Wallflowers (erysimum famility formerly cheirathus) start flowering from March and some, notably 'bowles mauve' keep flowering all summer. Their colours a range of warm tones from pinky-purples through rustic oranges and reds to bright yellows.
They are often grown almost as a single season plant; sown one year then dug up with and allowed to become semi-dry and sold as a tied bunch of tap roots to be planted in autumn or spring to provide vivid bedding flower for a year.
This method of growing provides masses of colour but is quite a lot of work. I prefer to grow them as short-lived shrub and find that if you cut them back after flowering ( or mid-season for 'bowles mauve' because it doesnt finish flowering until late) you get 3 or 4 years our of them before they get too woody to look attractive.
Some, including the native 'cheiri' - pictured above - self seed quite easily so if you are careful with our weeding the plant generates its own successors. This picture is or one that managed to seed itself into a crack in the paving just by my kitchen door and I am currently happily enjoying it everytime I go past.
For the bees, these early flowerers provide nectar and attract honey bees, bumble bees and some solitary bees; not necessarily in large numbers but over many weeks so the value is there.