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A polytunnel is a useful piece of kit when it comes to working in unpredictable British weather. It offers shelter for the gardener and precious plants of a more delicate constitution, it keeps off frost, warms up soil and generally makes the entire experience a little more productive (except for all the watering). But how does that actually translate into time – how many more material weeks of growing time are available to you?

Unfortunately with so much gardening, there is no exact science, instead, it is dependent on where you are in the UK, the weather that year, the positioning of your tunnel and plot etc.

6 Weeks

This is the industry standard at either end of the season, whether that is pulling plants out of dormancy early and before the frosts are finished or extending the season once the weather cools. While six weeks may not seem like it is going to make a significant impact on your carrot crop, when you consider it is either end of the season, 12 weeks in total, 3 months extra gives you a lot more growing time.

As your polytunnel gives you a six-week headstart on the season, it also intensifies it. Often if gardeners have additional space and left-over seed, they will sow the same crop at the same time, both inside and outside the polytunnel. Inevitably the crop within the tunnel will be larger or more fruitful.


A polytunnel gives more opportunities to overwinter in the environment some plants need. Some plants require a little protection from the most brutal elements of winter, however moving them into the house can be problematic due to a lack of light, too much heat or not enough space. A polytunnel allows you to store all these tender plants in one place.

There is also a plus side to overwintering crops. Planting crops in the autumn allows them to establish root systems before they fall dormant. This gives them a head start come the spring. Instead of waiting for the ground to warm up or starting off seedlings in cold frames, your crops will be starting to sprout.

Hardening Off

While not directly related to an extended growing season, your polytunnel offers your plants a ‘halfway home’ when it comes to hardening them off aka toughening them up, acclimatising them to cooler temperatures. Bringing your plants inside every evening and putting them out every morning can be an arduous task that is not always practical if the plants are big or cumbersome. Instead, you can use your tunnel as the perfect middle place between the shelter of a house and the exposure of the great indoors. It is less effort but you can still monitor your plants if you are in your tunnel regularly.