Polytunnel power: How to grow the best summer veg and fruit

In a world turned suddenly upside-down, food-growing is a wonderful way to anchor ourselves in safer waters and weather out the storm. In particular, if you’re lucky enough to own a polytunnel or glasshouse, then this is the very best time to put it to its most productive use, especially when it comes to sowing and/or growing many kinds of heat-loving vegetables, including tomatoes, climbing French beans, courgettes, aubergines, peppers, squash, sweetcorn and cucumbers. These are all crops that often struggle in our typically cool, damp Irish summers but thrive when grown in the sort of warm, bright, ultra-sheltered conditions that a polytunnel or glasshouse can provide. Not only that, they’re also the types of vegetables that may be increasingly hard to come by in the months ahead as disrupted supply chains creak under the pressure of the pandemic.

If there’s any growing space going a-begging, many different kinds of salad leaves also enjoy these sorts of sheltered conditions. So, of course, will seedlings of many other vegetables (including broccoli, cauliflower, cabbage, chard, perpetual spinach, swede, Florence fennel, leeks, scallions, spinach, runner beans and kohlrabi) that can be transplanted outdoors later in the growing season.

A careful watering regime is also crucial. Too little and you'll put plants under stress, but too much too often and you'll leave them vulnerable to disease

Certain tender but productive perennial and shrubby plants also do best when grown under this sort of protective cover. In my own cold garden, I’m glad of the lemon verbena (Aloysia citriodora) that flourishes in the polytunnel and whose sweetly perfumed, citrus-flavoured leaves can be used to make a delicious lemonade. In a world where an increasing number of us want to be self-sufficient when it comes to cut flowers, it’s also worth adding that many very ornamental crops, including dahlias and certain umbelliferous species vulnerable to carrot fly (for example, Ammi majus, Orlaya grandiflora and Daucus carota ‘Dara’), will also thrive in these sheltered growing conditions.

Considerable challenges

These many, very significant growing advantages aside, growing under cover also brings its own particular set of challenges. For example, monitoring temperature and humidity levels, both of which can dramatically fluctuate throughout the year, is key. Forget to throw open those doors or vents on a bright, still spring morning and you run the risk of exposing plants to such searing, sultry, tropical heat that some may not survive. Forget to close those same doors and vents on a cool, clear, still spring evening and those same plants and seedlings may suffer frost damage or be killed. As a general rule, try to prevent temperatures rising above 30 degrees and remember that seedlings and young plants in particular are especially vulnerable to sudden extreme swings in temperature.

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A careful watering regime is also crucial. Too little and you’ll put plants under stress, but too much too often and you’ll leave them vulnerable to disease as well as the possibility of reducing both flavour and productivity. Overhead sprinkling systems in particular are blunt instruments that often cause more problems than they solve. Instead it’s better to use drip-irrigation or to hand-water individual crops according to their specific needs, which can vary greatly according to the particular species as well as its stage of growth. Tomatoes, for example, are far more prone to disease when watered from above and instead need a careful watering regime that responds to the growing conditions/ weather conditions as these evolve throughout the growing season. Too much water can cause the plant’s roots to rot and raise the risk of diseases such as blight while erratic watering can reduce the quality and number of fruits produced and cause problems such as blossom-end rot.

Soil health

Managing and sustaining soil health and fertility is also crucial when you’re growing crops this intensively directly in the ground. Regular mulches of well-rotted manure, home-made garden compost and generous sprinkles of powdered seaweed will all help a huge amount to support soil health as will using fast-growing green manures.

Wondering why one of the seed trays had been tipped over, I lifted it up only to discover that a family of field mice had take up residence

Similarly careful crop rotation, good ventilation, good garden hygiene and vigilant pest and disease control are all vital, as various moulds, mildews, aphids, red spider mite and even rodents can also thrive in the sort of warm sheltered conditions that polytunnels and glasshouses provide. Some years ago I was checking on trays of baby seedlings growing on a home-made hot bench in my own polytunnel. Wondering why one of the seed trays had been tipped over, I lifted it up only to discover that a family of field mice had take up residence, lured there by the heat and seclusion. A gentle spray of water made them scamper quickly away but not all polytunnel pests are as easily managed. Getting a cat (much better than rodenticides, which shouldn’t be used in food-growing areas), promptly removing weeds as well as any dead, diseased or dying plants, hand picking any affected foliage (bag and burn these), and handpicking or squashing insect pests as soon as you spot the first signs of infestation are just some of the simple but effective ways to manage these risks.

Definitive book

For more detailed information on growing food and flowers under cover, written specifically with Irish growing conditions in mind, check out west Cork-based garden writer Joyce Russell's definitive book on the subject, The Polytunnel Book: Growing Fruit and Vegetables All Year Round. Dublin-based blogger and garden writer Nicky Kyle's website nickykylegardening.com is another excellent and regularly updated resource on polytunnel growing for Irish gardeners. While well-known Irish polytunnel suppliers Colm Warren polytunnels aren't offering an installation service during the current Covid-19 restrictions, they are delivering their polytunnels flat-packed for self-assembly along with other garden accessories including propagators, irrigation and ventilation systems and the Jiffy range of garden pots (polytunnelcompany.ie, cwp.ie). As regards the ongoing problem of sourcing other vital gardening supplies, some (not all) Irish garden centres are offering an online order/delivery service but expect possible delays of up to two or three weeks.