New dahlia plants should be in the ground now, protected against slugs. Thin out the growths of established plants, leaving four or five main stems to grow on. Tie the growths to strong stakes. The simplest way is to put a cordon of three stakes around each plant, as the basis for a corset of strong string. You can add extra rounds of string as the plants grow taller. A thick mulch after a thunderstorm will help retain the moisture they need.
Stop chrysanthemums set out last month, and guard against aphid attacks. Take out the dead flowers from lilac bushes and prune some of the scraggier growths. Grey-leaved senecio is coming into flower. If you grow it for the leaf rather than the flower, cut out the flower heads now.
Hoe regularly between rows of onions and other vegetables. If you have water to spare, give it to beans and courgettes, rather than tomatoes, unless the tomatoes are in growing bags. Keep pinching out the side shoots on staked tomatoes. Bush tomatoes can be left to their own devices.
Continue to make sowings every two weeks of salad crops, radishes and summer spinach. Nip flower buds off herbs to prolong leaf production. Basil can be kept quite satisfactorily in pots on a kitchen windowsill. Feed regularly with a liquid feed high in nitrogen.
Remember to water trees and shrubs that have been in place for less than a year. Autumn-planted specimens will be struggling less than spring-planted ones, but all need watching. One long soak is far better than several sprinkles.