January is a quiet time for tunnel growing and an ideal opportunity to sit back to relax and reflect. However, it can quickly become the most exciting time of the year as you begin to plot and plan for the coming year. When we make plans, we are filled with hope and excitement. We all know the best of plans can go a bit awry at times, but looking forward to a new season is always exciting!
First, I would look at what it is you wish to reap from your polytunnel or garden. Would you love lots of beautiful flowers? Your favourite salad items? Hearty winter veg for storing? Make a list of what you would like, and then check to see what growing conditions they require. The beauty of a polytunnel is that the growing season, yield, and produce quality are greatly increased. They can be increased even further using extras such as a heated bed for seedlings, frost protective fleece, or internal cloches.
Then it is time to look for seed or planting material. You may be able to source them locally, but there is also an astonishing variety of seed available online. You could wow your friends and neighbours with unusually coloured varieties of tomatoes, French beans, courgettes or even strawberries that you would not find in your local supermarket. There is nothing like salad leaves, spinach or sugar snap peas that have just been freshly harvested from your own efforts.
Don’t forget to look at the final size of what you are going to grow, including the height. Taller plants can be supported best in the centre of your tunnel unless you have straight sides. Also, some plant combinations make better companions than others. For instance, tomatoes work well with garlic (to repel aphids) or basil (to improve flavour) but won’t thrive as well next to cabbage or kale and shouldn’t be placed near potatoes as they are both susceptible to blight.
So, it’s time to get out your pen and paper, plot out your beds and start dreaming! Then make your dreams a reality. Life is an adventure and so is polytunnel gardening!
Early strawberries are one of the great pleasures of having a tunnel or glasshouse. Whether from pots or border soil, they taste like a promise of summer and so much fresher than shop-bought ones. Tidy up the plants, removing any dead leaves and bits. Check potted plants for vine weevils. Cut off any runners to divert the plant’s energy from growth into filling fruit. Keep the plants watered and give them as much light as possible, giving them some liquid feed as per the instructions on the packaging. Ventilate on sunny days to keep the temperature cooler and less stressful for the plants, and reduce the danger of fungus diseases. The flowers are more frost-sensitive than the leaves, so protect them from frost with fleece or cloches on cold nights. You will need to hand-pollinate the flowers as fewer insects visit plants under cover. Brush the flowers gently with a soft brush or cotton wool to transfer pollen from one flower to another. You need to repeat this on a number of days to ensure that each flower is fully fertilised and will form a complete strawberry. Continue feeding and watering, and watch out for pests such as aphids and slugs. Remove and destroy any mouldy fruit or leaves to prevent disease spreading. Pick and enjoy the fruit as soon as they are ready, but if slugs are a problem pick them a bit sooner and let them finish ripening in the kitchen.
Most people plant onions as sets; semi-mature bulbs which will hopefully grow bigger and be harvested before they go to seed. But you can also raise onions from seed sown in your greenhouse instead of from sets planted later. This is especially useful on cold, wet soils or in cold, wet springs, as onions cannot tolerate those conditions. Seed-raised plants are cheaper than sets, you have more varieties to choose from, and the plants are less likely to bolt or be pulled out of the ground by birds. On the downside, seedling onions need a longer growing season so February is definitely the last month for sowing, and the longer growing season means more time for pests and diseases to attack them.
Sow the black, angular seeds thinly in a seed-tray, or in modules at up to five seeds each. Keep them at about 10-15°C. The seedlings come up in loops, and you should prick them out from seed-trays into pots before they pull their tips out of the compost and straighten up. If multiple seedlings come up in modules, either leave them to push each other apart later as they grow, or single them with a fine-pointed scissors to give the one remaining plant room to grow a bigger onion. Keep the compost neither bone-dry nor saturated. Grow on the plants in bright, cool conditions until they have two or three leaves, harden them off well and plant them out at about 15cm apart each way. Wider spacing gives you bigger bulbs, and tighter spacing gives smaller ones.
You can also plant sets in pots and give them a head-start under cover, planting them out only when conditions improve.
Garlic has many health benefits, and growing our own is easy when you know how. Homegrown garlic has only the chemicals you apply, and you can eat the leaves and flower-stems when they are young. Planting garlic in a tunnel or glasshouse now can give you bigger and earlier bulbs than outdoor crops, but even starting a crop in modules or pots under cover and planting it out later gives some benefit. It stops the birds pulling them up too.
The ideal soil for garlic is light, well drained, moderately fertile and neutral or alkaline. Wet sticky clay can rot the bulbs and very rich soil encourages leafy growth at the expense of the bulbs. A good supply of potash helps, so dig in some wood ashes before planting. Garlic needs a long, cool growing season and a cold spell to stimulate maturity, so autumn planting is ideal. However, you need to choose your material carefully.
Do not plant shop-bought garlic and expect a decent crop; you may be lucky but it is sold for eating rather than planting and may be treated to inhibit growth, or carry plant diseases that will persist in your soil for many years. Get disease-free sets of varieties suitable for autumn planting from a garden shop or centre. Gently split them into cloves, and plant the bigger ones 2-5cm deep into the soil 15cm apart, in rows about 30cm apart, with the pointy end up and the flat base down. Plant the small cloves close together to produce leaves like chives. Water the crop lightly and let it dry out between waterings to prevent rotting. Dig up the bulbs gently when harvesting, and avoid pulling them. They bruise easily and then will not keep. Dry and store them in a cool, airy place.
Mizuna is one of the oriental greens that Europeans should grow and eat more. It’s best grown rather than bought because it needs to be eaten right after harvesting for maximum flavour and nutrient value, though it will keep for a couple of days in the fridge. Like lettuce, you can sow seed little and often all year round in a glasshouse or tunnel. September sowings will produce deeply cut leaves up to April or May: single leaves can be cut off after about three weeks and whole heads after six to eight weeks.
Mizuna likes moist rich soil, so dig in plenty of compost or other organic matter before sowing. Sow the seed about a centimetre (half an inch) deep in drills about 30cm (12 inches) apart. Protect the seedlings from slugs. Keep it well watered and ventilate on sunny days – it is prone to bolting in hot dry conditions. If it does bolt remove the flowering stems right away to keep it leafy and sow another batch to replace it. Don’t worry if it wilts after a frosty night: it is hardy and usually recovers. It is a member of the cabbage / brassica / crucifer family so don’t sow it in the same ground as any of its relatives for at least three years to prevent disease build-up
Mizuna has a milder flavour than either mibuna or rocket, and is good in mixed salads. Like spinach it can be steamed, boiled or stir-fried but it shrinks a lot, so harvest plenty. ‘Kyoto’ is a good variety to try.
Basil is easy to buy from the shops, but you can grow a wider range of types yourself at lower cost to you and the environment. The type sold in non-returnable pots is usually sweet basil suitable for salads and pesto, but there are also cinnamon and lemon basil, varieties with purple leaves that look great in green salads and spicy or liquorice-flavoured Thai varieties that combine well with Asian food.
Basil is a fast-growing, tender annual that needs lots of light, heat and water. Being very sensitive to cold, it is an ideal candidate for tunnels and glasshouses, especially in our unpredictable summers. It improves the flavour of tomatoes when served with them, and some say that it also does so when grown with them. It grows best in well-drained acid or neutral soil. Sow a succession of crops, but just a couple of plants each time is enough for regular use unless you want extra for pesto. Either sow them in cell-trays or in situ in the greenhouse soil, thinning them to single plants as soon as they are big enough to see. Use a scissors to avoid root disturbance. Thin or place the plants to about 30 centimetres apart. Harvest leaves as soon as the plants are about 20 cm tall, by cutting off shoot tips. Leave on the lower leaves until you discard the plant; they will produce the food to grow new shoots from the lower side-buds. Even if you need no basil, cut off the tips to prevent flowering (which makes the leaves bitter) and encourage fresh sprouts to grow. Surplus leaves can be either frozen for later use or composted, and the flower-buds are edible too.
You can grow sweet corn outdoors in a good summer, but for a more reliable crop you need a greenhouse. April sowings are best done in pots but in May and June seed can be sown directly into its final position. Sow one fresh seed about 1cm deep per tall pot of free-draining compost: if using older seed sow multiple seeds per pot and single the seedlings later with a scissors. Keep the compost warm and not too wet. Sweet corn roots go very deep and do not tolerate confinement, so plant out your crop as soon as possible. Choose the tallest space available, although it won’t matter if the tops are bent over at the roof. Dig large planting holes 30cm apart in a square block to aid pollination. Fill them twice with water and add in some compost when planting; corn needs plenty of water and feeding.
If space is short, you can plant corn between rows of greens due for harvesting soon. Keep watering and feeding. When the sticky tassels appear on the lower female flowers, shake or tap the plants gently each day to shower pollen down from the male flowers above and ensure well-filled cobs. Cross-pollination gives poorly set and less tasty cobs, so if growing different varieties try to plant them as far apart as possible. The cobs mature close together, so sow later crops to extend the season. The cobs are ripe when the tassels darken and wither, and punctured kernels leak milky fluid. They dry up and get starchy very soon, so harvest them promptly, cook then immediately for four minutes in boiling water, and enjoy.
Pollination occurs naturally outdoors, where wind and insects carry pollen from one flower to another to fertilise them and set fruit. Tunnels and glasshouses shelter plants from wind, and fewer insects go inside so natural pollination is sometimes not enough to set a full crop of fruit. Incomplete pollination of a flower can produce a fruit that only develops and grows on one side while the other side remains hard and misshapen. This can be a problem with strawberries. Peaches, nectarines, grapes, melons and aubergines among others can yield better with help.
Hand-pollination is the answer. When the first flowers are just fully open and conditions are dry, brush over the flowers gently with a very soft brush or a little cotton wool. In the past gardeners used rabbits’ tails, but animal welfare was not a consideration then. Paintbrushes are a bit too stiff for pollination, but the likes of a camera lens brush is perfect. Never use one on a lens afterwards, because it picks up oils from pollen that would smear the lens. Pollen grains have spines for gripping onto hairs such as on bees’ bodies, and they brush off onto the sticky stigmata of other flowers – job done. It is best to repeat for a few days running to ensure good pollination and catch the later flowers. Tomatoes are easy to pollinate by vibration; tap on the flower trusses or the supporting canes or wires, or water them from above with a coarse spray of water.
Cold frames are four-sided boxes with transparent covers sloping towards the sun. You can buy readymade ones or easily make your own with new or recycled materials. They are out of fashion nowadays because more people have tunnels or glasshouses, but still very useful.
You can use cold frames to hold plants for which there is no room in your greenhouse just now. They are good for hardening off vegetable plants or half-hardy annuals raised in the greenhouse before planting them out in the garden. You can sow seeds earlier than possible outdoors if greenhouse space is not yet available, or quarantine new plants, or keep plants that need warmer or cooler conditions than you have in the greenhouse. Cold frames are good for rooting cuttings, or warming up water or potting compost before use. If you need an extra degree or two of frost protection for plants inside the greenhouse, you can put a lightweight cold frame over them for the night.
Cold frames are best placed near your greenhouse for convenience, and facing as near south as possible. Their south walls should be low enough to let in the sun. To ventilate, prop open the downwind side of the top cover. If you raise its upwind side, a gust could flip it off, and if you slide it the gap may be on the wrong side letting in chilling draughts. It is harder to control their temperature due to their small volume, so you must anticipate the day’s weather and ventilate accordingly, closing the cover at night. If you expect hard frost, lay a quilt of weighted bubble-wrap or sacks stuffed with leaves over the cover for extra insulation. Water plants in the mornings to let the leaves and soil surface dry off before night.
As the days get shorter and colder, you might think about bringing an electrical power supply to your tunnel or glasshouse. Electricity can power lights, heaters, soil warming cables, propagators and climate controllers, letting you grow a wider range of plants over a longer season. But it’s not just a matter of running an extension cable out from your house; domestic cables and fittings are neither shockproof nor waterproof enough for safety in greenhouses. Electricity and water are a lethal combination. If you only need working lights you could use wireless battery or solar-powered lights with high-efficiency LED bulbs. If you already have a low-voltage garden lighting circuit nearby you could take a short spur off it, provided it can handle the extra load.
If you need mains power, you must get a registered electrical contractor to do the specialised wiring work. At the least, the supply from the house should be through Steel Wire Armoured (SWA) cable buried at least 50-60 centimetres underground with warning tape above it. It should pass through a Residual Current Device (RCD), which monitors the flow of current out from the distribution board and back and trips instantly if they are not the same (i.e. current shorting elsewhere). All sockets, plugs and fittings should be of heavy-duty industrial type with a much higher Index of Protection (IP) rating than domestic ones. They are not cheap, but neither is human life!