Are Greenhouses Frost Free?

A frost-free growing space is every gardener’s dream, allowing you to overwinter even sensitive plants successfully. It stands to reason that every fall many gardeners start to wonder whether greenhouses are frost free. That’s exactly what we’re going to address in this article.

Are Unheated Greenhouses Frost Free?

Let’s start with the bad news. Unheated greenhouses are not frost free in most instances, though this can depend on a number of factors. 

While generally speaking a good quality greenhouse will stay a few degrees warmer than the outside temperature there are a number of elements that can decide whether or not this constitutes being “frost free”. 

One major consideration, for example, is how low the temperature gets in your part of the world. The lower the temperatures you experience the less likely it is that your greenhouse will remain frost free. 

I’m in a temperate climate here in the UK and my greenhouse remains frost free for *most* of the winter. 

Just in the last few weeks however we’ve been hit with temperatures dropping to -5’c / 23‘F and during that period we have recorded temperatures down to -3 ‘C / 37‘F in our greenhouse. Hardly what you’d call frost-free, though as this was only for a very limited period of time I’m hoping the plants inside make it through OK. 

Another major consideration is how much winter sun you get in your part of the world. If skies normally remain cloudy and overcast then there will be little to warm your greenhouse during the day. In contrast, even in winter a sunny day can send temperatures skyrocketing in your greenhouse. 

Speaking again from my experience in recent weeks, while night-time temperatures hit unwelcome levels, at the same time my greenhouse reached 30’C / 85‘F during the day thanks to bright sunshine. 

It seems that the greenhouse managed to hold onto some of this heat, meaning that the lows experienced at night were only for short periods of time. Hopefully not long enough to do any significant damage. 

Do Plastic Greenhouses Protect From Frost?

Plastic greenhouses can offer rather more protection than glass greenhouses. Polycarbonate greenhouses benefit from two separate panes of plastic, joined between with a mesh of additional plastic. Air gets trapped between these two panes, and just like the double-glazing in your home and helps to insulate the interior from the worst of the cold. 

Even an unheated plastic greenhouse isn’t guaranteed to be frost free – as my own experience has shown me (we have a plastic greenhouse at present). However if you live an an area where temperatures rarely drop much below freezing even in winter then a plastic greenhouse *may* offer you better protection than a traditional glass model. 

If frost protection is a major concern of yours then a plastic greenhouse is likely to give you a better chance of success, which can be increased yet further with some of the tips outlined later in this article.

Can Plants Freeze in a Greenhouse?

It is entirely possible that plants can freeze in a greenhouse. During the most recent cold snap I spent some time in my own greenhouse, looking for signs of damage among my beloved plants. 

Sure enough, some foliage showed signs of frost damage. Stems had turned brown while some leaves seemed to have lost their structure and hung limply off the plants. More worrying for me were the containers that had frozen solid, potentially causing damage to sensitive roots. 

However while this all sounds like bad news, there are a few interesting observations I made that might help you to keep your plants safe. 

Firstly, the plants most-affected by the cold weather were those actually touching the sides of the greenhouse. To help protect plants in an unheated greenhouse then move them into the middle of the space so they are not in direct contact with the walls or roof.

Secondly, the plants where the compost itself froze solid were those sitting on the ground in the greenhouse. Those pots that were raised up off the ground on my greenhouse staging remained unaffected. 

Lastly, I was lucky that while nights have been cold, the days have been clear and sunny, rapidly warming up the inside of my greenhouse to the equivalent of a warm summer’s day. This seems to have allowed my plants to rebound rapidly. 

Some of the hardier plants such as lupins and forget-me-nots are actually showing healthy, fresh spring growth already, despite the cold snap. 

How Cold Is Too Cold for a Greenhouse?

For obvious reasons many people wonder how cold is too cold for a greenhouse. The answer is “it depends”. The plants that you’re growing will largely affect how cold is too cold. 

If you’re growing fully frost-hardy plants in your greenhouse, and are using it merely to offer protection from the harshest weather, then there probably is no such thing as “too cold”. 

Hardy plants can survive even freezing temperatures and will rejuvenate as the temperatures start to increase in spring. 

Keeping them in a greenhouse simply means that they will think it is spring sooner than if they were kept outdoors, thanks to the higher temperatures they experience. This, in turn, can lead to bigger, healthier and more vigorous plants, not to mention heavier yields of flowers, fruits and vegetables. 

Half-hardy plants are rather more fussy. You’re entering the “danger zone” as temperatures drop below 5’C / 40‘F. That’s not to say all your plants will succumb to the cold, but that the risks increase as the temperature drops. Below freezing the odds of damage go up even higher.

I have some banana plants that I currently overwinter in an unheated porch at the back of my house. Temperatures in the porch regularly fall to just 3’C / 37‘F at night in the winter. Like my greenhouse, the porch does warm up during daylight hours when the sun shines. 

But British winters can be cloudy and overcast so it can be weeks before there’s sufficient sunshine to really raise temperatures very high there.

Now, there’s no denying that the banana plants I overwinter in the unheated porch look pretty sorry for themselves by spring. Over the winter period the leaves slowly discolor and droop, showing brown spots where they have been damaged by the cold. 

I also keep other banana plants in the main house, treating them like house plants. They receive warmer temperatures and, due to their position, a bit more sunlight too. These plants look green, luscious, healthy and tropical all winter long. Which makes the “porch bananas” look all the sadder in comparison.

All the same, the reality is that the porch bananas survive. It doesn’t take long in the spring for them to spring back into life. The increasing temperatures and light soon have them shooting out healthy new green leaves. 

Just appreciate that there can be big differences between a plant “surviving” and one that is “thriving”. However the reason for discussing my banana plants is to show that you might surprise yourself with what is possible depending on how long your winters are, how low the temperature drops in your area and how much winter sunshine you receive.

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