The question of whether greenhouses need to be heated is an important one. The reality is that greenhouses do not necessarily need to be heated, depending on your intended use.
There are a number of factors that will determine whether your greenhouse needs heating. Here are some of the most important elements to consider:
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Outside Temperature
The colder it is outside during the winter the more important greenhouse heating can become. Unheated greenhouses can reach astonishing temperatures during the day when exposed to even modest sunshine. Here in the UK my greenhouse routinely reaches 30’C (85‘F) or more on bright days in February. This is of course more than sufficient for many plants.
The problem comes when the sun sets. At night, with no sunlight to warm the interior, temperatures can plunge to within just a few degrees of the outside temperature.
Whilst I live in a part of the UK that has a relatively temperate climate, which barely drops more than a few degrees below zero, readers in the northern USA, Canada or Scandinavia may find that an unheated greenhouse simply gets too cold on winter nights.
Adding heating can immediately transform your growing space from a cold, damp, unwelcome place to spend time to somewhere warm, cosy and conducive to plant growth.
Insulation Levels
Greenhouses can vary significantly in their ability to retain heat on cold nights. For example polycarbonate (plastic) greenhouses tend to stay warmer for longer than pure glass greenhouses.
There are other considerations too. Greenhouses with some form of heat sink will also stay warmer for longer after sunset. Examples might be a tiled floor or brick footings that warm up during the day, then gently release their warmth back into the atmosphere at night.
Lastly, it is entirely possible to add more insulation to a greenhouse. Bubblewrap is one of the most common forms of insulation added to greenhouses in cold weather. It can be bought in large sheets and attached to the inside of your greenhouse to keep it warm.
Focusing on design elements that will naturally keep your greenhouse warmer may negate the need to add heating to your growing space.
Plant Selection
Plants vary significantly in their cold hardiness, so it follows that this too can affect whether greenhouse heating is necessary.
Should you be using your greenhouse to protect frost-hardy plants then they should survive – even thrive – in an unheated greenhouse all winter. In contrast, more sensitive plants from warmer areas may die if the temperature drops too low. Many citrus, for example, can struggle once temperatures drop below around 5’C / 40‘F.
When deciding whether your greenhouse needs heating, spend some time researching the cold-hardiness of your plants to make conclusions about how well they would cope with cold temperatures.
Growth Vs Maintenance
There are a surprising number of reasons to purchase a greenhouse. For example you might want to extend your growing season, starting seeds earlier in the year, and continuing to harvest crops right through till late fall. When the winter rolls around perhaps your greenhouse is allowed to lie fallow till the following spring.
On the other hand some greenhouse owners want to continue growing healthy crops right through the winter months.
Still others use their greenhouse as somewhere to protect perennial plants from the worst of the winter weather – the cold, the winds, the rain and snow.
Your intended winter use for your greenhouse will have an impact on whether heating is needed. For example if you’re happy to let your greenhouse go largely unused in the winter months then heating is likely unnecessary. Any remaining plants can simply be left to their own devices in the coldest months.
On the other hand, if you want your plants to grow and flourish even in December or January then some additional heating is likely to be necessary. Under these circumstances, appreciate that additional lighting may also be needed, as day lengths in the winter may not provide enough sunlight to encourage growth in some plants.
Personal Activity Levels in the Winter
Lastly consider what you plan to do in your greenhouse in the colder months.
An unheated greenhouse in winter is not always a pleasant place to be – especially on cloudy, overcast days. Under these circumstances an unheated greenhouse can be cold, damp and unwelcoming. If you’re hoping to spend time in your greenhouse year-round then some gentle background heating can make all the difference to your experience.
On the other hand, if you’re happy to just quickly check your plants from time-to-time but rarely intend to spend long periods of time in the greenhouse then no heating may be necessary.
Conclusion
In temperate areas of the world greenhouses rarely need heating in the summer months, as they are naturally warmed by the sun and can retain this heat late into the night. In the winter, however, things can be rather different.
Whether your specific greenhouse needs heating is a matter of personal opinion. Many people – myself included – choose not to heat their greenhouse. This helps to reduce costs but does mean a shorter growing season and more careful plant selection than those who heat their space.
A heated greenhouse can be a wonderful place to spend time at any time of year, and opens up a whole new world of potential plants you can grow. Heating can extend your growing season even further, and can make it easier to produce fresh fruits and vegetables even in the coldest weather. It does, of course, come with a cost however.