Echium fastuosum
An exotic shrub. That should be any oxymoron. How could something as exotic and beautiful as this be described as something as dull as 'a shrub'? This can. No alternative. It's woody and the trunk and branches tend to create the most wonderful shapes under its canopy of floppy glaucous leaves and in early spring starts producing it's crowd of perpendicular electric blue flower spikes.
Fully out in April. The plant might reach 6ft or more (still doesn't qualify as a tree) and 6ft across and the flowers about 1ft tall. If you have a garden mild enough for this (within 200 metres of the beech or Central London), one of its many engaging traits is its tendency to produce the flowers even in the depths of winter.
A harbinger of spring, a nice reminder that winter won't last forever. It's just before Christmas as I write and the plants at the nursery (in the unheated greenhouse) are already making it obvious where they're going to flower.
It needs plenty of light, good ventilation and a reasonably well drained soil. If you can give it winter protection (a shed or greenhouse), try it in a pot but make it as big as you can manage. They grow quite fast but don't live particular long (10 years?). A roof garden in Central London.