Sowing and planting
Plant out summer bedding and seed-raised plants, if not already done so. Make sure they are well watered in and keep moist during dry weather.
Plant out cannas and dahlias once danger of frost has passed.
Gaps in herbaceous borders are best filled with annual bedding at this stage in the season. One exception is wood anemones (e.g. Anemone blanda or Anemone coronaria), which take about three months to flower after planting. The rhizomes are best planted now, after an overnight soaking, 5cm (2in) deep in a suitably moist but free-draining soil or compost with lots of added leaf mould or organic matter.
It is not too late to direct sow a few fast growing, late-flowering hardy annuals such as Calendula, Godetia and Clarkia.
Perennials such as Alcea (hollyhock), Delphinium and Lupinus can be sown directly into drills outside once the seed heads have ripened and started to split naturally. If garden space is limited you can sow them into pots and place them in a cold frame or by the base of a sheltered wall in filtered sunlight.
Thin out direct sowings of hardy annuals. This is best done in two or three stages at fortnightly intervals. Final spacings 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, potting them on and then planting them out once they are robust.
If you want to grow your own spring bedding for next year, many (including wallflowers, pansies, and Bellis perennis) need to be sown between May and July in order to flower next spring.
Polyanthus primulas are best sown only when temperatures are reliably warm, as they need a constant temperature of around 15°C (60°F). A sheltered cold frame in June or July provides the right environment.
Winter bedding plants for the following winter can also be sown from May until July. Attractive choices include ornamental cabbages, kales and winter pansies.
Tubs can be planted up with summer bedding if not done already.
Hellebore seed can be harvested once the seed heads have ripened (i.e. when squeezing the pod causes it to split and release seeds). Seed needs to be sown immediately, while fresh, and need a winter’s cold season in order to break their dormancy in order for germination to occur. Do remember that seed grown plants will differ from the parent plant.
Cutting back, pruning and dividing
Spreading and trailing plants, can become tatty and patchy. Trimming them back after flowering encourages fresh growth and new flowers.
Cut back dead bulb foliage if not done already. It is important to wait until the foliage dies down naturally, as cutting back too early can lead to blindness next year.
Cutting back clumps of spring-flowering perennials can encourage a fresh flush of foliage.
Cut back and deadhead Oriental poppies after flowering. Cutting them right back to ground level will stimulate growth of fresh new foliage, and perhaps even some new blooms. Mulching and feeding will help to support this new growth.
Euphorbias looks a lot better if spent flowers are removed, cutting the flowered stem back to ground level. This can be especially important with Euphorbia amygdaloides var. robbiae, as old stems of this cultivar are particularly prone to powdery mildew when grown in dry soils.
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.
Divide hostas as they come into growth. Divide Primula (primroses) after flowering, planting them in a nursery bed until they are ready for planting out again in the autumn.
Lift and divide overcrowded clumps of bulbs after they have finished flowering.
Perennials that are showing new shoots from the crown can still be propagated via basal stem cuttings.
Take cuttings from garden pinks (Dianthus). They can be pulled off the parent plant by pulling with thumb and forefinger while holding a suitable non-flowering shoot four pairs of leaves from the tip. Treat as softwood cuttings.
Hoe borders to prevent annual and perennial weeds from spreading and seeding themselves.
Tackle bindweed when it appears in a border.
Lift clumps of forget-me-not once the display wanes, and before too many seeds are released. They can become invasive if left unchecked.
Stake tall perennials to prevent wind damage to flower spikes.
Sweet peas need training and tying in to their supports to encourage them to climb and make a good display.
Liquid feed containerised plants 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 rootbound.
Pest and disease watch
Inspect lilies for the scarlet lily beetle whose larvae can strip plants in days.
Vine weevil larvae can be a serious pest of containerised plants, and become active this month. There are various chemical and biological controls available.
Aphids multiply rapidly in summer. Remove early infestations by hand to prevent the problem getting out of control. Aphids can transmit viruses, as can other sap-sucking insects.
Continue to protect lily, delphiniums, hostas and other susceptible plants from slugs and snails.
Hellebores can develop hellebore leaf spot on old leaves.