Watering your polytunnel is one thing that it can be very important to get right, and it is something that is quite often done wrongly by beginner polytunnel gardeners. It is vital to the success of polytunnel crops to understand that watering is something that will vary considerably throughout the year. The frequency of watering, and the quantity of water you use, should be very, very different in the middle of summer and the middle of winter, with almost infinite gradations in between.
Of course, exactly how often you water in your polytunnel will depend on where it is sited. In a polytunnel on sandy soil, for example, with ground that slopes away beyond it, water will be carried away through the soil far more quickly and even once you have improved the soil by adding a large quantity of organic matter, you will likely still require more water than will be required on other sites, while a heavy clay site, especially one in a dip, will need far less since the water will be stored more effectively in the soil for the plants to uptake.
How much you need to water your polytunnel will also depend on the plants that you are growing and how advanced in growth they are. Some plants, it should go without saying, need far more water than others.
Finally, the amount of water you use will depend on the weather and the temperatures that you are experiencing in a given year. One winter, for example, the temperatures will rarely go below zero degrees, while in other winters, you can regularly be struck by temperatures well below freezing for weeks on end. Some winters, there is plenty of precipitation, while others are surprisingly dry and crisp.
When deciding how often to water your polytunnel in winter, you will have to take all of the above factors into account. Remember that while the surface of the soil may appear dry, scratch the surface and you may still find that it is very moist below. Though, of course, no rain or snow will fall inside your polytunnel, you should remember that water will still track in, to a degree, below the soil.
As a general rule, you will usually not have to water in winter more often than once a fortnight or so – though this is only a very rough guideline and it is better to make up your own mind based on all the factors discussed above. Over winter, it is a general rule, also, that plants will need far less water than most people may imagine. Over-watering is usually worse than not watering enough, so it is always better to err on the side of caution.