What to do in the garden in May

Pink perennial — False Goatsbeard or Astilbe x arendsii 'Look at Me' from Thompson & Morgan

With the days getting longer in May, find out what you should be doing in the garden this month
Image: Astilbe x arendsii 'Look at Me' from Thompson & Morgan/Copyright: Visions BV, Netherlands

May has arrived and the days are getting warmer and longer. Summer is on its way and it's time to tidy up spring plants, plant out summer flowers and get planning for autumn.

There are plenty of flowers and vegetables to sow and grow in May. Or read on for a comprehensive list of other garden jobs to get on with this month:

Timely Tips for May

open greenhouse vents and windows on a hot day

May's the perfect time to air your greenhouse out
Image: val lawless/Shutterstock

    1. Start planting out summer bedding plants towards the end of this month in warmer parts of the country. For more information about growing bedding plants, head over to our hub page.
    2. Look after your finished spring bulbs for next year. Once they’ve gone over, resist the temptation to cut back the foliage. Instead, let it die and break down on its own and add liquid fertiliser all around the clumps. This will give you an even better display next spring.
    3. Remember to open greenhouse vents and doors on warm days. You can also damp down your greenhouse on hot days to increase humidity and deter red spider mites.
    4. Optimise your watering regime — watering early and late to get the most out of your water — and start collecting and recycling water whenever possible.

In the flower garden in May

Lavender 'Hidcote' from T&M

Keep your lavender plants healthy with regular pruning
Image: Lavender 'Hidcote' from Thompson & Morgan

  • Thin out drifts of hardy annuals.
  • Harden off half-hardy plants by leaving them outside during the day and bringing them back under cover at night for 7 to 10 days before planting outdoors.
  • Plant summer hanging baskets, adding good-quality compost, slow-release fertiliser and water-retaining crystals, to keep them in top condition. Protect them from late frost under cover. Try planting petunia 'Pegasus Wine Splash' for cascading stems of brightly spotted blooms. Find more advice and tips about keeping your hanging baskets looking amazing across the seasons on our hub page.
  • Harden off dahlias and tender exotics such as canna ‘Tropicanna’ lilies for planting as soon as the risk of frost has passed.
  • Continue dividing herbaceous border perennials to improve vigour and create new plants.
  • Divide established clumps of hostas as they come into growth.
  • Trim back spreading plants such as aubrieta, alyssum and candytuft (Iberis sp.) after they’ve flowered, to encourage fresh new growth and more blooms.
  • Towards the end of the month, keep an eye out for powdery mildew on flowering shrubs and flowers, like rhododendrons. Treat with fungicide to prevent further spread.
  • Lift forget-me-nots to prevent heavy self-seeding and reduce spreading.
  • Prune penstemons now — cut all the old shoots back to the base, providing there is new growth at the bottom of the plant. If there are no new shoots at the base, cut just above the lowest set of leaves.
  • Lightly cut back & tidy up late-flowering honeysuckle. Leave any big pruning jobs until winter.
  • You've still got time to plant any container-grown evergreen climbing shrubs now the risk of frost has passed.
  • Take cuttings of tender perennials, such as fuchsia and pelargoniums (tender geraniums). The new shoots of hardy perennials can also be used for cuttings.
  • Take softwood cuttings of shrubby herbs (such as sage and lemon verbena).
  • Prune out overcrowded and dead stems ofearly-flowering clematis (C. alpina, C. cirrhosa, C. macropetala, C. armandii, and their cultivars) after flowering.
  • Tie in rambling and climbing roses. Laying the stems horizontally will help to produce more flowers.
  • Tie in sweet pea plants with sweet pea support rings to encourage them to climb.
  • Prune spring-flowering shrubs after flowering.
  • Cut back flowered shoots of choisya to promote a second flush of flowers in autumn.
  • Trim lavender plants, cutting off old flower heads and about 2.5 cm (1 inch) of the current year's growth.
  • Feed and water container plants.
  • Top-dress permanent pot plants to refresh the compost.
  • Supplement container plants with balanced liquid feed every 2-4 weeks to promote healthy growth.
  • Closely inspect plants for pests and diseases — early prevention is much easier than curing an infestation.
  • Pick off any larvae or lily beetle from rosemary or viburnum plants as soon as you spot them to prevent an infestation.
  • Look out for signs of blackspot on roses. If discovered, treat it with a systemic fungicide.
  • Continue to weed beds and borders. This reduces your plants’ competition for water and nutrients.
  • Lift and divide overcrowded clumps of daffodils and other spring-flowering bulbs.

In the vegetable garden in May

closeup of green asparagus spears — Asparagus officinalis 'Mondeo' by Thompson & Morgan

Garnish your springtime meals with freshly harvested asparagus
Image: Asparagus officinalis 'Mondeo' from Thompson & Morgan

  • Continue earthing up potatoes. Read our potato growing guide for detailed advice on how best to do this.
  • Harvest asparagus spears when they are no more than 18 cm tall.
  • Thin out direct-sown vegetables such as spinach, carrot and lettuce seedlings, then water the rows well.
  • Harden off outdoor tomatoes, courgettes and pumpkins for planting early next month.
  • Protect carrots from carrot fly by covering with horticultural fleece or enviromesh.
  • Pinch out the growing points of broad beans as soon as beans start to appear at the base of the plant to reduce the risk of blackfly attack.
  • Weed around your onions and garlic to reduce competition for nutrients and water. Apply an onion fertiliser to boost growth.
  • Make supports for your runner beans, using 8 ft (2.4 m) bamboo canes.
  • Support pea plants with twiggy sticks or pea netting.
  • Keep on top of weeding — weeds will compete with your vegetables for precious water, light and nutrients.

In the fruit garden in May

Strawberry 'Just Add Cream' from T&M

Keep pests away from your strawberry crop
Image: Strawberry 'Just Add Cream' from Thompson & Morgan

  • Protect strawberry plants with straw (to control weeds and lift the berries off the ground) and netting (to keep birds off the fruit). Visit our comprehensive hub page for more info and advice on growing strawberries.
  • Harvest rhubarb, picking only one-third of the total amount of stems.
  • Hang pheromone traps in stone fruit trees from May to August to monitor plum-fruit-moth activity.
  • Hang pheromone traps in apple trees to reduce codling moth.
  • Keep young fruit trees well watered while they are putting on rapid growth.
  • Remove blossom and fruits from newly planted fruit trees to help them establish properly during their first year.
  • Regularly check the centre of gooseberry bushes for green gooseberry sawfly caterpillars — they will quickly skeletonize leaves if not removed.
  • Erect garden netting around soft fruit plants to prevent birds eating your crop.

Looking after your lawn in May

  • Apply lawn weedkiller to your lawn this month.
  • Feed your lawn with a high-nitrogen fertiliser to encourage healthy green growth.
  • Water the grass during hot weather — that’s particularly important for newly seeded or turfed lawns. Never allow new lawns to dry out.
  • Lower mower blades to their regular summer-cut height.
  • Continue sowing lawn seed and repairing bare patches in the early part of the month.
  • Postpone mowing newly sown grass until it reaches 3 inches in height and make sure the mower blades are on a high setting.
  • Install lawn edging to keep edges tidy and smarten up beds and borders.

Other jobs about the garden in May

Japanese koi fish underwater with fish food

Don't forget about the fish!
Image: Glenn Young

  • Feed pond fish little and often.
  • Remove duckweed and blanket weed from ponds, and thin out both pondside perennials (if necessary) and submerged oxygenating pond plants. Leave any removed weed beside your pond overnight to allow insects, snails and other pondlife to escape back into the water.
  • Clip evergreen hedges or low growing hedging now.
  • Control the heat in your greenhouse with blinds or shade paint.
  • Hang fly traps throughout the greenhouse to monitor levels of whitefly, thrips and other pests.

From your armchair in May

Sunflower SunBelievable™ 'Brown Eyed Girl' from Thompson & Morgan

Relax and enjoy the garden now that warmer weather has arrived
Image: Sunflower SunBelievable® 'Brown Eyed Girl' from Thompson & Morgan

  • Keep a diary of which seeds you’re sowing and planting to help you stay on top of things throughout the year. Refer back to our handy seed sowing times guide for more information.
  • Take time out from all your hard work to relax and enjoy the garden now that the warmer weather has arrived.

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