Still plenty of overwintered greens
April is such a month of promise. Seedlings are growing fast and the weather might even be hot. Inside the polytunnel it will be warmer still. It’s easy to imagine the space filled with melons and grapes, or to look at those small tomato plants and imagine picking a bumper crop. The next few weeks are crucial ones: plants get put out into the borders and growth steps up a notch or two. It can seem hectic at times, with lots of small tasks to attend to, but things will calm down before too long.
Weather report
April might bring showers and even snow, but the year is on the move. Sunshine levels are increasing and there is a strong possibility of some good hot days. Night frosts might disappear in some parts of the country by the end of the month, but don’t be in too much of a hurry to declare the last frost passed – more can appear in May.
April jobs in brief
Water carefully
Open the propagator on warm days
Keep sowing
Keep potting on seedlings and small plants
Soak the subsoil before planting out
Harden off tomatoes and courgettes
Plant out tomatoes, courgettes and French beans
Feed strawberries
Organize growbags and pots
Aid pollination
Clear old crops
Open doors to provide ventilation
Replace old and torn polythene
Watch out for greenfly
Cucumbers
Sweetcorn
Pumpkins
Climbing French beans
Spinach
Tomatoes
Salad leaves
Lettuce
Kohl rabi
Basil
Fennel
Marigolds
Courgettes
Broccoli
Sprouts
Kale
April sowings and buying in plants
• Seed sown in April should germinate quickly and seedlings usually suffer fewer setbacks to growth as temperatures start to settle. Plants won’t quite catch up with February and early March sowings and so will not produce the earliest crops, but they won’t be too many weeks behind.
• If you haven’t sown peppers and aubergines by now, it is probably best to buy in plants. These stand the best chance of producing a summer crop. There is still time to try sowing melons at the beginning of April, but only if using a fast, early and reliable variety, like ‘Sweetheart’. If the summer is hot these will bear fruit. Otherwise, buy plants and keep them warm until they are ready to go out.
• Tomato seed can be sown up until the end of April, but it might be best to buy in a few plants of medium or large varieties. Early April sowings of cherry varieties will start to crop in mid- to late summer and may well give the best autumn fruit.
• Sow basil if you didn’t do so in March. In fact, it can be wise to make a second sowing in April just in case anything goes wrong with the March sowing and you need a few extra plants.
• Try sowing Florence fennel in small pots or cells around mid-April. This crop gets off to a great start in the polytunnel. Put two seeds in each pot – you will thin these later to leave the strongest seedling to grow on. For good germination fennel needs a bit of heat, but not too much (12–15˚C/54–59˚F is fine). If nights are cold, germinate seed inside the house, or find a space at the edge of the propagator. This sowing will usually be destined to grow on outdoors, but summer sowings of fennel can be grown on in the polytunnel.
• Climbing French beans will stretch up to the roof. Sow early in April for plants that will keep cropping right through the summer and even into October. Climbing French beans come in a variety of colours. Sow a few each of purple, green and yellow varieties for a really attractive display. ‘Blauhilde’, ‘Neckar Queen’ and ‘Goldfield’ will give the full range.
• In a large polytunnel, it makes sense to sow lettuce and rows of salad leaves right through the summer. If the tunnel is small, however, save the space for more exotic crops and grow salad outdoors. The same applies to spinach and kohl rabi, although both of these do exceptionally well in a polytunnel.
• Early April is the best time to sow sprouting broccoli, kale and Brussels sprouts. These plants will not find a permanent home in the polytunnel until much later in the year, and in the meantime they will have to grow on in large pots outside the tunnel door. Sowing in April will give the fastest-cropping, largest plants, but May or even June sowings will still give good crops. For a simpler option, buy in plants in plugs in a few weeks’ time.
Propagator full of small plants
Careful watering
All plants need water, but too much can be as bad as too little. Waterlogged seedlings will collapse and disease is always a danger; even mature plants hate a soggy root run. It’s best to assess how much more water a pot really needs, rather than soaking at random with a hosepipe. Push a finger into pots, or lift them up to check how damp the compost is around the drainage holes. Peat-based composts tend to dry out on the surface while the underneath might still be damp. On the other hand, propagators heat from underneath, so the bottom of the pot may be dry even though top layers are damp.
As a rule, small plants usually do better if kept slightly dry rather than soggy. This is especially true for cucumbers and melons – too wet a compost will lead to collapse. Tomatoes are an exception: stand pots in a bucket of water, from time to time, so that the compost is thoroughly wetted from the bottom up.
Keeping plants warm
Most tender plants such as peppers, aubergines, cucumbers and melons will suffer if the temperature falls below 10˚C/50˚F. Ideally, to crop at their best, they shouldn’t be exposed to temperatures below 15˚C/59˚F, but this can be hard to guarantee. Tomatoes are a little hardier and can cope at 12˚C/54˚F quite happily (or even lower once they get their roots into the soil). Remember: you can carry vulnerable plants in pots indoors at night and put them back out in the polytunnel in the morning when things warm up. This ensures that they have plenty of light and adequate warmth in the absence of a propagator. It may seem like a lot of work, but it’s only for a short while until outdoor temperatures settle down.
Tomatoes and courgettes can be left out of the propagator from the middle of the month if night-time temperatures are above 12˚C/54˚F. This will help harden them off, and it will also free up propagator space for other plants. If space is needed, and temperatures aren’t reliably high at night, use a cloche to cover the plants. Drape an old duvet or blanket over the top at night and temperatures won’t fall too low underneath.
Once you put plants out into the border soil, the stems and leaves have to suffer what fluctuations of air temperature are thrown at them, but the roots will be kept at a more steady temperature in the soil. If nights are cold, cover rows of plants, or wrap a few sheets of newspaper, a sack, or a piece of fleece around individuals, but remember to remove coverings during the day.
Remember to open the propagator to keep temperatures from getting too high on sunny days. Close it at night to keep plants warm.
Cucumbers
It is worth making a second sowing of cucumbers in April. Sow single seeds 1cm/½in deep per 8cm/3in pot. These will grow rapidly to give a good summer crop. A sowing this month safeguards against any failure of earlier plants. Try an all-female variety like ‘Tiffany’ or ‘Flamingo’; or for a less temperamental option try ‘Burpless Tasty Green’, which produces male flowers, but they don’t have to be removed, so there is no extra work attached.
Young plants from March sowings should be growing rapidly. Be fanatical about keeping the compost on the dry side of damp, in order to avoid root rot, and keep potting on into larger pots rather than risk a crowded root system in a small pot.
It isn’t usually warm enough to plant cucumbers out in an unheated polytunnel in April. However, if plants go into a hotbed (see May) and if night temperatures are above 12˚C/54˚F you can plant cucumbers out at the end of the month. If temperatures are much lower than this, keep plants warm and wait until May.
Young cucumber plants
Sweetcorn
The first sowings this month will give ripe cobs in late July and early August. Make a second sowing three weeks later for a staggered crop. Cross-pollination between some varieties can lead to poorly filled cobs and in supersweet varieties the kernels can be less sweet. To avoid this, sow one variety at a time. Most modern varieties produce wellfilled cobs with yellow kernels. Try the variety ‘Indian Summer’ for multicoloured cobs and a really sweet flavour.
Sweetcorn needs a bit of warmth for good germination, but avoid cold wet compost, as otherwise seedlings will flop. Sow one seed 1cm/½in deep per 8cm/3in pot, or use root-trainers. Sweetcorn puts down long roots, so anything you use for raising seed should be as deep as possible. Keep at 20˚C/68˚F until shoots appear, and then try to maintain a temperature above 12˚C/54˚F.
Pumpkins and squash
Sow a couple of pumpkins (or squash) at the same time as sowing sweetcorn. Sow more if you plan to raise extra plants for growing outside. Pumpkins can take up a lot of room, but less so if they are twined among the growing stalks of corn. The pumpkin foliage covers the ground and keeps it moist – a bonus for the corn.
Sow each seed on its side in a 12cm/5in pot. Plants grow quickly and a large pot saves potting on.
Varieties that can be slow to fruit outdoors can do really well in a polytunnel. Try ‘Butternut Squash’ and ‘Uchiki Kuri’, or ‘Small Sugar’ for the most fruit of all.
Soak the subsoil before planting out
Soil can really dry out in the borders of a polytunnel, and especially so if these have been left empty and unwatered for several months. A sprinkling of water over the top of dry soil can run through without wetting, or it will simply dampen the surface layer. Some plants, like tomatoes and sweetcorn, can grow extensive root systems that will search for the essential moisture, but it is important to provide them with a good starting stock. So take a little time to get the subsoil thoroughly wetted before planting out thirsty crops.
To do this, dig a hole about 30cm/12in wide and deep for each plant. Fill this with water, let it drain and then fill it again. Repeat this until the subsoil is thoroughly soaked, as well as the surface layers. Do this about a week before planting tomatoes to allow excess water to drain before you put young plants out. At planting time the hole will be filled with damp compost, but by then the surrounding soil will have a store of water that will last for weeks.
The same technique can be used with plants grown in a trench. Simply fill with water, and allow to soak before refilling again.
Soaking the subsoil
Tomatoes
These are the main polytunnel crop for many people. They can be spectacular producers and the taste of a home-grown tomato is quite superb. Success with this crop depends on the variety to a certain extent. It depends much more on how plants are fed and watered, and if they have adequate support and enough room for air and light to circulate. You can establish good patterns for all these with good planting techniques.
Tomato plants can be spectacular producers
Top Tip
I always plant my tomatoes out into an unheated polytunnel by the end of April. I also always dither about the exact date, wondering if it is warm enough, if plants are big enough and if they have been hardened off enough. The answer to all these, in my garden, is always yes. The tomatoes not only survive but also thrive once they get their roots down into a rich damp medium and I have never lost any. The secret is good soil preparation and strong plants. Of course a lot depends on how far north you live. If in doubt, ask a local, experienced gardener. You should also prepare a stack of materials (newspaper, crop cover, woven sacks, etc.) to wrap around individual plants if nights get cold.
Young tomato plants should have been out of the propagator during the day for a few weeks before planting out. You need to wean them off night-time propagator heat as well. Do this when night temperatures seem settled and not too cold. Plants should never be exposed to sudden jumps in temperature; use fleece, cloches, etc., if necessary, to acclimatize them to lower temperatures.
Preparing the bed and planting out tomatoes
• Dig manure into the bed well in advance of planting, if possible. Then dig out large planting holes, fill with water and allow to drain until the subsoil is soaked (see above).
• Push a support cane or stick into the soil at the bottom of each hole. Use a crowbar if necessary, so that you can push the cane well down. Sticks and canes need to be long enough to support tall tomato plants, but they mustn’t rub against the polythene. They also need to be strong enough so that they won’t break or bend under the weight of the crop. String is an alternative to canes: simply wind this around the root ball when planting and tie the other end up to the frame of the polytunnel. However, this method works only if tomatoes are to be grown beneath the hoops of the frame, or if you string a wire between the hoops, so that you can tie the strings in.
String made from natural fibres may rot and break under the weight of the crop. For a strong support man-made fibres are a safer bet.
• Fill each planting hole with compost or well-rotted manure. Add a bit of dried seaweed, or wood ash, to provide extra potash for these greedy plants.
• Tomato plants should be 15–20cm/6–8in tall at planting time. They should have strong-looking stems and deep green leaves. Reject any spindly or discoloured plants. A very slight tinge of yellow isn’t too bad: this means the plants have exhausted the compost in the pot and are ready to move on.
• Tip the root ball out of the pot and check that the roots look healthy. If roots have wound around the pot, tease them free so that they can gain easy access to the new planting medium.
• Allow plenty of room between plants: 45cm/18in apart in the row is fine, but allow at least 75cm/30in between rows. If rows are closer, plants must be further apart in the row.
• Young plants should be tied to the canes or strings to support the growing stem. Do this roughly every 30cm/12in as the plant grows. Don’t pinch out side shoots until the young plants have established themselves in their final home – the side shoots won’t adversely affect growth if left in place for the first week after planting out.
• Tomatoes can be planted into growbags, or 25cm/10in pots, with great success. Cut slits in the bottom of a growbag, so that roots can work their way down to the soil beneath. This isn’t essential, but a sealed bag means you have to pay greater attention to watering and feeding all summer long.
• Another way to boost feeding power is to cut large round holes in the growbags and push 20cm/8in pots (with plants) into these. Roots will grow down through the pot and into the growbag.
NOTE: If conditions aren’t right for planting out, always pot up into a larger pot rather than risk restricting growth.
Planting tomatoes
Planting French beans
Dwarf varieties, sown in pots last month, can be planted out as soon as they have two proper leaves, but wait a little longer if temperatures are below 8˚C/46˚F. Plant in a double row over a trench filled with compost, the rows 20cm/8in apart, and allow 15cm/6in between each plant. Move as much of the root ball as possible without damaging roots. If the soil is acidic, scatter a little hydrated lime, or wood ash, over the soil around the plants.
I find that French beans can survive temperatures as low as 5˚C/41˚F, if a cold night comes out of the blue, but the plants will die if temperatures dip much below this. A double layer of fleece is usually enough to keep plants safe on cold nights.
Planting courgettes
Courgettes sown last month can be planted out in mid-April. Allow plenty of space for the plants to grow – they often get much larger than ones grown outdoors. Dig a hole about 30cm/12in square and deep. Thoroughly wet the subsoil (see April) before filling the hole with compost. Courgettes are thirsty plants, so it is worth leaving a slight depression around the young plant. This will act as a reservoir when you water in the months ahead.
Always put a stick in the ground to mark where the stem goes into the ground. By the time the courgette plant is fully grown it can be hard to find the best point to water among all those leaves.
Courgettes are cold sensitive, but far less so than some other plants. Air temperatures can go down to 5˚C/41˚F in the tunnel with no obvious setback, provided this doesn’t persist for more than two or three nights. If the night is to be really cold, throw an extra layer of fleece over the plants and they should be fine. Better still, to maintain steady growth, cover the plant with a large, clear plastic tub or container. This will act as an easily removable cloche.
Top Tip
If space is limited, try growing a courgette plant in a large pot or bucket and move this outdoors once the weather warms up.
Potting on aubergines and peppers
These plants grow slowly and at times it may look as if they haven’t moved much at all. Check to see if roots are restricted in the pot and if that’s the case pot them on. Aubergines and peppers won’t go out into an unheated polytunnel until well into May, or even June, but growth mustn’t be held back in the meantime. These will probably be the last plants to leave the propagator – they really do need heat if they are to flower early and to crop well.
While handling each plant, give it a health check. A little attention now may help avoid future problems. Watch out for greenfly, which seem to be particularly fond of young pepper plants. If you spot that leaves are curled or distorted, this could be the problem. Squash the greenfly between finger and thumb, or take the plants outdoors and give them a good squirt of water to dislodge the pests. The fallen greenfly will stay outdoors when you return the plants to the tunnel. This is safer than using chemicals on sensitive young plants.
Plant out lettuce, spinach and kohl rabi
If you sowed these in pots in February or early March, they will be ready to plant out when 3–5cm/1¼–2in high. Make drills about 5cm/2in deep in the border soil. Mark the row with sticks and string. Fill each drill almost to the top with compost. Lift each plant carefully, keeping the roots intact. Firm them into the compostfilled drills, allowing 15–20cm/6–8in between plants and 25–30cm/10–12in between rows.
Top Tip
If you plant lettuce in blocks, be sure to allow enough access at each side. You should be able to reach in to weed, or to pick, without trampling on the growing plants.
Leeks and celeriac sown in February will have produced a flush of healthy seedlings by the beginning of April. These should be pricked out and potted on into deep tubs of compost. There is no need to keep them in the polytunnel once they are established, but it is best to keep them just outside the door. That way you can pop them back inside until fully hardened off.
Small beetroot plants in cells should be put outside to harden off at the start of the month. These will soon be ready to plant in an outdoor bed. Try growing a few inside the polytunnel if there is space – they will be ready a few weeks before the outdoor crop.
Feed strawberries
Plants in pots appreciate a liquid feed while the fruit is swelling. Apply this every seven days to give the best chance of growing lots of large, juicy strawberries. Try using a liquid feed made from comfrey and horse manure. Liquid seaweed works well too.
Don’t let pots dry out at this point, as if you do the fruit will be small and poorly formed. Check leaves for mottling caused by virus disease and remove any severely affected plants, which will never crop well and may infect other healthy plants.
In a warm spring, a few fruits may ripen at the end of the month. Enjoy!
Strawberry plants in flower
Top Tip
You may need to use netting to protect ripe fruit from adventurous birds, which will fly in through open polytunnel doors or squeeze under closed ones.
Grape vines
If you didn’t feed vines last month, it’s not too late to do the job now. A layer of manure, as a mulch around the stem, is the best way to provide a good supply of nitrogen.
New young shoots will grow rapidly. On newly planted vines, nip out the growing points of all except the lead shoot. This should be done after the side shoots have made six leaves.
Think about where to train the vine. Up and along the ridge is usually the best choice, provided it doesn’t create too much shade.
Crops in containers
Carrots should be growing well in their containers and putting up plenty of leaf. Once carrot foliage is around 10cm/4in tall, slugs seem to leave it alone. Keep the carrot pot watered and fed while the young roots swell.
Potatoes in buckets need earthing up around the stem as the foliage grows. The bucket will be only half full of compost at planting time. March and April should be the right time to top up January and February sowings. Fill up to the top with compost, leaving just enough room to make watering easy. When filling the tub hold stems aside so that they aren’t damaged, and try not to bury leaves – a good clump of foliage must remain above soil level. If necessary, earth up in two stages as the foliage grows.
Early crops may be ready to harvest.
Tomato plants can start to flower at the end of April. Mist them lightly with water around midday. This helps to set the first trusses of fruit.
Mangetout peas sown last October should be covered in flowers in April. The occasional bee might show an interest, but peas are self-pollinating, so don’t need any insect help.
Strawberry flowers do need pollinating. If there are no suitable insects doing the job, use a soft fluffy paintbrush to transfer pollen. Incomplete pollination will lead to misshapen fruits.
Broad beans don’t need any help outdoors, but they give a better set of early pods in a polytunnel if the plants are near an open door. A paintbrush might help, but it can be hard to get a brush into the flowers.
First flowers on tomato plants
Make room for summer crops
Remove any crops that have finished producing, or have gone bitter, as the space will be needed for summer crops. It’s still not too late to dig some well-rotted manure, or compost, into the beds after crops clear. Now is the time to decide which crops have finished and can easily be removed, and which will grow on to provide pickings for a few more weeks.
Check over rows of overwintered salad crops. Tear a leaf off and taste it! Some lettuce can turn horribly bitter and some of the oriental salad mixtures can get quite hot. Most leaves get tougher the larger they grow. For an extra spicy flavour, the leaves of crops like rocket and mustard greens can still be picked even when the plants are in full flower.
A bit of fresh air
Ventilation is really important in April. Avoid cold draughts, but try to prevent temperatures rising much above 30˚C/86˚F. This will mean judicious opening of doors on any sunny day. If it’s windy outdoors, but temperatures are rising in the polytunnel, open the most sheltered door and use a heavy weight to stop it flapping. Always make sure that wind has a way to get out of the polytunnel if it blows straight in.
A new cover for the tunnel?
It may be the last thing that anyone wants to think about when the polytunnel is full of young plants, but April is about the best time to replace a polythene cover. If you leave this job until much later, the tunnel will be filled with large crops. Do it any earlier and the temperature will be too cold to ensure a good fit.
It is always best to choose a warm, calm, dry day for replacing polythene. This means the polythene is supple and expanded. Get the best fit possible on a warm day and when temperatures drop the polythene will shrink back to an even tighter fit. This helps avoid ‘floppy polythene syndrome’ and so can extend the life of the covering.
First mangetout peas
First broad beans
Salad leaves
Lettuce
Spring cabbage
First strawberries
Kohl rabi
Spinach
Swiss chard
Sprouting broccoli
Oriental leaves
Beetroot
Baby carrots from autumn sowings
First early potatoes from December planting
Mangetout peas
Broad beans
April harvest
Some harvesting hints
• Mangetout peas sown last autumn should be covered in flowers in April, as should broad beans. Keep plants well watered while they are in flower. The first pods will be ready for picking from the middle of the month onwards. Pick while these are small and more will keep coming. The first pods are lovely raw in salads.
• Spring cabbage comes into its own and fills the traditional ‘hungry gap’, although of course there shouldn’t be any gap at all if you have made regular sowings in the polytunnel. Use cabbage while the leaves are tasty and tender; if these are left too long the outer leaves become tough.
• Kohl rabi should be used at tennis-ball size or smaller. They will still be juicy and full of nutty flavour at this stage, but may run to seed as temperatures rise.
• Swiss chard ‘Bright Lights’ will keep cropping through the month, lending a splash of colour. Use both leaves and stems.
• Spinach may look tatty but it will keep producing until the new sowings take over.
• And, of course, there should always be plenty of salad leaves.
• Sprouting broccoli should have been cropping for several weeks at this stage. Make a last picking before evicting exhausted plants.
• Beetroot will be coming to an end from an autumn sowing. Pick and eat before plants bolt and the roots turn stringy.