Clearing old crops makes room for new ones
May is often the nicest month of the year. There is heat in the sun and everything grows with an enthusiasm that is hard to match. The beginning of May is a busy time for the polytunnel gardener, with young plants jostling for attention and space. Remember your planting plan. Try to rotate crops even in a small space. Use growbags and containers if the soil is likely to carry disease. Cold nights might still be an issue, but don’t panic: the workload will ease up by the end of the month as temperatures settle down and all those plants in small pots get settled into a more permanent home.
Weather report
May can be one of the driest and sunniest months of the year, but there are no guarantees! Watch out for late frosts this month. Temperatures of less than 5˚C/41˚F can do serious damage to tender plants raised in the expectation of heat. If days are bright and fine, nights are often clear and cold. Frosts can be at their worst around the full moon, particularly if this falls early in May. Once this has passed, it might well take the last frost with it.
May jobs in brief
Track down plants if extra are needed
Keep sowing for late summer and autumn crops
Tie tomatoes in to supports and nip out side shoots
Mist early blossom to aid fruit set
Plant out tomatoes, sweetcorn, pumpkins, cucumbers and basil
If it is warm enough, plant peppers, aubergines and melons
Eat ripe strawberries!
Remember to water grape vines
Tie vines so that they don’t touch the polythene
Grow climbing French beans to provide a bit of shade
Plant marigolds to protect tomatoes from whitefly
Harvest regularly
Clear the last of overwintered crops
Ventilate and water to maintain optimum conditions
Damp everything down on hot days
Time to sow
Cucumber
Sweetcorn
Basil
Pumpkins
French beans
Lettuce and salad leaves
Kohl rabi
Spinach
Lettuce seedlings
Thoughts on May sowings
The polytunnel might be almost completely planted up by the end of this month, but there are still one or two important crops that can be started now.
• If cucumber plants have collapsed through cold or root rot there is still time to sow seed in May. Try sowing ‘Burpless Tasty Green’, singly in 12cm/5in pots. Keep these at a steady temperature of 15–20˚C/59–68˚F and watch them grow. They won’t bear fruit as quickly as earlier sowings, but the plants will certainly get full marks in the growing race. By the time August comes around, there will be as many cucumbers on these plants as on the ones sown in March.
• May brings a last chance to sow basil and almost the last to sow sweetcorn to give late crops. Sweetcorn does best from April or May sowings, but remember that cobs from one sowing tend to all become ready within a few days of one another. Some people love this and invite all their friends for a fresh sweetcorn feast. Another option is to make two sowings – one in April and another in May. This will give a staggered crop. Sow sweetcorn in the place where it is to grow now, so that the long roots can grow unrestricted into the ground.
• There’s no problem in sowing a pumpkin in May to grow on in the polytunnel. Sow in situ and it will grow so fast that it might even catch up with April-sown ones.
• Keep sowing dwarf French beans. These crop really heavily under cover. It is well worth having a few short rows if space permits.
• Make regular sowings of rocket and other salad cut-and-come-again crops. These will be ready to cut in a few weeks. Don’t cherish plants once leaves become straggly or bitter; the idea is to have a fast turnover of tasty leaves. These crops can be sown outdoors now, but they will grow faster, and suffer less damage, in the polytunnel.
• If kohl rabi is a favourite, make regular sowings, but if space is short, save this for a winter crop.
Acquiring young plants
Garden centres will stock some vegetable plants, but they may not have many tomatoes, peppers, cucumbers or aubergines left in May. Some seed companies also supply plug plants and although these should have been ordered in advance over the last couple of months, it might be worth phoning round if things are desperate. Another option is to ask a greenhouse or polytunnel grower near by: they might be happy to find a good home for a few surplus plants.
Health check before planting
All plants should be checked over before you plant them out in the polytunnel, and always choose the strongest ones for growing on. (This also applies when buying plants at a garden centre.) Height isn’t always the issue – a shorter, well-coloured plant with a strong stem is often the best choice. Having said this, some weak-looking plants can go on to perform miracles once they get their roots into a decent growing medium.
Watch out for disease. If a young pot-grown plant is already showing signs, dispose of it. Keep only healthy plants in the polytunnel.
Planting out
Once temperatures settle down – often around the middle of May, or after the month’s full moon has passed – you might judge it safe to plant out tender plants. This can be a tricky time to gauge, but at some point it comes down to just going for it. Plants in small pots are vulnerable, since the compost can get too warm and damp, or dry out too quickly. It is better to get plants into the border soil, or large pots, and hence provide a more stable environment for the roots. If this means covering with fleece, so be it.
Become a weather fanatic and keep an eye on forecasts from day to day. Knowing what the next week has in store might influence your planting plans.
If you have grown a few spare plants, leave these in pots. You might need them to replace any that fail from early planting.
Tomatoes
If you haven’t already planted out tomatoes into their final home, do so in May (for planting technique, see April). Remember to plant a row of French marigolds near tomatoes to keep whitefly away. Plants begin to flower this month – try to avoid pot-bound specimens struggling into bloom.
Tomatoes are surprisingly tolerant of lower temperatures, so don’t worry too much if temperatures dip down on the odd cold night. However, if temperatures below 10˚C/50˚F are forecast, it’s worth providing a little extra protection. Wrap a sack, bag or bit of fleece around each plant in the evening and remove it in the morning once the sun has warmed things up.
Tomatoes in pots should be watered from the base. Stand them in a trough or large tub. The worst problems occur when the top layer of compost is damp: it gives the illusion of being adequately watered, but roots may be dry.
Tomatoes are thirsty plants and I like to sink a cut-off milk carton or plastic drink bottle into the soil next to each plant. This makes watering a much faster job in the summer months.
French marigolds keep whitefly away from tomatoes
Tomato side shoots
Most polytunnel tomatoes are trained as a single stem up a cane or string. However, they will try to grow into more bushy plants and so produce side shoots. If these shoots are left to grow on, plants will grow lots of leaf at the expense of fruit. Look in the joints where leaves meet the main stem. Cut, or nip out, any small shoots as soon as they appear there. Check for larger side shoots that have grown unnoticed. Larger shoots will make a bigger wound, so use a sharp knife or scissors to cut them out, leaving as clean a cut as possible. You can nip out smaller shoots between finger and thumb. It’s worth checking every couple of days and removing side shoots while small. A large wound can allow disease to enter the plant.
Always check round the base of the stem. New shoots will keep appearing there.
If you grow plants up canes, you should tie them in every 30cm/12in or so. Ties should be loose enough to allow the stem to expand as it grows, but not so loose that they slide down the cane. If you use strings for support it is possible to twist the stem around the string as it grows. An occasional tie will be needed to prevent a laden stem from slipping down.
Bush varieties don’t need to be supported to the same extent (nor do they need to have side shoots removed). These varieties perform well in growbags, or large pots, and can be moved outdoors for the summer months; they don’t make use of the space in a polytunnel in the same way that taller varieties do.
Nipping out a tomato side shoot
Tomatoes: early flower set
Cherry varieties of tomato produce plenty of flowers in May, but there can be problems in getting these earliest flowers to set fruit. Later trusses set without a problem, but the first ones may need a little help. This doesn’t mean going around with a paintbrush, since the flowers will self-pollinate, but you can aid the process by adjusting humidity a little. Mist plants overhead with a fine spray of water in the morning and again in the evening. This ensures the pollen is just slightly damp (not saturated) and most likely to set fruit. This won’t be necessary for second and subsequent trusses, but the first ones always benefit from a helping hand.
With luck, the first green fruit should start swelling in May. Use a high-potash liquid feed, such as one based on seaweed, once a week through the next few months.
Always dilute liquid feeds with plenty of water and never apply them to dry soil.
Sweetcorn
Sowings made last month will be ready for planting out in May. Sweetcorn puts down long roots and doesn’t like anything that hinders that process. Because of this, it is best to get pot-grown plants into the border as soon as possible.
Sweetcorn never does as well if grown in containers, but if you want to try it, choose the deepest ones you can find.
Plants should be planted 30cm/12in apart in each direction. Such close planting is fine if the plants are away from the polythene sides, but use a mulch so that it isn’t a struggle to remove weeds in between. Planting in blocks aids pollination and helps ensure full cobs. Put plenty of well-rotted manure, or compost, into each hole at planting time. Don’t stint on this and don’t stint on water either – fill and refill the hole with water before planting so that it can drain to wet the subsoil. Sweetcorn is a greedy and thirsty plant, but if treated well it can produce two cobs (or sometimes three) per plant.
Plants from one sowing will all crop within a short time. If you want to extend the cropping season, make a second sowing in May.
When planting corn apply a scattering of dried seaweed on the surface of the soil.
Sweetcorn ready to plant out
Pumpkins and squash
If you sowed seed last month, plants will have grown to fill their pots and be ready for planting out towards the end of May. It is fine to sow this month, but sow two seeds where each plant will grow – you will thin these later to allow the strongest one to grow on. Pumpkins need a lot of room and they are greedy feeders. One or two plants would be the maximum that an ordinary polytunnel could cope with. Even then, it might be necessary to trim back foliage and train the stems to grow out through the door if things get really excessive.
Make a hole 30cm/12in square and the same depth, among the sweetcorn block, half fill this with compost, put the plant in and backfill with soil. It is a good idea to make a shallow dip all around the plant to help with watering. Use a long stick to mark the point where you planted the pumpkin. It may seem obvious now, but once lots of foliage grows it will be hard to find where to water.
Butternut squash often does much better in a polytunnel than it does outdoors.
Pumpkin and squash plants
Cucumbers: planting out
It can be hard to decide when to plant out the first cucumbers. Cucumbers are not as hardy as tomatoes, yet not as delicate as aubergines. Early May is a good guess. This choice will be driven by the fact that March-sown plants can be 30cm/12in tall and starting to flower, or even fruit. Also, there is always the fear that large plants in pots can flop overnight if roots start to rot. Cucumber plants hate to be in a pot of warm soggy compost, so keep them fairly dry in the pot. The rot starts underneath and plants are often too far gone before you notice the signs. Once plants get their roots into the more balanced temperatures of a larger bed, rot is less of a problem.
Cucumbers that have been raised in heat should be hardened off for a week before planting out. Too sudden a temperature drop could mean losing the lot. It might be warm enough in the polytunnel simply to spread the plants out during the day and to cover them over at night, but if temperatures are low, the plants will have to hop back into the propagator at night until things warm up.
Cucumbers can be planted into large pots (25cm/10in minimum) or growbags, but they perform best of all in a hotbed (see part 7). Put a layer of compost on top of a pile of fresh strawy manure and leave for a few days, so that the first burst of heat cools, before planting out the cucumbers (45cm/18in apart). This keeps the roots warm but well drained, which helps avoid problems with root rot. If nights get colder after you have planted cucumbers out, drape fleece over the bed for a little extra protection. You can raise the fleece on sticks to act like a cloche and contain some of the heat rising from the bed.
If a hotbed isn’t an option, at least make a mound of soil and plant the cucumber on top of this to aid drainage.
Planting cucumbers in a hotbed
Supporting growing cucumber plants
As plants grow they will need some support. Push a stick into the ground next to each one and tie a string to this. Tie the other end of the string up to the frame of the polytunnel (or to a wire stretched between two hoops). You can twirl the growing cucumber stem around the string. An occasional tie will keep a laden plant from sliding down the string.
Alternatively, tie a stone to the lower end of the string to weight it.
February-sown cucumbers will certainly flower and might well produce fruit in May. Don’t let a small plant bear large cucumbers too early. A good solution is to pick fruit while still quite small and let only one or two grow at a time. By June, the plants will be large and strong enough to carry a much heavier crop. Remember that many older varieties produce bitter fruit unless the male flowers are removed regularly. The seed packet should state if this is the case.
February-sown plants will usually be at peak cropping from June through August. It is worth sowing more seed in early May to get plants that will be at their peak in September and October.
Female cucumber flower and immature fruit
Hardening off peppers, aubergines and melons
These young plants are less cold tolerant than tomatoes and cucumbers, so wait until the weather is settled and cold nights have passed before planting them out. This might be in the latter half of May, or even into June. Never plant out directly from the propagator into an unheated polytunnel, as the shock caused by the drop in temperatures can set plants back. Always harden off in stages. Each stage will take a few days or longer – it is hard to be precise, as this is so weather dependent.
1. Uncover the plants in the propagator during the day. If temperatures drop, heat will still kick in from underneath. This stage might start as early as a warm March or April.
2. Lift the plants out on to a sheet of polystyrene during the day. This helps keep the pots at a steady temperature. Put them back in the covered propagator at night.
3. Lift the plants out during the day, as above, and leave them out, but covered with insulating material such as fleece or crop cover at night.
4. Put the plants in their final growing position. Still cover them at night if temperatures drop below 15˚C/59˚F.
Nothing will be gained by planting out too early. If nights are cold, keep these tender plants warm until night-time temperatures are reliably above 12˚C/54˚F and preferably closer to 15˚C/59˚F. This may mean waiting until June before leaving them out in an unheated structure.
Covering tender plants with a cloche for a few weeks, or even for months, after planting out can provide extra heat to ensure good crops.
Aubergines
If aubergines haven’t been a success in previous years, try planting two per organic growbag (or one per 30cm/12in pot). Slit the base of the bag and sit this on soil that you have enriched with manure or compost. The plants love the warm growbag compost for an early start, but the roots will find their way down into the enriched soil in the summer months.
If plants are to go in the border, try to get hold of some well-rotted horse manure. This can work wonders if dug into the soil before planting. Allow a minimum of 45cm/18in between plants.
Water aubergines at the base of the plant, rather than wetting the leaves and flowers. The exception to this is in really hot, dry weather, when red spider mite can get out of control in the polytunnel: aubergines are particularly susceptible to this pest and the best way to avoid it is to keep everything damped down.
Peppers
Sweet peppers can produce great crops when grown in the border soil. To grow well they like plenty of organic material and potash. The organic material holds moisture in the soil. Peppers are thirsty plants and they will not crop well if denied access to water. Dig in an equal mix of compost and manure if possible. Manure alone contains too much nitrogen and the plants will produce leaf at the expense of fruit. Compost alone can be too low in nitrogen and leaves will be pale and plants stunted. Add a scattering of seaweed meal to provide potash. Plants should be 45–60cm/18–24in apart, depending on the size of the variety. Push a support stick down into the soil at planting time. The plant can be tied into this as it grows.
Peppers can also be grown in large pots or growbags (one plant per 30cm/12in pot and two plants per growbag). Chilli varieties particularly like this treatment, but they will exhaust available nutrients around eight weeks after planting. Since plants can crop right through to December, it’s important to feed them in the months ahead and to be vigilant about keeping compost moist during flowering to ensure good fruit set.
Chilli varieties do really well in large pots. I always plant one small variety in a 30cm/12in pot. I grow this on in the polytunnel, but bring it indoors in October. This provides fresh peppers and a splash of colour in the house through the winter.
Melons
Melons seem to suffer most if temperatures fall too low. If the weather is settled and night-time temperatures in the polytunnel don’t fall below 15˚C/59˚F, you can plant melons into the border soil, or three per growbag, or one per large pot. If there is any doubt about the weather, it is safest to keep the young plants growing with a little added heat. Pot on into larger pots if necessary and plant out in June, if that’s what the weather dictates.
Dig some manure into the soil, and then heap up a small mound around 10cm/4in high, so that the planted melon is elevated a little. This provides a better drainage system for temperamental roots. Alternatively, fill a wooden frame with compost, or make a hotbed and plant melons in this. Plants can be planted 30cm/12in apart if you intend to train the stem upwards – in which case make sure a string can be tied into the polytunnel hoops, or use a cane. If plants are to ramble over the ground they need a wider spacing (45cm/18in).
I usually ignore advice to pinch out growing points on melon plants, as anything that delays fruiting means the plants may not get a long-enough season to produce ripe fruit.
Root rot is a problem in a cold, wet root run and stems can rot at ground level. To avoid this, I make a small hole in the side of the mound and water into this. This technique gets water down around lower roots, but keeps compost drier at ground level.
Basil
It’s a good idea to get plants into the soil as soon as the weather warms up. They can tolerate a little cold much better than a restricted root ball in a pot of warm compost can. In fact, healthy young plants may start to flop if they don’t get planted out soon. Even if young plants struggle for a week or so after planting, they quickly pick up and grow on well.
Don’t split up clumps of small plants. Basil likes company, so plant two or three together and allow 30cm/12in between clumps.
Soil for a good basil crop should be enriched with compost or well-rotted manure. Basil doesn’t do so well on impoverished or heavy soils. Don’t overwater; try to mimic the drier conditions of the Mediterranean, where basil grows so well.
Strawberries
There should be plenty of ripe fruit this month, but a lot depends on the amount of sun. If May is hot and dry, berries will ripen earlier than they will if the month is dull and grey. Pick fruit as it ripens and cover with netting if birds are getting into the polytunnel and pecking at the berries. Watch out for dogs and small children as well – both of these pinch strawberries! Remember to water and apply a weak liquid feed while the berries are swelling. If the compost dries out the plants will produce small hard fruit.
Always remove any berries that show signs of grey mould.
Grape vines
Keep the vine under control. It is better to cut out some shoots than shade the entire polytunnel with a mass of leaves. Tie the main shoot in as it grows, but allow enough length in the tie so that foliage hangs away from the polythene. Remember that grapes need lots of water. Ideally roots should be outside the structure and so benefit from rainfall, but if the weather is dry, water roots well for a good crop of juicy fruit.
Small clusters of embryo fruit should form on the vine during May. You will need to trim back the shoots that grow these in June.
French beans
The first French beans might be ready for picking by the end of May. Pods dangle under the leaves in clusters, so take a careful look. Pick pods while small and firm and plants should continue to crop for several weeks.
If you sowed climbing beans in pots last month, they will be ready for planting out now; or you can sow early in May for a slightly later crop.
Fill a trench with plenty of compost. Use some from the heap – it doesn’t matter if it is part-rotted. Beans don’t need a lot of nitrogen, but they do like a nice moist root run. If the soil is acid, scatter hydrated lime on it at planting time.
Plants of climbing varieties can be planted 20cm/8in apart in a single row. Push a stick into the ground next to each one and tie a string from this up to the roof structure. Beans will naturally twine around the string, but they may need a little help in getting started.
Some plants, such as lettuce, don’t like too much direct sun. If you plant them in the shade formed by a curtain of climbing beans, they will grow better than if they are exposed to the full heat of the polytunnel.
Clear seeding crops
Any bolting overwintered crops should be pulled early in May to make room for new planting. Don’t treasure those old greens for too long – there should be plenty of delectable fresh pickings coming in.
Interplanting is always an option until plants clear. This can even provide some protection to new plants that you put out in between the old.
Ventilation and temperature
Along with watering, ventilation rates top of the list for success when you are growing under cover. With no ventilation, temperatures can get high enough to kill plants. With too much ventilation, cold and wind chill will do the same job. Open and close doors as necessary to maintain a steady growing temperature. Try not to let this go over 30˚C/86˚F or under 15˚C/59˚F ideally (or 10˚C/50˚F more practically) and most polytunnel crops will grow very well.
Plants in small pots can be killed and even large plants, with roots in the earth, can wilt if temperatures reach 40˚C/104˚F, but the beginning of the month can still bring cold nights. Shut doors in the evening and open them in the morning to avoid dramatic temperature swings.
Watering
Correct watering is crucial to healthy growth. Keep borders damp, spray overhead to keep plants cool and, if it is really hot, water paths: as water evaporates it will cool things down.
New potatoes
Baby carrots
Courgettes
Cucumber
Salad leaves
Lettuce
Strawberries
Mangetout peas
Broad beans
Kohl rabi
Spinach
French beans
Pick ripe strawberries
Some harvesting hints
• There should be plenty of healthy vibrant spinach and salad leaves all bursting with the sweet taste of a fast-grown crop. If there is a good crop of lettuce, start using the first plants when small; try not to end up with lots of huge plants at risk of going to seed.
• Early strawberries will ripen this month from pots brought into the polytunnel earlier in the year. Pick as the fruit ripens and keep an eye out for mildew, removing any affected fruit.
• Keep picking courgettes while they are small and still a treat. Check under leaves for lurking marrows – these may inhibit new fruit from growing.
• Mangetout peas and broad beans will still be cropping abundantly under cover. Evict these at the end of the month if outdoor ones are producing well, and especially if you need the space for other crops.
• Baby carrots and early potatoes can be harvested now from early sowings. These may be small, but oh so delicious! If growing potatoes in buckets, simply slip a hand down the side and remove a few roots while leaving the plant to grow more. Carrots aren’t so easy to remove: turn the pot upside down and shake it hard.
• Pick the last of the spring cabbage before summer catches up. If this crop is taking up too much space, plant sweetcorn or tomatoes in gaps in between. Space will be freed up when the cabbage is removed.