January brings a moment of calm, with Christmas a thing of the past and some New Year gardening resolutions firmly in place. The months to come will get busier and busier, so use this one well to ensure that everything is organized for the year ahead.
If the polytunnel is empty now, it’s worth trying a few early sowings. It will be several weeks before any of these are big enough to harvest, but the sight of seeds freshly sown in compost is the first exciting hint of the bountiful crops to come.
Weather report
January can bring some of the coldest weather of the year and some of the hardest frosts. Temperatures below 0˚C/32˚F are common enough and there is a strong possibility of snow in all but a few milder coastal extremes.
January jobs in brief
Keep a diary, make a plan and order seeds
Make a few early sowings if weather permits
Sow aubergines in pots at 20˚C/68˚F
Plant out lettuce sown last November
Plant out mangetout peas and broad beans sown last autumn
Keep light levels as high as possible
Keep on top of repairs to the polythene
Water sparingly and only if necessary
Start feeding crops
Bring strawberry pots in from the cold
Finish pruning grape vines
Try sowing in January
A few early carrots
Mangetout peas
Salad leaves – rocket, mizuna, mibuna
Spring lettuce varieties
Radishes
Sowing mangetout peas
Making plans
Spades and hoes may be obvious tools for the garden, but don’t forget the notebook and pen: keep a diary of the polytunnel year as it unfolds and this will be an invaluable tool in the years ahead. Make a note of when you sow seeds and how well they germinate, when crops are ready to eat, what things do well and what fails. If there are problems with particular pests and diseases, write these down as well, and always date each entry. The more information, the merrier – or at least, the more use the notebook will be next year, when the process starts all over again.
It’s a good idea to dedicate the first page to a wish list of all that you would like to grow in the polytunnel. On the next page, draw a sketch plan showing how all these crops will fit in – or not, in which case you may have to cross some things off the list.
If possible, the plan of the tunnel should be divided into four quarters, so that you can practise crop rotation on a four-year cycle (see Part 6). A crop like tomatoes may occupy one of the quarters, in which case this crop will not be grown in the same spot for another four years. In a small polytunnel, tomatoes may take up half of the space, but these can at least be moved to the opposite side every other year.
Top Tip
Tomatoes, peppers and aubergines grow better in a drier atmosphere, whereas cucumbers, courgettes and melons prefer a humid one. It isn’t really possible to provide both sets of ideal conditions at once, but when drawing up a planting plan try to group accordingly.
Is it too early to sow seeds?
There are three things to consider before making any sowings in an unheated polytunnel in January:
• What temperature do the seeds need to germinate?
• How warm is the soil inside the polytunnel?
• Might this be an anomaly, or is the temperature likely to stay roughly the same at this time of the year, in this part of the country?
Of course, it can be hard to predict what the weather will do for more than a few days, and the possibilities can vary wildly from year to year. However, it is usually possible to make some educated guesses based on local knowledge. In southern parts of the UK and Ireland it is often possible to make a few sowings in mid-January. In the Midlands it may be best to wait until the end of the month and in the north of Scotland buy a propagator, or wait a few weeks more before sowing without heat.
Having said that, there is little to lose by trying a few early sowings and the gains can be great in a mild year. A polytunnel really does make a difference: it’s amazing to watch seedlings push through in weather that seems too cold to allow anything to grow. If the first sowings don’t work, try again next month, but don’t leave it too late for a shot at the earliest crops. Wait until a cold snap is over and choose the right moment. So long as seed germinates and seedlings push through, it doesn’t much matter if growth is then slowed by another cold spell.
Top Tip
Soil in the polytunnel will usually be a degree or two warmer than that outside. To make it warmer still, and to protect seeds in trays, use cloches, bubble wrap or fleece to hold in the heat.
Carrots
If the polytunnel is large enough, you can sow carrots directly into the border. It isn’t worth having later varieties taking up space, but the earliest ones will give tasty young roots several weeks ahead of ones grown outdoors. ‘Early Nantes’ and ‘Amsterdam Forcing’ perform particularly well from late January sowings, but if the winter is cold wait until February.
Possibly the best way to get a crop of sweet young roots is to sow seed thickly in a 30cm/12in pot. Just cover the seed with compost, water lightly and cover the pot with a triple layer of fleece until seedlings emerge. Not all seed will germinate, and not all seedlings will survive, but a good scattering of slim leaves should start to appear in two to three weeks’ time.
Top Tip
To repel slugs, raise the pot on a brick or stones, raised above a moat of water. These pests adore young carrot seedlings and can munch through an entire pot as soon as shoots appear.
Potatoes in the border
Some people like to grow a crop of early potatoes in the border soil. Allow 30cm/12in between plants and the same between rows. If plants are protected from frost, and if they are well fed and watered, a January sowing can give a good return of tasty spuds in late April or May – much earlier than ones sown outdoors. However, if temperatures are consistently lower than 5˚C/41˚F, leave potatoes to chit in the shed and wait until February before planting them out in the polytunnel.
A potato crop will break up new soil and create a more friable, workable structure for later crops, but there are a couple of drawbacks:
• The potatoes may not be ready to harvest before the space is needed for more valuable crops such as cucumbers and tomatoes.
• If the potatoes become infected with blight, this disease can spread from the potato leaves to the more valuable tomato crop growing near by.
Top Tip
Try mulching around potatoes with black polythene or straw. This warms the soil and suppresses weeds. You will raise a faster crop this way, but watch out for slugs hiding underneath the mulch.
Potatoes in containers
In an average-sized polytunnel, there probably isn’t enough space to grow potatoes in the border. The best solution is to grow some in buckets or large containers. These can be moved outdoors, if necessary, as the weather improves. Put a 5cm/2in-deep layer of compost in the bottom of a bucket. Choose an early variety, like ‘Orla’, and put one or two chitted seed potatoes (ones with green shoots just emerging from the tuber) on top of the layer of compost. There should be some holes in the bottom of the container to allow good drainage. A bucket with a split across the bottom will drain well enough; otherwise use a drill, or melt holes with a hot nail. Take care: steel conducts heat! Use pliers to hold the nail and be careful not to burn anyone in the process.
Cover the seed potatoes with a further 5cm/2in of compost and cover with bubble polythene or fleece if nights are cold. Young shoots will take a few weeks to appear.
Mizuna, mibuna and rocket
These tasty salad leaves do extremely well from a January sowing. Sow directly into drills filled with compost, marking the rows with sticks and string. Use a good pinch of seed for every 25cm/10in of row. Remember that seed can be slower to germinate when the soil is cold and more seed will fail now than in a couple of months’ time. Cover the seed with a 5mm layer of soil.
NOTE: Slugs and snails will eat emerging mizuna seedlings in preference to mibuna or rocket. The latter two have a spicier flavour, which may not suit a mollusc’s taste. Once mizuna plants are a few centimetres tall, slugs seem to leave them alone.
Mizuna and mibuna
Lettuce
Choose looseleaf, butterhead or cos varieties to sow at this time of year. These are reasonably hardy and do surprisingly well from an early sowing. It may be cold when you sow the seed, but the days are lengthening and soil in a polytunnel soon starts to warm up.
Choose two or three varieties to give a greater chance of success. ‘Salad Bowl’ is a good early looseleaf variety that deserves its popularity. It is a good performer for the beginner or experienced gardener, although the green variety usually does better than the red from such an early sowing. ‘Little Gem’ is a favourite cos type that is quick to mature and gives crunchy leaves in late April or May. Choose ‘All the Year Round’ for a butterhead lettuce from such an early start. These are reliable performers in an average year. Try sowing other favourite varieties, as they may work equally well, but leave sowings of crisphead and iceberg varieties until February or March.
Top Tip
Such early sowings of lettuce do best if seedlings are raised in trays or pots and planted out to their final growing position when 4cm/1½in tall. This method avoids gaps in rows and makes it easier to provide extra protection on cold nights. It also allows you to select the strongest seedlings for planting out and, in a mild winter, it is easier to keep young plants away from the ravages of slugs and snails.
Sowing lettuce
Sow seed thinly in pots or tubs, and just cover with a scattering of compost. Water lightly so that the compost is damp, but not soggy, and cover with a layer of bubble polythene. If the temperature in the polytunnel falls below 5˚C/41˚F, cover with a clear plastic tub or cloche, to retain some extra warmth. You can use a propagator, but set it no higher than 10˚C/50˚F. Or you can start seeds on a window ledge in the house, but don’t leave them there too long: if you do, the seedlings will grow leggy and weak. The aim is to produce strong, hardy plants. Germination takes six to ten days. It can be slower if it is really cold. Remove the bubble polythene once seedlings are up, and the cloche once temperatures rise above 10˚C/50˚F.
Sowing lettuce
Tender plants
It can be tempting to sow tomatoes, peppers, melons, cucumbers, etc., in January and this would indeed give plants an early start. However, young plants will soon outgrow the propagator or window ledge – and they can do this several weeks before the polytunnel is warm enough for you to plant them out. The earliest that some of these crops can be planted out is late April or early May. In a cold year, or a cold part of the country, planting might have to wait until late May, or even early June. If you put a heater in the polytunnel for a few weeks in March, April and May, or if you use a very large propagator, by all means sow early, but nothing will be lost by waiting until February or early March to get most tender crops under way.
Top Tip
There are exceptions to any rule and aubergines are the one tender plant I would sow in January.
Aubergines
Some people find aubergines easy to grow and some find them difficult. An early start is one of the tricks of getting good fruit. These plants usually crop best in a rich, fertile growing medium. Ideally, temperatures shouldn’t fall below 15˚C/59˚F and there should be plenty of sun while the fruit swells.
Sowing an early variety such as ‘Moneymaker’ or ‘Black Enorma’ in January allows plants to flower early enough in the summer to give the best chance of fruit. Plants will have to be nurtured through a few cold months. If this seems like too much work, buy young plants instead in a few weeks’ time.
Sow seed in a pot of compost in the last week of January (or first week of February if that’s how it works out). Sprinkle compost over the top to a depth of 6mm. Few people need more than three or four aubergine plants, so sow twice that many and choose the best to grow on. The compost should be damp, but not wet. Place the pot inside a plastic bag and use a rubber band to hold the bag in place. Aim to keep the pot at a temperature of 20–25˚C/68–77˚F, but in practice a warm window ledge will do the job, even if temperatures fluctuate a little outside this range. Germination can take 7–21 days.
Plant out some sowings from last year
If your enthusiasm for polytunnel growing didn’t wane towards the end of the previous year, you should have young plants in pots all ready to go into their permanent home.
Winter lettuce planted out now will be ready to eat in a few weeks’ time. Whether it goes into a growbag, a larger pot or the border soil, any young plant will appreciate the move; roots should never be left to grow until restricted and deprived of nutrients, for leaves will yellow and plants will bolt while small.
Use a table fork to ease young plants out of the pot with as much of the root ball as possible. For winter lettuce, allow 20cm/8in between plants and the same between rows; these do not grow quite as large as their summer counterparts. Water lightly but don’t soak the soil.
Kohl rabi can be planted 20cm/8in apart in rows 25cm/10in apart. This is slightly closer than it says on the packets, but it works fine for producing tennis-ball-sized tasty roots.
Kohl rabi
Top Tip
Before planting out small plants, fill drills with a mix of two parts well-rotted compost to one part leafmould. This will not only feed them but also keep the roots in a moist growing medium.
Mangetout peas
If you sowed seed in pots last autumn, you may well have planted out seedlings in December. If not, the young plants will now be around 10cm/4in tall and starting to flop in the pots. These should go into the ground as soon as possible.
Dig a trench to one spade’s depth and half fill this with compost from the garden heap. It doesn’t matter if the compost is fairly coarse – it will continue to rot in the trench and will hold a good supply of water. Peas and beans can make their own nitrogen, but they need this layer of moisture-retaining material to keep them supplied throughout the thirsty times ahead.
Fill the top of the trench with loose soil and plant the peas into this. Try to tip the whole root ball out of the pot. If the seed was multiple-sown in tubs, the root ball should come out intact and the peas should be at an appropriate spacing. It is possible to move individual plants a little – aim for a double row 12cm/5in apart with roughly 5cm/2in between plants. Make a depression in the soil to fit plants and roots at the same level, or just slightly deeper than they were in the pot. Firm gently with both hands so that the young plants aren’t loose in the soil. No seeds or roots should be visible.
Planting mangetout peas
Top Tip
If you didn’t sow mangetout peas last autumn it is still worth making a January sowing in the polytunnel. ‘Oregon Sugar Pod’ is a good variety, as it withstands low temperatures. Either sow in tubs, or seed can go directly into the ground at this time of year. If sowing direct, make a double row with 12cm/5in between the two rows and 5cm/2in between peas in each row. Cover with 5cm/2in soil.
Top Tip
Peas and beans prefer a neutral to alkaline soil. If the soil is acid (you can buy a kit to test for this), scatter a little hydrated lime, or wood ash, along the row.
Broad beans
If you sowed a winter-hardy variety of broad bean, such as ‘Aquadulce Claudia’, in pots in November or December, young plants will be ready for planting out in January. Make sure you choose a spot close to a door, in order to get the best set of early flowers; broad beans will self-pollinate, but pollination is more successful if bees can have easy access to the flowers. Make a trench and plant out as for peas (above), allowing 20cm/8in between the double row and 15cm/6in between plants.
NOTE: There is no advantage to sowing broad bean seed in January for a polytunnel crop. Plants take up space and will not crop at a significantly different time to ones sown under a cloche outdoors. However, it might be worth sowing one or two seeds in pots to fill any gaps in outdoor rows.
Strawberries
Strawberries grow really well in a polytunnel and can produce extra-early crops. Young plants in pots should have been out of the tunnel since last summer. They need a touch of frost to help them flower well, but by January they should have had this and they will be ready to bring back under cover. Give all plants a health check before bringing them into the tunnel. Remove discoloured leaves and check the root ball of failing plants to see if any vine weevil grubs are in residence. If so, squash each and every grub before repotting all plants in fresh compost.
Lay a thick layer of manure on top of the border soil and put the pots in a row on top of this. If you use fresh manure it will help heat the pots as it rots. Roots take several weeks to grow out of the pot and into the manure, by which time it will have broken down enough to cause no harm. If the timing is right, the manure provides a good nutrient boost just when the fruit is swelling. Plants brought indoors at the beginning of January should ripen fruit in early May.
Top Tip
Keep pots pretty dry for a couple of weeks until new growth starts.
Grape vines
If you didn’t prune and tidy the vine at the end of last year, do the job now before it starts into new growth. Vines fruit on new growth, so prune back shoots from the previous year to leave one fat bud. It’s always worth checking and removing any dieback after a period of temperatures below 0˚C/32˚F – there may be some dead wood that you missed last year.
Frozen veg!
Plant growth stops at temperatures below 5˚C/41˚F and if it is cold enough, for long enough, plants can freeze inside a polytunnel. If this happens, fill a watering can with cool (not freezing or warm) water and pour this over any affected foliage. The idea is to melt any ice before the sun hits. Plants may look a bit sorry for themselves for a few days, but many will make a full recovery.
Of course, if a really cold spell is forecast it’s best to cover plants with an extra insulating layer.
Light levels
Days are short at this time of year and the amount of light getting through to growing plants can be limited. A small fall of snow is no problem, even if it covers the tunnel: it will often slip free and slide down the polythene. If snow covers the structure for more than one day, however, or if it builds up to a weight that may cause old polythene to split, it is a problem. Use a soft, long-handled brush to sweep snow away. Alternatively, use the brush to lift the polythene from the inside and the snow should slide off. Use the latter technique only if polythene is sound.
The winter months are a good time to remove any branches and prune back any hedges that shade the polytunnel. This is always much harder after a spurt of spring growth, or when trees and shrubs are full of leaf. Take care not to damage polythene with falling branches, especially if these have spines or thorns. Larger branches may need the attention of a tree surgeon – always call in a professional if in doubt.
Repair any rips
Look after the polythene cover of the tunnel. With care, this can last ten years and twenty isn’t unknown. Replacing the cover is an expensive and time-consuming job, but regular checks and repairs can defer the date when it needs to be replaced (for how to repair polythene, see Part 6).
Time for a liquid feed
As overwintered plants start into new growth, they need an extra bit of feed to push them along. Don’t rely on the leftover nutrients in a tired bed. Start using the liquid manures you made at the end of last year, or mix up a new batch early in January for use at the end of the month (for making liquid feeds, see Part 7). It’s amazing how late-winter crops can get moving over the next few weeks.
Top Tip
Wait until soil temperatures are above 5˚C/41˚F before applying a liquid feed.
Spinach
Swiss chard
Pak choi
Kohl rabi
Florence fennel
Spring cabbage
Beetroot
Turnip
Broccoli
Kale
Parsley
Lettuce
Oriental and salad leaves
Some harvesting hints
• If temperatures are below 0˚C/32˚F for a long period, some of the less hardy crops, such as Florence fennel, may not survive. Cover with several layers of fleece to give them the best chance until temperatures rise. Eat fennel while young and juicy. If using the foliage, take a little from each plant.
• If the large outer leaves of crops such as spinach, Swiss chard and beetroot are covered with grey spots that have reddish edges, remove affected leaves. These are unsightly and tough and are best not eaten. New leaves will grow to replace them.
• Keep picking cut-and-come-again crops even if leaves are small.
• Never strip a plant completely, or take out the growing point.
• Take outer leaves off lettuce if you don’t need the whole plant. More will grow at the centre.
• Be patient with sprouting broccoli: don’t nip out the first central spike until it bulks out and a few side shoots grow.