Greenhouses

Strawberries

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.

Onions from seed

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.

Plant garlic now

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 – tips on growing from horticulturalist Peter Whyte

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.

Hand Pollination

By Internet Archive Book Images - Image from page 78 of "Luther Burbank, his methods and discoveries and their practical application; prepared from his original field notes covering more than 100,000 experiments made during forty years devoted to plant improvement" (1914), CC BY-SA 2.0, https://commons.wikimedia.org/w/index.php?curid=34472756Pollination 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

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.

Mangetout Peas

We associate mangetout peas with early summer, but you can harvest them from April onwards if you sow them now. They are surprisingly hardy.  Sow about four seeds of ‘Oregon Sugar Pod’ or any other variety in four-inch (10 cm) pots of good compost and place them in your tunnel or glasshouse.  Protect them from mice until they germinate, and from slugs thereafter.  Don’t water them too heavily, and keep them covered with bubble-wrap or fleece at nights to speed up germination.  Plant them into a composted trench in the border soil when the plants are about 3” (8 cm) tall.  Push in short pea-sticks on either side of the row to support the plants and keep their growing tips off the soil surface, where slugs would eat them.  Ventilate well during the day to reduce mildew infections.  Cover over the rows at night with fleece or bubble-wrap to protect them on very cold nights.  Add in longer pea-sticks if the plants need them.  The plants should be ready to flower by April, and generous watering and some feed will help boost yield.  Harvest the pods often to keep the plants flowering.  You can sow a second, later crop to extend cropping until the outdoor crop comes in.  Enjoy!

Mibuna

Mibuna is a traditional Japanese green crop, ideal for greenhouses in winter. Like mizuna, it is a member of the cabbage family and produces green leaves for salads and cooking over a long period.  It is grown in much the same way.  It is less hardy and productive than mizuna, but its narrow strap-like leaves (often with a white midrib) have a more spicy flavour that gardeners enjoy and some slugs don’t (though some like it hot).  It can be harvested as whole plants, but excels as a cut-and-come-again crop.

Seed sown now in containers or borders can produce leaves from October to April or May next. Add in plenty of compost, and sow seed thinly in drills 1cm deep.  Border rows should be about 30cm apart.  Keep the soil moist: dryness encourages bolting, especially in spring-sown crops.  Watch out for flea beetles and slugs.  Cut off and compost overgrown leaves and any flowering stems that appear; it diverts the plant’s energy into growing more fresh leaves.  Use leaves while they are still young.  Eat them raw in salads or lightly steamed or pickled.

Image courtesy of GIY

Late Vegetables

This summer’s drought may make vegetables scarce and expensive until next year. Plan now to maximise the cropping potential of your tunnel or glasshouse and to leave no square metre unproductive over the winter.  It is too late to sow spring cabbage seed outdoors for planting out in the autumn, but raising plants in your greenhouse now may bring on seedlings faster and have them big enough to plant out on time.  If you do, harden them off before planting out and protect them from hungry pigeons.  Alternatively, plant them in the greenhouse for an earlier crop.  Plant single seed potatoes in large pots now, and they should still have time to mature before midwinter.  Beetroot, white turnips and kohl rabi will produce small but tasty roots, and you can eat their young leaves too.  Rocket, oriental greens such as Pak Choi, kale, winter lettuce, corn salad, Texel greens and winter purslane can be sown in succession for salads.  Calabrese, French beans and Florence fennel can all yield well in greenhouses from August sowings if the early winter is not too cold.

If your greenhouse is full already, bring on seedlings in cell-trays and then small pots until planting room becomes available. You can sow radishes, baby leaf greens and other short-term crops between rows of slower growing crops, as they will be harvested before the space is needed.  You can still have your five a day!

Sweet Potato – great idea for Polytunnel growing

Sweet Potato

Sweet potatoes are related to morning glory and not ordinary potatoes. It’s possible and fun to grow your own.  Plants are usually raised from ‘slips’ taken from sprouting tubers.  You can sometimes buy them online or in a garden centre; ‘Beauregarde’ and ‘Georgia Jet’ (with orange flesh) and ‘T65’ (white flesh) are suitable varieties.  ‘O’Henry’ produces tubers close to its base and is good for container growing.

Bought tubers can be from varieties less suitable for Irish conditions, but you can still get a small crop from their shoots. Organic tubers unsprayed with sprout inhibitor are best.  Stand the tubers half-submerged in lukewarm water with their narrow ends down.  Place them in a warm sunny spot to sprout, changing the water daily.  Cut off the sprouts when they are about 15cm long.  If still rootless, root them in warm water or cutting compost and plant when rooted.  Soak bought-in slips overnight in lukewarm water before planting.

Plant them deeply (to encourage tubers to form at the nodes) in light, well-drained, fertile, neutral or acid soil in full sun. Always keep plants and tubers above 10°C/50°F.  Give warmed water when needed, and feed weekly with tomato feed.  Tie up the sprawling stems or they will root into the ground at the nodes.  Green leaves can be eaten in salads or lightly cooked like spinach, but don’t take too many or you will reduce the tuber crop.  Dig up the tubers after 3-4 months when the foliage turns yellow and dies back.  Avoid damaging them as they bruise easily.  Dry the roots in the sun for a few hours and then cure them for five days at 30-32°C / 85-90°F and 85-90% humidity.  Store them above 10°C/50°F in good ventilation and either eat them soon or blanch and freeze them.

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