What to do in the garden in May

There's always something to be doing in the garden, whether it's pruning, tidying or sowing, so we've put together our top gardening tasks for May.

 In the flower garden

• • Thin out drifts of hardy annuals.
• • Harden off half-hardy plants by leaving them outside during the day and bringing back under cover at night for 7 to 10 days before planting outdoors.
• • Harden off dahlias and tender exotics such as Canna for planting as soon as the risk of frost has passed.
• • You can still divide herbaceous perennials now to improve their vigour and create new plants.
• • Divide Hostas as they come into growth.
• • Trim back spreading plants such as aubrieta, alyssum and candytuft after they have flowered to encourage fresh new growth and more flowers.
• • Don't be tempted to cut down or tie up the foliage of spring-flowering bulbs, let them die down naturally.
• • To reduce the spread of forget-me-not, lift the plants now to prevent heavy self-seeding.
• • Prune your Penstemons now - cut all the old shoots back to the base provided 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 your 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 now, cutting off any old flower heads and about 2.5cm (1 inch) of the current year's growth.
• • Feed and water container plants.
• • Top-dress permanent pot plants to refresh the compost.
• • Give your container plants a balanced liquid feed every two to four weeks to promote healthy growth.
• • Plant up pots and baskets of summer bedding and harden off before placing in position. In cold areas wait until June.
• • Start to closely inspect your plants for pests and diseases - early prevention is easier than curing an infestation.
• • Pick off larvae of rosemary, viburnum and lily beetles as soon as they are seen.
• • Look out for signs of blackspot on roses. If discovered, Blackspot can be treated with a systemic fungicide.
• • Continue to weed beds and borders to prevent competition for water and nutrients.

 In the vegetable garden

• • Continue earthing-up potatoes. Read our potato growing guide for information about how to earth up potatoes.
• • Harvest asparagus spears when they are no more than 18cm tall.
• • Harvest rhubarb, picking only a third of the total amount of stems.
• • 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.
• • Plant out brassicas and leeks to their final positions.
• • Protect crops 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. Applying an onion fetiliser will boost growth.
• •Use 8ft (2.4m) bamboo canes to make supports for your runner beans.
• • Support your pea plants with twiggy sticks or pea netting.
• • Be vigilant with weeding - weeds will compete for precious water, light and nutrients.

 In the fruit garden

• • 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 whilst they are putting on rapid growth.
• • Remove blossom and fruits from newly planted fruit trees to allow them to establish properly during the ir first year.
• • Surround strawberries with straw to protect fruit. Net them to keep birds off of the fruit.
• • Regularly check the centre of your gooseberry bushes for the green gooseberry sawfly caterpillars. They will quickly skeletonise the leaves if not removed.
• • Erect netting around your soft fruit plants to prevent birds eating your crop.

 In the greenhouse

• • Harden off half-hardy bedding plants to acclimatise them to outdoor conditions.
• • Continue pricking out and potting on seedlings and cuttings.
• • Plant glasshouse tomatoes in beds or growing bags.
• • Ventilate glasshouses on warm days.
• • Use blinds or apply shade paint to avoid large fluctuations of temperature.
• • Damp down your greenhouse on hot days to increase humidity and deter red spider mites.
• • Hang fly traps throughout the greenhouse to monitor levels of whitefly, thrips and other pests.

 Looking after your lawn

• • Now is a great time to apply lawn weedkiller to your lawn.
• • Feed your lawn with a high nitrogen fertiliser to encourage healthy green growth.
• • Water your lawn during hot weather, particularly newly seeded or turfed lawns. Do not allow new lawns to dry out.
• • Sowing lawn seed and repairing bare patches can still be carried out in the early part of the month.
• • Mower blades should be lowered this month to their regular summer cut height.
• • Don't mow newly sown grass until it reaches 3 inches in height and make sure the mower blades are on a high setting.

 From your armchair

• • Use a diary to keep track of which seeds you are sowing and when they were sown and planted out - it really helps later in the year.
• • Take time out from the hard work to relax and enjoy your garden now that warmer weather has arrived.

 Other jobs about the garden

• • Feed pond fish, little and often.
• • Remove duckweed and blanketweed from ponds, and thin excessive growth of submerged oxygenators. Lay removed weed beside the pond overnight to allow insects, snails and other pondlife to escape back to the water.
• • Keep on top of weeds to prevent them spreading.
• • Clip evergreen hedges now.

What to do in the garden archive

• Back to 'In the Garden this Month'
• What to do in the garden in June
• What to do in the garden in July
• What to do in the garden in August
• What to do in the garden in September
• What to do in the garden in October
• What to do in the garden in November
• What to do in the garden in December
• What to do in the garden in January
• What to do in the garden in February
• What to do in the garden in March
• What to do in the garden in April
• What to do in the garden in May

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