What to do in the garden in May
May is the month the garden really takes off. The main jobs are planting out tender plants once the risk of frost has passed, staking perennials before they flop, sowing tender crops outdoors towards the end of the month, and keeping on top of weeds and watering while everything grows at full speed. Get these done now and you set the garden up for a long, generous summer.
Harden off and plant out tender plants
Dahlias, salvias, cosmos, tomatoes, courgettes and other frost-tender plants raised indoors all want to go outside this month, but timing matters. In much of southern England the last frosts have usually passed by mid to late May, while in the north, in Scotland and in colder inland valleys it can be the end of the month or even early June before it is safe. Keep an eye on the local forecast rather than the calendar.
Before planting, spend a week or two hardening plants off. Put them outside during the day and bring them in at night, or move them into a cold frame and open the lid a little more each day. This gradual change toughens up the leaves and avoids the check in growth that comes from going straight from a warm windowsill into a cool border. Water them in well and keep a sheet of fleece handy for any surprise cold nights.
Stake perennials before they need it
The best time to support tall perennials is before they look as though they need it. Push in twiggy hazel or birch pea sticks, or set up a simple grid of canes and twine, around peonies, delphiniums, asters, heleniums and taller sanguisorbas now. The new growth will grow up through the supports and hide them within a few weeks, and you will avoid the tricky job of propping up a collapsed plant in July.
Give late perennials the Chelsea chop
Cutting back perennials in the last week of May, the week of the RHS Chelsea Flower Show, is one of the most useful tricks in the planting year. It is known as the Chelsea chop. Cut back some of the stems of late-flowering perennials such as sedums (Hylotelephium), heleniums, phlox, rudbeckias and Echinacea by a third to a half. The plants flower a little later, on shorter, sturdier, bushier stems that rarely need staking.
You do not have to cut a whole clump. Chop just the front third of a group and you get a staggered display, with the back stems flowering first and the front ones following on a couple of weeks later.
Sow and plant in the vegetable garden
Once the soil has warmed up, usually in the second half of the month, you can sow runner beans, French beans, courgettes, squash and sweetcorn directly outdoors, or plant out ones raised under cover. Keep sowing short rows of salad leaves, radish and beetroot every couple of weeks for a steady supply rather than a glut. Earth up potatoes as the shoots appear, drawing soil up around the stems to protect them from late frost and stop the tubers turning green.
Look after spring bulbs
Deadhead tulips and daffodils as the flowers fade so they put their energy into the bulb rather than into seed. Leave the foliage to die back naturally for at least six weeks before tidying it away, because the leaves are feeding next year's flowers. If you grow tulips in pots and want to reuse the bulbs, you can lift them once the leaves yellow and store them somewhere dry and airy, although many tulips flower best in their first year, so treat any repeat display as a bonus.
Weed, water and feed
Weeds grow as fast as everything else in May, so little and often is the way to win. Run a hoe through the borders on a dry, breezy day and the seedlings will shrivel on the surface. Water new plantings and containers deeply rather than giving them a daily sprinkle, which encourages roots to grow down rather than staying near the surface. Start feeding pots and hanging baskets with a liquid feed every week or two once they are in active growth.
If you are building a new border this spring, the advice in how to plant a border that looks good all year will help you choose plants that carry on well past May.
May checklist
Harden off tender plants and plant them out once frosts have passed.
Stake tall perennials now, before they flop.
Chop back late-flowering perennials at the end of the month.
Sow beans, courgettes and squash outside once the soil is warm.
Earth up potatoes and keep sowing salads little and often.
Deadhead spring bulbs and let the leaves die back naturally.
Hoe weeds on dry days and water new plants deeply.
Sow biennials such as foxgloves and wallflowers for next spring.
Prune spring shrubs and sow biennials
Shrubs that flower on last year's growth, such as flowering currant (Ribes sanguineum), spring-flowering Clematis montana and early Spiraea, can be pruned as soon as they finish flowering. Removing a few of the oldest stems keeps them full of flower. Late May is also a good time to sow biennials such as foxgloves, sweet williams, honesty and wallflowers in a seed bed or in modules. They will build up into strong plants over summer, ready to be moved into place in autumn and flower next spring.
If your borders are starting to feel tired or patchy as they fill out, May is a brilliant month to notice where the gaps are. Take a few photographs now and you will have a clear plan when it comes to planting in autumn. Looking further ahead, the June jobs pick up where these leave off.
Quick answers
When is it safe to plant out dahlias and bedding in the UK?
Usually from mid to late May in the south and from late May to early June in the north and in colder gardens. Wait until the local forecast shows no more frost, and harden plants off for a week or two first.
What does cutting back perennials in May do?
Cutting back late-flowering perennials by a third to a half in late May, around the time of the RHS Chelsea Flower Show, means the plants flower a little later on shorter, sturdier stems that rarely need staking.
Should I cut off daffodil leaves after flowering?
No. Deadhead the flowers but leave the leaves for at least six weeks so they can feed the bulb for next year. Once they turn yellow, you can tidy them away.
