If some of your herbaceous plants did not flower well last year, it could be that they need to be rejuvenated by being divided and replanted. Dig them up, and throw out woody and unproductive parts at the centre of the clump, likewise any bits that look diseased or doubtful. Divide the clumps into portions containing about six or eight healthy shoots and replant in soil that you have replenished with garden compost or well-rotted manure. Work in a handful of bloodfish-and-bone- or seaweedmeal. Water well and make sure the soil does not dry out if there are prolonged, rainless spells in April.
Don't divide grasses and red hot pokers until the weather warms up in May or so.