The polytunnel takes pride of place in the garden of many a vegetable grower. It’s a particularly important structure for cooler climates, where it allows you to extend the growing season at both ends. It can also protect your crops from unpredictable spells of weather. With a tunnel you’re essentially creating your own inner microclimate for vegetable growing. You can further optimise this climate by paying attention to ventilation, light and shade, and humidity. Once you get into the groove of polytunnel growing, you probably won’t look back!
Compared to their close cousin the greenhouse, a polytunnel covers a lot of square feet - allowing you to grow a range of crops. They can vary in size from ‘mini polytunnels’ to 12 foot structures to massive commercial polytunnels. For the home vegetable grower, you should be careful not to exceed the size recommendations (if only so as not to fall foul of neighbour complaints!). Most importantly, it should not exceed 3 metres in height: most domestic polytunnels (including the ones we sell) fall well under that, so fear not. Over the years, polytunnels have become more and more durable and - provided you take care when setting them up - can withstand very stormy weather.
A typical polytunnel features a metal or steel frame with a high quality polythene cover. The polythene cover may need replacing at intervals of 4 to 7 years, but if the tunnel is in a well sheltered area it can last for a decade. Providing the tunnel with a natural windbreak such as hedging or a ‘shrub barrier’ can bolster your defences against extreme weather. Meanwhile, you can increase the lifespan of the polythene by being proactive and immediately tackling any small tears with some polytunnel repair tape. Clearing any debris, branches or snowfall from the roof is also a good habit to get into. Avoid the temptation to place the polytunnel too close to tree cover: sharp branches can pierce the polythene.
Base Rail vs Trench Method
The method you use to anchor the polytunnel will play a role in how well it stands up to weather conditions. One of the more common traditional ways to do this is known as the ‘trench’ method: this is where you dig a trench roughly 8 inches deep (and the same wide) around the polytunnel. When the polythene cover is stretched tight around the frame, the excess polythene is then inserted into the trench before covering it with shovelfuls of soil. This will provide a natural anchor, although it can be an awkward task - with preferably more than two people required to hold the polythene tight and anchor it at the same time. Burying the polythene at at least a spade’s depth will protect the structure against pests, weeds and drought.
On the other hand, an increasingly popular method of anchoring the tunnel is to use the base rail system. With this system the polythene is fixed to a movable rail which can be slid up or down. As well as being a quicker and easier method, the base rail allows for more precise adjustments of polythene tension. It also allows you to set up a polytunnel on harder ground, including concrete. Ultimately it’s all down to personal preference, but at Quickcrop we believe that the base rail system is the best - especially for a DIY build.
When situating your polytunnel, some factors to take into account include:
Sunlight: Observe your outdoor area to see the areas that receive the most sunlight. To work its magic, a polytunnel will need to see as much sunlight as you can provide for it - especially in our climate where the sun can sometimes be shy.
Orientation: Positioning the tunnel along a north-south axis is recommended. This way both ends of the tunnel will receive equal amounts of sunlight. However, you can get creative with this as well by planting crops in such a way that shade-tolerant plants reside in the shaded areas.
Accessibility and Irrigation: Keep in mind that you will be using your polytunnel a lot, and it pays to have a nearby water source rather than marching back and forth with containers.
Polytunnels can heat up deceptively fast during the warmer months. To prevent heat stress to plants, ventilation and cooling will play an important role in polytunnel gardening. The entrance door to the polytunnel will be your main point of ventilation. During warmer spells it can be best to leave the tunnel door open throughout the day, and even during the night if temperatures remain warm. The polytunnels we supply here at Quickcrop go one better by having doors at both ends. We believe this is essential to provide good levels of airflow during hot weather, thus decreasing the chances of fungal disease gaining a foothold within your tunnel. It also encourages the entry of beneficial pollinators.
Raised Beds in a Polytunnel
Poly tunnels lend themselves well to growing in raised beds. A 12 ft tunnel should have space for three rows of raised beds, with paths in between for ease of access. Aside from making it more comfortable for you to tend to your crops, this lends a nice air of symmetry to the polytunnel environment. Climbing plants like tomatoes and cucumbers are better in the centre, as this will be the highest point of your tunnel. For narrower tunnels, it may be necessary to settle for two rows on either side of a central path. A raised bed set-up allows you to optimise the soil without needing to worry about the soil quality or make-up underneath the tunnel.
Polytunnels won’t provide you with a completely pest-free environment, but the (mostly) enclosed space can make them less of a hindrance. Mesh or net coverings placed over door panels can work as a compromise between the need for pest deterrence and the need for ventilation.
‘Supertherm’ is not the latest Marvel franchise, but a thermic polythene which helps retain heat in the polytunnel during spells of cooler weather. It can also add further protection against frost damage when outside temperatures drop below freezing. Maybe most impressively, the increased thermal efficiency doesn’t come at the cost of light transmission, as you might expect: the tunnel will retain 90% light transmission, scattering light evenly throughout the structure. These supertherm polytunnels are sourced from First Tunnels, one of the leading tunnel suppliers.
Spare Parts or Essential Parts?
When buying a polytunnel, you will often find that some parts are available as ‘extras’. We believe that some of these extras are pretty essential to success, so we have included them as part of the polytunnel kit from the get-go. What parts are we referring to?
Crop Bars: You will recognise these as the horizontal bars just below the roof of the polytunnel. They are attached to the tunnel hoops and can be used to support climbing plants, which adds an all-important height dimension to your growing space. These climbing plants are often ones that tend to thrive in a polytunnel, so they are a vital addition in our opinion.
Anchor Plates: These are square steel plates which are fixed around the foundation tubes. As you can imagine, they will hold the tunnel hoops firmly in place and prevent lifting or sinking. Since they’re removable, they also leave you the option of moving the tunnel to a new location.
Base Rail: As mentioned above, we think the base rail system will make life easier for you, allowing for easier installation and adjustment, as well as less manpower when erecting the polytunnel..