Top crops Make the most of a polytunnel

POLYTUNNELS extend the growing season and may let you grow veg you couldn’t attempt outdoors but they need your love and attention to work well.

Living at a height of 200 metres (around 650ft) in the Borders, outdoor sweetcorn, tomatoes and cucumbers are a no-no.

And overwintering broccoli, cauliflowers and even spring cabbage depend on a mild, relatively dry winter. Believe it or not, even tough broad beans are challenging.

With a good dose of TLC, they all thrive in a tunnel’s protective bubble. I use every square inch for as much of the year as possible. Although a shorter growing season limits me to one major crop per bed outdoors, two or more are possible in a tunnel.

The minute winter crops, beans, brassicas, chard and endives, are over summer residents like tomatoes, cucumber, sweetcorn and cucumelon, take over.

Like outdoors, you need to plan a rotation, so brassicas must move, as do tomatoes, close relatives of potatoes. Peas and beans form the third group, with sweetcorn, fennel and assorted greens a fourth. Juggling everything in a small space ain’t easy and I sometimes have to break traditional rotation rules.

Everything happens more quickly in a tunnel. Veg matures at speed and, like an early sowing of mange-tout peas, reaches twice the height of outdoor ones.

But the same applies to our problems. Disease spreads quickly, so good preventative hygiene is essential.

Many gardeners are overly tidy in a garden and sweep away beneficials along with pests, destroying this pest-predator relationship. Nonetheless, meticulous cleanliness is essential in an artificial tunnel.

You’ll get into places more easily just now, without soaring sweetcorn or skyward cucumbers. I’ll clean the wooden edges against plastic walls. I remove the built up gunge that’s become a haven for slugs and their eggs. Moss and algae thrive in these damp nooks and crannies. Inevitably some soil and plant debris accumulates along path edges. I lift my fairly light recycled rubber mats from their sand base. I scrape off any soil and thinly sprinkle some fresh sand before replacing the mats.

Regular pest control over winter is important. It’s all too easy to pass the tunnel for several days, without going in, only to then find voles have scissored through half of your spring cabbages and slugs feasted on the rest. A regular sprinkling of organic slug pellets and a daily inspection of a mouse trap prevents this. I cover the trap to keep any birds safe.

I’ll use a warm sponge to clean the inner plastic, before repeating the task outdoors. It’s best to do this during a mild spell. Letting more light into the tunnel is so worthwhile, as I saw last year. Over time the plastic had become pretty opaque, but the extra light through a new skin revolutionised

how well the plants grew and fruits ripened. Unfortunately the benign conditions we provide for winter crops appeal equally to weeds. Although there’s no alternative to hand weeding closely planted greens, other bare patches can be mulched.

I naturally wouldn’t encourage you to use plastic in the tunnel, or anywhere else for that matter, as there are plenty of biodegradable alternatives. As well as the usual straw, mushroom compost, leafmould and coir, cardboard or thick layers of newspaper work perfectly well. A tunnel is not a place of beauty and there are no strong winds, so leave damp cardboard uncovered. It prevents weed germination, keeps soil moist and is easily lifted to check whether slugs are lurking underneath.

During a mild settled spell, open part of the tunnel during the day to allow air circulation; as well as when you’re going in and out.