Sowing and planting
Plant out cannas and dahlias (such as Dahlia 'David Howard', left) when the danger of frost has passed. Tubs can be planted up with summer bedding in milder areas. In colder areas further north or at high altitudes, it is advised to 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 (including wallflowers, pansies, and daisies, Bellis perennis) need to be sown between now and July in order to flower next spring, as they are biennials.
Winter bedding plants can also be sown from now until July.
Remove faded wallflowers and spring bedding from beds and containers, to make space for summer plantings.
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.
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.
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 and vegetables such as radishes (left). 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.
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 red lily beetles (left) 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.