Sweet rocket, Hesperis matronalis, is a pretty biennial, bearing white or purple flowers similar to honesty. A packet of sweet rocket seeds is likely to produce flowers varying in colour from deep rich purple, through all the paler lilac shades to white – leave them to seed themselves and they will maintain this diversity indefinitely, gradually naturalizing over a large area. The flowers’ fragrance is as sweet as a violet’s, and most pronounced in the evening. It looks good when allowed to drift, naturally through an informal sunny or partially shaded border, especially in a cottage garden or wildlife garden scheme. Sow Hesperis matronalis seed in late-spring where you want it to flower. Deadhead spent blooms to prolong flowering, but allow some flowers to develop seeds so plants can self-seed around the garden.