Sowing and planting:
Sow seeds of fast-growing hardy annuals, such as Eschscholzia (California poppy) or poached-egg plant, to fill gaps. Click here to visit our Seeds & Propagation page.
Now is also the perfect time to plant perennials. These delightful plants give excellent value for money, flowering year after year and can also be divided to make more plants as they grow. Click here to visit our garden perennials page.
Plant up hanging baskets, and leave them in a sheltered spot before hanging fully outside.
Begin planting summer bedding in the last week of May as long as there is no forecast of frost. An hour before planting water the plants in their containers and remember to water after planting. Click here to visit our Summer Bedding Plants page.
Spring is also the ideal time to plant shrubs as the soil is moist and warm enough for new root growth to help the plants become established quickly. Click here to visit our Shrubs page.
In the Greenhouse:
Apply shade paint to the outside of the glass or use blinds on sunny days to prevent temperatures from soaring.
Open doors and vent on greenhouses to increase ventilation on warm, sunny days.
Damp down the floor of the greenhouse regularly on hot days, to increase humidity levels. This benefits plant growth and also reduces the risk of pest problems such as glasshouse red spider mite.
Don’t forget to give greenhouse plants more space as they put on new growth. This will help to prevent disease and to contain early pest infestations.
Harden off your half-hardy bedding plants that were started off early under cover. By placing them outside for a short period only at the warmest time of day and then gradually increasing the length of time they are outside you can avoid the ‘shock’ that they otherwise experience when moved outside suddenly and permanently.
Check plants at least every few days to see if they need watering. Seedlings will need daily attention. Use rain, grey or recycled water wherever possible.
Continue to prick out and pot on new seedlings and cuttings.
Hydrangeas and fuchsias can be propagated from softwood stem tip cuttings.
Pest and disease watch:
Regularly inspect plants and also the structures of the greenhouse and conservatory for glasshouse red spider mite, whitefly, thrips and other pests. Paying careful attention to the undersides of the leaves and to each plant in turn can spot early infestations that would otherwise be missed. Control with approved treatments and hang yellow sticky traps to help monitor numbers of flying pests.
Check roses for signs of blackspot (left), aphids and leaf-rolling sawfly damage.
Damp down the floor of the glasshouse regularly on hot days to reduce the risk of glasshouse red spider mite.
Brush up fallen compost and debris and pick off dead leaves from plants. This will help prevent pests and disease spreading.
Inspect lilies for red lily beetles as the larvae can strip plants in days.
Vine weevil larvae can be a serious pest of containerised plants and become active this month. Tip out the rootball of suspect plants and inspect for the creamy orange-headed maggots which tend to curl up into a ‘C’ shape. There are various chemical and biological controls available.
Aphids can multiply rapidly during mild spells. Remove early infestations by hand to prevent the problem getting out of hand. Protect sweet pea plants in particular, as they can get sweet pea viruses.
Early May – sow (under cover) beans, marrows courgettes and squashes, two per pot, and thin out the weakest seedling to leave the strongest plant.
Early to mid May – sow seeds of herbs, such as parsley, chives, coriander, garlic, basil, dill, fennel, mint, thyme, sage and lovage.
Late May – sow beetroot, lettuce, watercress, rocket, radish, spring onions, coriander, parsley, chives, carrots, swedes, turnip, leaf beet, spinach, cabbage, peas and mangetout, endive, marrows and courgettes, turnips and chicory. Sow a few seeds of salad leaves and stir-fry leaf crops every two to three weeks to ensure a regular supply.
Sweetcorn, courgettes, marrows, pumpkins, cucumbers, tomatoes, French beans and Runner beans are temperature sensitive crops. If they are sown in soil that is too cold, germination will be poor and any seedlings that do appear will not crop as well as those started in warmer soils. The critical period is when the seeds are taking up water in order to start the germination process. Once this is complete, lower temperatures are acceptable.
Potatoes will grow very quickly under warm and moist conditions. When they are 10cm tall, the leafy shoots can be mounded around with soil to their full height, a process known as ‘earthing up’. Earthing up potatoes will increase the length of underground stems that will bear potatoes. This mounding can be repeated once or twice more at two to three week intervals to ensure the best crop, with the added benefit of smothering any competing weeds.
Plant out crops being raised in pots under glass, such as marrows, courgettes and tomatoes end of month.
Pinch off strawberry runners as soon as they develop, to stop them competing with developing fruit for nutrients.
Plant out leeks into their final position once they are pencil thick in late May.
Spray peaches and nectarines with fungicide to prevent an attack of peach leaf curl.
Pick rhubarb stems as they develop, and water clumps with a generous helping of liquid feed.
Trees and shrubs:
Ensure newly planted trees and shrubs do not dry out. Water with rainwater or recycled water wherever possible. Click here to visit our Ornamental Trees page.
Loosen any tree ties that are digging into the bark or could do so soon as the trunk girth expands.
Cut back tender shrubs and sub-shrubs such as Penstemon, Caryopteris and Fuchsia after the danger of frost has passed.
Clip evergreen hedges. If not too woody shredded clippings can be added to the compost heap, ideally in combination with soft material such as grass clippings.
Prune spring-flowering shrubs such as japonica or Japanese quince (Chaenomeles), Choisya and Ribes after flowering. Remove one stem in three from Kerria and Spiraea ‘Arguta’ and shorten the other flowered stems to a suitable sideshoot. Evergreens such as Viburnum tinus can also still be trimmed this month.
Prune overcrowded, dead or diseased stems of Clematis montana once it has finished flowering. Untangling the stems can be fiddly but once you can see where you are cutting you need not worry about pruning this plant – it will take even hard cutting back very well.
Prune out frost damage from affected evergreen shrubs. Click here to visit our Shrubs page.
Tie in climbing and rambling roses as near to horizontal as possible. This will restrict sap flow causing more side-shoots to grow along the length of stem and more flowers will be produced.
Twining climbers (such as honeysuckle and Clematis) need regular tying in and twining around their supports. Click here to visit our Climbing Plants page.
Mow regularly and continue adding clippings to the compost heap.
Keep the lawn edges trimmed.
Use the half-moon edging iron or a spade to create a 7.5cm (3in) gutter around the lawn edge. This will prevent grass from creeping into the border from the main lawn.
Apply a high nitrogen summer lawn fertiliser to encourage a healthy-looking lawn, taking care to avoid any runoff as this may cause pollution. Always follow the manufacturer’s instructions carefully.
Sowing new lawns or over-seeding dead patches can still be carried out in early May. Prepare the ground for sowing by cultivating, levelling and lightly firming beforehand. Do not walk over or mow newly sown grass until it has reached a height of 5-7.5cm (2-3in) and then only give it a light trim at the highest setting.
Ensure new lawns (either from turf or seed) do not dry out during dry spells. Keep off them for as long as possible to allow establishment. Don’t worry over a flush of weed seedlings in newly seeded turf. These will disappear once regular mowing begins.
If moss is a problem, choose a combined fertiliser and mosskiller when feeding the lawn.
Selective lawn weedkillers will kill the weeds but not the grass or any naturalised bulbs. However, be warned – they will kill wild flowers growing in the turf, and damage border plants they come into contact with.
Apply a liquid fertiliser to spring bulbs after they have flowered to encourage good flowering next year and help prevent daffodil blindness. Allow the foliage of daffodils and other spring-flowering bulbs to die down naturally. Click here to visit our Fertilisers page.
Liquid feed plants in containers every two to four weeks.
Prune spring-flowering shrubs immediately after flowering, such as forsythia, ribes, chaenomeles, pyracantha and kerria.
Pinching out the tips of your fuchsia will encourage it to become more bushier. For each tip you take off two side shoots will develop and in turn will produce flowers.
Take cuttings from fuchsias, geraniums and other tender plants.
Put supports in place for herbaceous plants before they are too tall, or for those – like peonies – that produce heavy blooms.
Hoe borders to prevent annual and perennial weeds from spreading and seeding themselves.
Pinch out the leading shoots on plants such as Chrysanthemum and Helianthus to encourage bushy plants. If tall thin sprays are preferred they can be left un-pinched – perhaps removing a few buds (known as ‘disbudding’) to encourage larger blooms.
Keep tubs, hanging baskets and alpine troughs well watered. Use collected rainwater, or recycled grey water wherever possible. Click here to visit our Watering page.
Pot on plants showing signs of being root bound. You can tip out the root balls of unhappy looking containerised specimens to see if they are indeed pot bound or if they are suffering from some other problem.