Deadhead dahlias as each bloom finishes. Dahlias are one of the most profusely flowering plants in the garden, but in order to keep them going for a long season, it's critical to remove spent flowers regularly. Just like with roses, by taking off the old blooms you send a signal to the plant that the season is not yet over. To deadhead, just trim each spent flower back to the main stem. When all of the flowers on a stem have finished, cut the stem back to a set of leaves so the dahlia will continue putting out fresh, new growth.