Flowers – Sowing and planting
Plant out cannas and dahlias when danger of frost is past. Tubs can be planted up with summer bedding in milder areas. In cold areas, it is advised to wait until well into June, or until all risk of frost has passed.
If you want to grow your own spring bedding for next year, many common choices (including wallflowers, pansies, and Bellis perennis) need to be sown between now and July in order to flower next spring, as they are biennials.
Winter bedding plants for the following winter can also be sown from now until July.
Cutting back, pruning and dividing
Divide clumps of herbaceous perennials that you want to propagate. Bamboos and clumps of bulbs or rhizomes can be divided in the same way. Cutting back clumps of spring-flowering perennials such as Pulmonaria and Doronicum can encourage a fresh flush of foliage.
Divide Primula (primroses) after flowering, planting them in a nursery bed until they are ready for planting out again in the autumn, for a display the following spring.
Divide hostas as they come into growth.
Spreading and trailing plants such as the annual Lobularia (sweet alyssum), and the perennials Alyssum and Aubrieta, can become tatty and patchy. Trimming them back after flowering encourages fresh growth and new flowers.
Lift and divide over crowded clumps of daffodils after they have flowered.
Deadhead tulips and daffodils.
Propagation
Take softwood cuttings of tender perennials like Argyranthemum, Pelargonium and Fuchsia. They will provide new plants for display later this summer.
Perennials that are showing new shoots from the crown can be propagated via basal stem cuttings.
General maintenance
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.
Lift clumps of forget-me-not once the display wanes, and before too many seeds are released. They can become invasive if left unchecked.
Put supports in place for herbaceous plants before they are too tall, or for those – like peonies – that produce heavy blooms.
Harden off plants raised from seed and cuttings by leaving them outside for gradually increasing periods of time. Start with only the warmest part of the day, and build up to overnight exposure. Doing this for 10-14 days before planting them outdoors permanently (whenever the risk of frost has passed), will reduce any check to their growth while establishing in their final position.
Thin out direct sowings of hardy annuals. This is best done in two or three stages at fortnightly intervals. Final spacing should be between 10-20cm (4-8in), using the upper limit for tall or spreading plants, and the lower limit for smaller plants. Prick out indoor sowings when they are large enough to handle without damage.
Hoe borders to prevent annual and perennial weeds from spreading and seeding themselves.
Sweet peas need training and tying in to their supports to encourage them to climb and make a good display.
Pinch out the leading shoots on plants such as Chrysanthemum and Helianthus to encourage bushy plants. However, if tall thin sprays are preferred, they can be left un-pinched, perhaps removing a few buds (known as ‘disbudding’) to encourage larger blooms.
Liquid feed plants in containers every two to four weeks.
Keep tubs, hanging baskets and alpine troughs well watered. Use collected rainwater, or recycled grey water wherever possible.
Remove faded wallflowers and spring bedding from beds and containers, to make space for summer plantings.
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.
Pest and disease watch
Inspect lilies for the scarlet lily beetle 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..
Continue to protect lily, delphinium, hosta and other susceptible plants from slugs and snails.
Remove dead leaves from around the basal rosettes of alpine plants to prevent rotting.
Top dress spring-flowering alpines with grit or gravel to show off the plants and help prevent rotting around the neck.
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Trees and shrubs - Pruning and training
Cut back tender shrubs such as Penstemon, Caryopteris and hardy Fuchsia after danger of frosts has past.
Clip evergreen hedges. If not too woody, shredded clippings can be added to the compost heap.
Prune spring-flowering shrubs such as Berberis, 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.
Late spring is a good time to coppice or pollard Eucalyptus.
Prune out frost damage from affected evergreen shrubs.
Young mimosa trees (Acacia dealbata) can also be cut back now. Mature trees respond less well to pruning.
Remove any reverted green shoots on hardy variegated evergreens, to prevent reversion taking over.
Prune wall-trained pyracanthas, removing any shoots coming out from the wall, and shortening other new growth to about 8cm (3in). This encourages spur formation, and increased flowering relative to green growth.
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. Therefore more flowers will be produced.
Twining climbers (such as honeysuckle and Clematis) need regular tying in and twining around their supports.
General maintenance
Ensure newly planted trees and shrubs do not dry out. Water with rain, grey or recycled water wherever possible.
Loosen any tree ties that are digging into the bark, or could do so soon as the trunk girth expands.
Propagation
Take softwood cuttings of deciduous shrubs, including Forsythia, Fuchsia, Hydrangea macrophylla, Philadelphus and Spiraea.
Layering is a good way to propagate climbers and lax-stemmed shrubs. Layers should root by next spring, especially if attention to watering is given during dry weather. Examples to try include Philadelphus, Forsythia, Hydrangea and Lonicera.
Pest and disease watch
Viburnum beetle grubs start nibbling holes in the leaves this month, giving plants a tattered appearance. Inspect V. tinus and V. opulus regularly and spray or pick off the grubs by hand.
Check roses for signs of blackspot, aphids and leaf-rolling sawfly damage.
Inspect sick looking box and holly trees for signs of blight.
Phytophthora root rots can cause die back on mature trees and shrubs. Wet winter weather followed by a hot spring and summer can encourage this problem on susceptible woody plants.
Caterpillars, aphids and other fly pests can all be problematic at this time of year. Early infestations can be managed by hand removal, but approved insecticides are necessary for more serious attacks.
Check for damage or cankers on deciduous trees.
Camellias sometimes fail to perform as well as hoped. This can be down to insufficient watering in the previous summer or current spring, but can be also be due to problems with frost, wind, or poor soil conditions. Try improving cultural conditions before concluding that the problem is a pest, fungus, or virus.
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Lawns
Mow regularly and continue adding clippings to the compost heap.
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.
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.
Troubleshooting
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.
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Ponds
Continue to plant up bog gardens. Tidy and mulch with composted bark or garden compost.
Thin out, cut back or divide excessive new growth on established aquatic plants. You can still plant new aquatic plants this month. Plant vigorous specimens in aquatic plant baskets to contain them. Top the surface with a layer of gravel to prevent the fish from stirring up the compost.
Begin stocking ponds with fish once new plantings have established. Avoid introducing goldfish to wildlife ponds. They will eat frogspawn and so upset the natural balance.
Troubleshooting
Remove blanket weed by twirling around a rough stick. Skim off floating weeds such as duckweed with a net. Leave weeds on the pond side for 24 hours to allow trapped creatures to return to the water before adding to the compost heap.
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Greenhouse, conservatory and houseplants – Houseplants
Move conservatory plants, such as Cymbidium, Ficus and Citrus outdoors during warm days but bring back inside if cold nights are expected.
Clean shiny-leaved plants with a damp cloth and spiky plants with a soft brush.
Pot up houseplants showing signs of being rootbound or top dress large containers with fresh compost.
Cool conditions and regular watering will help keep potted indoor azaleas looking good for longer. Remember to water azaleas with rainwater collected in a rain butt, not with tap water.
Pruning and training
Train the new stems of passionflowers over frames or on horizontal wires.
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.
Try growing on plug plants in your greenhouse. They are a relatively cheap source of large numbers of plants, and avoid the need for propagation facilities and time-consuming pricking out.
Hydrangeas and fuchsias can be propagated from softwood stem tip cuttings.
Pot on any Begonia, Gloxinia and Achimenes plants that you started off earlier in smaller pots, but which are now large enough to re-pot in the next size up.
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. 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.
Damp down the floor of the glasshouse regularly on hot days, to reduce the risk of glasshouse red spider mite.
Damping off of seedlings can be a problem with sowings under glass. Clean equipment and, where necessary, use of fungicides such as Bio Cheshunt Compound can help to control this problem.
Brush up fallen compost and debris, and pick off dead leaves from plants. This will help prevent pests and disease spreading.
