Ipomoea Heavenly Blue

True sky blue is almost unheard of in a flower. Most blues lean violet or indigo. This one is the real thing: a clear azure trumpet eight to ten centimetres across with a crisp white throat, on a vine that will cover three metres of fence in a season. It holds an RHS Award of Garden Merit.

Every flower opens at dawn and is finished by mid-afternoon, which is where the name comes from. Fresh ones open every morning from July to the frosts, so the display is continuous even though each individual bloom lasts half a day. It is a plant to look at with the first cup of tea rather than after work.

Two things go wrong for people. The seed coat is hard, so soak it overnight in warm water before sowing or germination is slow and patchy. And it is genuinely frost-tender and hates cold soil, so do not plant it out until late May or June. It cannot support itself, so the trellis, wires or strings go up before the plants do.

The seeds are toxic if eaten. Keep the packet away from children and pets.