What to do in the garden in May
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.
If you’d like to know what to sow and grow this month, check out this page. Or read on for your comprehensive list of May’s other garden jobs:
May's the perfect time to air your greenhouse out
Image: val lawless
- Start planting out summer bedding towards the end of this month in warmer parts of the country.
- 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.
- 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.
- 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
Keep your lavender plants healthy with regular pruning
- 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.
- Harden off dahlias and tender exotics such as canna for planting as soon as the risk of frost has passed.
- Continue dividing herbaceous perennials to improve vigour and create new plants.
- Divide hostas as they come into growth.
- Trim back spreading plants such as aubrieta, alyssum and candytuft after they’ve flowered, to encourage fresh new growth and more blooms.
- 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.
Take cuttings of tender perennials, such as fuchsia, argyranthemum and pelargoniums (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 of early-flowering clematis (C. alpina, C. cirrhosa, C. macropetala, C. armandii, and their cultivars) after flowering.
- Tie in climbing and rambling roses. Laying the stems horizontally will help to produce more flowers.
Tie in sweet peas with plant 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 of rosemary, viburnum and lily beetle as soon as you spot them.
- Look out for signs of blackspot on roses. If discovered, treat it with a systemic fungicide.
- Continue to weed beds and borders to prevent competition for water and nutrients.
- Lift and divide overcrowded clumps of daffodils and other spring-flowering bulbs.
In the vegetable garden
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 for precious water, light and nutrients.
In the fruit garden
- Protect strawberries with straw (to control weeds and lift the berries off the ground) and netting (to keep birds off the fruit).
- Harvest rhubarb, picking only one-third of the total amount of stems.
- Hang pheromone traps in plum 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 skeletonise leaves if not removed.
- Erect netting around soft fruit plants to prevent birds eating your crop.
Looking after your lawn
- 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.
Other jobs about the garden
- Feed pond fish little and often.
- Remove duckweed and blanket weed from ponds, and thin out submerged oxygenators. Lay any removed weed beside your pond overnight to allow insects, snails and other pondlife to escape back into the water.
- Keep on top of weeds to prevent them spreading.
- Clip evergreen hedges 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
Write down what you sow & grow throughout the year
- Keep a diary of which seeds you’re sowing and planting to help you stay on top of things throughout the year.
- Take time out from the all your hard work to relax and enjoy the garden now that the warmer weather has arrived!