There’s always a lull before the main summer
rush arrives, when there’s plenty of foliage but not enough flower to satisfy the gardener or the pollinator. However some perennial plants flower in the June gap, lifting the garden and carrying it towards summer.
Oriental poppies. These long-lived poppies come in a wide range of colours. After flowering they become summer dormant, leaving an unsightly gap. Surround them with penstemons or cosmos as a follow up act. These tap-rooted, long-lived poppies compensate by having good winter foliage and an ability to thrive on poor soil. Good varieties include the beetroot-blotched, small-flowered shell-pink ‘Karine’, the upright bright-pink ‘Raspberry Queen’ and the early bright-red ‘Beauty of Livermere’. Widely available.
Astrantia. Hattie’s pin cushion sums up the gentility of this remontant umbellifer which always flowers lavishly
now as well as a little in September. ‘Hadspen Blood’, a deep-red form from the famous Somerset garden, loves the same dappled shade and cool position as the oriental hellebore and they make good partners. ‘Ruby Wedding’  is a clearer red. The pale-pink, sun-seeking ‘Buckland’ will flower almost continually. Always deadhead as the flowers age badly and inferior seedlings can be a problem. Widely available.
Hardy geraniums. Certain hardy geraniums begin their long flowering season in May. ‘Patricia’ is black-eyed magenta-pink that forms a substantial mound. ‘Orion’, also substantial, is the finest of the blues. Cut both back hard as they finish their first flush and they will leaf up in a week and flower five weeks later. 
Woodland geraniums also peak now. Among the best are the sprawling G. macrorrhizum and the more upright G. phaeum. Be warned though: the latter self-seeds. Widely available.
London Pride. This provides a foot-high froth of pale-pink or white flowers above crimped green rosettes.
There are some very fine forms on offer.‘Miss Chambers’ (Cotswold Garden Flowers) has red-eyed pale flowers and darker rosettes and S. x geum Dixter (Beth Chatto) form are both aristicratic forms that could line a path, or mingle with similar plants that like a bit of moisture. Divide and replant every third year or so, to keep up vigour.
Viola cornuta. The truly perennial winged violas are long lived as long as they are sheared back in early September to form a tight mat of foliage. They will not overwinter if left untrimmed. The white form, ‘Alba’ is divine close to box, but there are also wispy blues such as the pallid pink ‘Victoria’s Blush’, the sky-blue ‘Belmont Blue’ and the more solid-petalled species Viola cornuta. 
Anthemis ‘Susanna Mitchell’. This sprawling pale-yellow anthemis with the grey-green filigree foliage begins flowering from mid-May onwards, making a fine partner to Nepeta ‘Six Hills Giant’. It can tumble over a wall, or cover the ground and it’s hardy in well-drained soil. Cuttings need to taken regularly as, like all anthemis, this tends to be short-lived. 
Until tomorrow: Some people don’t like change, but you need to embrace change if the alternative is disaster.