Sowing and planting
Plant out cannas and dahlias (such as Dahlia ‘David Howard’, pictured) when the danger of frost has passed.
Remove faded wallflowers and spring bedding from beds and containers. These can be planted up with summer bedding in milder areas. In colder areas further north or at high altitudes, wait until early June, or until all risk of frost has passed.
If you want to grow your own spring bedding for next year, many common choices are biennials (including wallflowers, pansies, and daisies or Bellis), so need to be sown between now and July to flower next spring.
Winter bedding plants such as primroses, Polyanthus and winter pansies can also be sown from mid-May 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. They can be replanted to bulk up the display, or if using them in containers, plant them in a spare area until they are ready for moving to their final positions in the autumn for a spring display.
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 patchy. Trimming them back after flowering encourages fresh growth and new flowers.
Deadhead tulips and daffodils, leaving the foliage and flower stems, which put energy back into the bulbs.
Lift and divide overcrowded clumps of daffodils after they have flowered to improve displays in the coming years.
Propagation
Take softwood cuttings of tender perennials like Argyranthemum, Pelargonium (geraniums) and fuchsias. They will provide new plants that will flower later this summer.
Perennials that are showing new shoots from the crown can be propagated by basal stem cuttings.
General maintenance
Apply a seaweed liquid fertiliser to spring bulbs after they have flowered, to encourage good flowering next year, and help prevent daffodil blindness. Allow the foliage to die down naturally.
Lift clumps of forget-me-not once the display fades, and before most of the seeds are released. You can end up with too many if they’re left unchecked.
Put supports in place for herbaceous plants before they grow too tall, or for those that produce heavy blooms, like peonies.
Harden off plants raised from seed and cuttings by leaving them outside for gradually longer periods of time. Start by putting them outside during the warmest part of the day, and build up to leaving them out overnight. Doing this for 10-14 days before planting them outdoors permanently (whenever the risk of frost has passed), will help them establish quickly in their final positions.
Thin out direct sowings of hardy annuals and vegetables such as radishes (pictured). This is best done in two or three stages at fortnightly intervals. Final spacing should be 10-20cm (4-8in), with the wider spacing for tall or spreading plants, and the closer spacing for smaller plants. Prick out indoor sowings when they are large enough to handle.
Hoe borders to prevent annual and perennial weeds from spreading and self-seeding.
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 chrysanthemums and sunflowers to encourage bushy plants. However, if tall thin sprays are preferred, such as for cut flowers, 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 with homemade fertilisers.
Keep tubs, hanging baskets and alpine troughs well watered. Use collected rainwater, or recycled grey water wherever possible.
Pot on plants showing signs of being rootbound. You can tip out the rootballs of unhappy-looking containerised plants, to see if they are indeed potbound or if they are suffering from some other problem.
Problems
Inspect lilies for red lily beetles (pictured) as the larvae can strip plants in days. Remove them by hand. Log lily beetle sightings here to help our scientists with their research.
Vine weevil larvae can affect containerised plants, and become active this month. If you suspect a plant may have vine weevil (usually a wilted appearance despite damp compost), tip out the rootball and inspect for the creamy, orange-headed maggots, which tend to curl up into a ‘C’ shape. There are biological controls available.
Aphids can multiply rapidly during mild weather. Where needed, remove infestations by hand to prevent the problem increasing. Protect sweet pea plants in particular, as they can get sweet pea viruses. Aphids are essential food for young birds so only remove where necessary and don’t use any sprays.
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.
