May is probably the main sowing month for me, in Yorkshire. I can catch up on all the sowings I wanted to make in April (especially brassica, broad beans and peas), and continue successional sowings (lettuce, spring onions, radishes, beetroot, chard, carrots, leeks, spinach, and turnip).
Still a considerable risk of frost, at least until mid-month, but usually I'll take the risk and start sowing frost-tender crops (sweetcorn, French beans, cucurbits) in the second half of the month.
General Tasks
» Weeding by hoe to discourage slugs.
» Cut comfrey to make up a comfrey tea - you will need it soon.
Potato/Tomato family (Solanums)
» Finish sowing potatoes.
» Continue earthing up potatoes.
» Tomatoes should be re-potted; or even, if they are big enough, for hardening off and planting out.
Bean/Pea family (Legumes)
» Support main-crop peas and broad beans.
» Sow more peas.
» Sow French and Runner Beans - through to July.
Brassica family
» Thin Brassica seedlings; they may be tall enough to transplant this month.
» Sow Kale, Broccoli, Kohl Rabi, Savoy and Winter Cabbage - as March/April brassicas in protected seed bed.
» Thin swedes and turnips
» Harvesting - last of Broccoli, Spring Cabbages and Kale; Radishes, Rocket.
Root vegetables
» Thin Carrots (if necessary) and Parsnips. Do not allow to dry out.
» Clean out carrots showing signs of carrot fly.
» Sow Salsify and Scorzonera.
Onion family (Alliums)
» Still not too late to sow leeks.
Other vegetable families
» Thin Beetroot, Chard and Spinach. Do not allow to dry out.
» Sow Cucumbers and Squashes (week 4, or early June) in prepared beds - or earlier under cloches.
» Harden off and plant out sweetcorn sown indoors; sow sweetcorn direct in bed (week 4).
» Harvesting - Asparagus.
Herbs
» Most herbs can be sown now direct - but sow Basil inside or under cover.
» Basil can be hardened off and planted outside.
Fruit
» Protect strawberries with mulch.
» Weed rasberry canes and fruit bushes.
» Harvesting - Rhubarb.