Pinch off any long, leggy stems or stragglers that grow out of the side. Pinch the stem off back down to the leaf nearest an appropriate height (at the level of the rest of the stems). Pinching back in this manner will give the subshrub a more uniform shape and encourage bushy growth.
Prune any dead or diseased foliage 2 inches into healthy tissue. Check for diseased tissue once every two weeks or so. Deteriorating foliage attracts disease, especially in densely growing plants like Argyranthemum frutescens.
Cut off all of the Argyranthemum frutescens' flowers in mid-spring with a pair of sharp lopping shears after most of the flowers have faded. Cut low enough to trim off any flowers and leggy growth. Deadheading the plant in this manner will stimulate the Argyranthemum to produce an new flush of flowers. Continue to deadhead each time the flowers fade. The plant will continue to produce flowers until fall.
Prune the Argyranthemum frutescens down to within a few inches of the ground at the end of the season when most of the foliage has died.