In the flower garden…
• 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 7-10 days before planting outdoors.
• Harden off dahlias and tender exotics such as cannas 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 aubretia, alyssum and candytuft after they have flowered to encourage new growth and more flowers.
• Let the foliage of spring bulbs die down naturally, don’t be tempted to cut it back.
• Prune back any penstemons now that haven’t already been done. Cut back all the old shoots to the base provided there is new growth at the bottom of the plant. If there are no new shoots at the base, cut back just above the lowest set of leaves.
• Prune out overcrowded and dead stems of early-flowering clematis after they have flowered.
• Tie in climbing and rambling roses. Laying the stems horizontally will help to produce more flowers.
• Prune spring flowering shrubs after flowering.
• Trim lavender plants now, cutting off any old flower heads and about 2.5cm/1″ of current 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 weeks to promote healthy growth.
• Continue to weed beds and borders to prevent competition for water and nutrients.
In the vegetable garden…
• May is still a perfect month for planting asparagus and seed potatoes.
• Continue to earthing-up planted potatoes.
• 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 early next month.
• Plant out brassicas and leeks to their final positions.
• Protect crops from carrot fly by covering with horticultural fleece.
• Put supports/cane sin place for your runner beans.
• Support your pea plants with twiggy sticks or pea netting.
Looking after your lawn…
• Now is a great time to apply a 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″ in height and make sure the mower blades are on a high setting.
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 greenhouses on warm days.
• Hang fly traps in your greenhouse to monitor levels of whitefly, aphids and other pests.