May brings rapid growth and the first taste of summer crops, but late frost is still possible in parts of the UK. Make room for tender plants while keeping a close eye on warmth, water and ventilation.
MAY IN THE POLYTUNNEL: AT A GLANCE
Sprouting broccoli, cabbage, carrot, cauliflower, coriander, courgette, cucumber, daikon, fennel, kohlrabi, lettuce, melon and watermelon, radish, rocket, spinach, spring onion, sweetcorn. Don’t forget to sow some sweetcorn! Polytunnel plants will be ready three to four weeks ahead of those planted outside, extending what is surely one of the best crops of the year.
Established crops from earlier sowings.
Broad bean, sprouting broccoli, cabbage, carrot, cauliflower, chard, coriander, kohlrabi, lettuce, onion, pak choi, peas, radish, rocket, spinach, spring onion, strawberries, turnip. (I’m leaving out ‘growing’ for this month as it’s all the plants on the ‘sowing’ and ‘harvesting’ lists, and should be for the next few months.)
MAY IN THE POLYTUNNEL
May is one the most exciting months for the polytunnel gardener. All the plants suddenly seem to be quite capable of taking care of themselves, or even taking over completely if you let them. So, one important ‘thing to do’ in May is – don’t let them!
Any remaining overwintering plants, apart from broad beans and peas, are probably bolting like crazy. Most of them should have been removed by now as they either become woody, lose their flavour or both. Chard, on the other hand, stays tender right up to the point of collecting seeds, by which time it can be well over 2 metres tall and casting a very big shadow indeed. So, it’s unlikely to be something you allow to go to seed every year – and of course you won’t need to. A single chard plant will produce enough seeds for years, as well as plenty for your friends.
If you haven’t tried saving seeds before now, do give it a try this year. It’s easy, fun and incredibly productive. And, you are very likely to end up with better plants as a result. Don’t bother with hybrids (‘F1’ varieties) as the seed won’t breed ‘true’. Instead, look for ‘open pollinated’ (i.e. pollinated by wind or insects) varieties. So long as you don’t allow them to cross with other species (some will if you let them, such as carrots with Queen Anne’s Lace), these will breed ‘true’ – which is to say, they adapt. Instead of perfect clones, you get slight variations within a particular type. If you only save seeds from the best plants, i.e. those which were the last to bolt, and/or the healthiest and most productive – you might end up with something really worthwhile, and possibly even a whole new variety.
Lettuce, peas, tomatoes, climbing French beans – all these produce self-pollinating flowers and there’s very little need to worry about them crossing. Others are more difficult and may require isolation or protection. For lots more information on just how to save seeds from specific varieties as well as a list of useful seed suppliers, see ‘How To Grow Food In Your Polytunnel’.
Local conditions matter: These are possible crops, not guaranteed harvests. Timing depends on the crop variety, earlier sowing dates, light, temperature and whether your tunnel is heated.
WHAT TO SOW IN MAY
Sprouting broccoli, cabbage, carrot, cauliflower, coriander, courgette, cucumber, daikon, fennel, kohlrabi, lettuce, melon and watermelon, radish, rocket, spinach, spring onion, sweetcorn. Don’t forget to sow some sweetcorn! Polytunnel plants will be ready three to four weeks ahead of those planted outside, extending what is surely one of the best crops of the year.
WHAT TO HARVEST IN MAY
Broad bean, sprouting broccoli, cabbage, carrot, cauliflower, chard, coriander, kohlrabi, lettuce, onion, pak choi, peas, radish, rocket, spinach, spring onion, strawberries, turnip. (I’m leaving out ‘growing’ for this month as it’s all the plants on the ‘sowing’ and ‘harvesting’ lists, and should be for the next few months.)
ESSENTIAL POLYTUNNEL JOBS FOR MAY
Tip 1
Many insects take to the air in May. Some are good for the garden (hover flies, bees, ladybirds) and some are bad (butterflies, aphids, carrot fly, flea beetles). All of them will find your polytunnel, and some might decide it’s nice enough to stay. By hanging weighted mesh curtains across the polytunnel door openings, air can still circulate but you can keep most of the insects out. However, be warned: If you completely exclude insects, you will either have to go without particular crops (strawberries, for example) or pollinate the flowers by hand. This can usually be accomplished with a fine paintbrush, but it’s very time-consuming. Instead, you might try a compromise: a reasonably coarse mesh will allow everything but butterflies through, so at least you can get a decent cauliflower crop!
Tip 2
Keep up the evening ‘slug patrol’ as this is their peak breeding time. Water in the mornings, so that leaf surfaces have time to dry before the evening. That makes it harder for slugs to get around, and there is less likelihood of mould developing. For the same reasons, don’t let plants crowd each other. The health of your polytunnel depends on good air circulation as well as water and nutrients.
YOUR MAY CHECKLIST
MAY POLYTUNNEL GROWING FAQS
Can tender crops go into the tunnel in May?
Check your local frost risk and overnight temperature first. Harden off plants and protect them if a cold night is forecast.
What should I do with bolting winter crops?
Harvest usable leaves and remove plants whose quality has declined, making space for summer crops.
Find individual crop advice in our Top of the Crops guide, or browse all monthly growing guides.
