Greenhouses naturally warm up when sunlight passes through the glass or plastic panes and gets trapped inside. This means that the more sunlight your greenhouse emits, the better.

Putting aside the obvious step of putting your greenhouse in the sunniest spot available, a good first step to keeping your greenhouse warm in winter is giving it a good clean. Remove any algae, dirt, dust or bird mess to ensure as much winter sun can pass through as possible.

There are specialist greenhouse cleaning liquids that are plant-safe to use. Alternatively if you think your greenhouse can stand up to it a pressure washer may be used. 

Minimize Draughts

Uncontrolled draughts can be your enemy when it comes to retaining heat in a greenhouse. After all, we know from school science classes that hot air rises. Combine this with windy winter weather and a draughty greenhouse and it’s clear a lot of heat can be lost unnecessarily. 

Before the worst of the winter weather sets in it can be a good idea to inspect your greenhouse for any potential draughts. For example, have any glazing panels moved slightly during the season? Have any bolts become loose, or any parts of the frame become misshapen? 

Also consider how well doors and roof vents fit, and if necessary replace insulation strips around them, just as you might for double-glazed windows and doors in your home.

If necessary use a silicone sealant to cover gaps and reduce the loss of warm air. 

Add Extra Insulation

Another way to retain warmth in your greenhouse is to consider adding more insulation. Two common materials are horticultural fleece or bubble wrap. Both can be fitted to the inside of your greenhouse using specialist fittings available from most good garden centers. 

Utilize Organic Matter

Rotting organic matter can produce a surprising amount of heat. 

Setting up a compost bin inside your greenhouse can help to produce warmth all winter long. Fill it with leaf litter gathered from around your yard, vegetable peelings, grass clippings and so on. 

The more it rots down, the greater the warmth will be, so aim to start the process nice and early. 

Alternatively if you have an existing compost heap then give it a good turn in the fall to reveal the well-rotted organic matter lower down and transport some of it into your greenhouse for the winter months. 

Add a Heat Store

Some materials quickly heat up on sunny days and then gently release their warmth over an extended period of time as the temperature drops. Good examples of this would be paving slabs, concrete and rocks. 

Properly utilized, they’ll help to “store” the sun’s heat in your greenhouse, keeping it warm long after the sun begins to set. 

Use Candles

One simple and low-cost trick utilized by many gardeners is candles. To experiment with this solution you’re going to want to buy some slow-burning, squat candles. Don’t buy the tall, thin candles that you might use on a dinner table.

Take a terracotta plant tray, place one or more candles on it, light them, and then cover the candles with an upturned terracotta pot. You may need to experiment with lifting the pot up slightly with stones or rocks to ensure the lit candles continue to burn well.

The candles then gently warm up the terracotta pot, and the air within it. The single hole typically found in terracotta pots also acts like a “chimney” with the warm air filtering out through it. 

The effects of candles used in this way are relatively meagre so this technique is best used in smaller greenhouses, or whether only a modest increase in temperature is required. For those situations, however, a few candles can make all the difference.  

Paraffin Heaters 

Paraffin heaters are the “classic” form of greenhouse heating. As the name suggests, these heaters burn paraffin as a fuel, producing a pleasant background heat that keeps frost at bay.

The fact that paraffin heaters have been around for long means that there are literally dozens of different versions available. While historically paraffin greenhouse heaters were only used for smaller spaces, these days one can buy much larger heaters suitable for much larger spaces.

When selecting a paraffin heater, be sure to do your research thoroughly and read as many reviews as possible. You’ll want to remain mindful of the size of your greenhouse and ensure that any heater you consider is suitable for that amount of space.

An additional benefit of paraffin heaters is of course they work well where no electrical source is available – such as in a greenhouse that is down the end of your garden. 

When using paraffin heaters great care should be taken about positioning, especially in plastic greenhouses. The last thing you want is anything melting or catching fire while you’re indoors. 

Paraffin heaters are cheap to buy, but the costs to run them can start to add up in larger greenhouses. For this reason some gardeners opt to install a power point and make use of electric-powered heaters.

Tubular Heaters

Electric tubular heaters are a safer alternative to paraffin heaters when fitted correctly. While they will require an electrical point to function they do away with the need for a naked flame. 

Like paraffin heaters, the heat output tends to be quite gentle. This makes them ideal for keeping your greenhouse frost-free, but they may be less effective if you’re trying to keep sensitive plants like citrus growing throughout the cold winter months. 

Electric Fan Heaters

Electric fan heaters are the “gold standard” of greenhouse heaters and offer many benefits if you have a power socket available. For one thing, many of them are thermostatically controlled, allowing you to accurately set your chosen temperature. 

For another, the fact that they contain a fan helps to move air around your greenhouse. Not only does this reduce the chances of any cold pockets remaining (as can happen with other forms of greenhouse heating) but also minimizes the chances of mold or fungus attacking your plants in winter.

Lastly many greenhouse fan heaters will also function as a cool fan in the summer months, when overheating is more likely than the chill of cold winter weather. 

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