The risk of frost has usually passed by the end of May, so it’ll soon be safe to start planting tender and half-hardy plants outdoors without protection. For now, while hardening off tender seedlings, keep some horticultural fleece to hand in case frost is forecast.
1. Keep floppy annuals and perennials upright by putting in supports and stakes now. Their foliage will quickly grow to hide the framework. For a more rustic look, use hazel sticks or other straight branches from the garden.
2. Temperatures in protected glazed areas such as greenhouses and conservatories can get very hot in May. Watch that delicate plants don’t become scorched or overheated by keeping vents or windows open during the heat of the day and closing them again at night. Take care with watering too.
3. Prune shrubs and climbers that have already flowered for spring. Examples include Clematis montana, Kerria japonica, and spirea ‘Arguta’.
4. Trim formal evergreen hedging such as box, making sure no birds are nesting in it first. Small-leaved box will become more dense with pruning, helping to create crisp green topiary edges.
5. Watch for the slow build up of pests in the garden. Aphids start to become more active in late spring, and molluscs such as snails and slugs will start to take fresh interest in any seedlings. Use insecticides carefully and avoid spraying flowers in bloom as this can lead to harming beneficial pollinators.