Be wary of claims about plants that will flower all summer long: very few will and long-flowering doesn’t necessarily make a great garden plant. Bindweed will flower for weeks on end. Salvia ‘Caradonna’, though, is an exceptional plant that produces lots of flowers throughout the summer and at the same time is stunningly beautiful. The stems are dark purple and the flowers are violet-blue. Together they create an intensity of colour that is particularly striking when you grow several plants together. Bees and hoverflies love the flowers. To keep it flowering even longer, cut down the flower spikes once they start to fade.