First, once the flower dies, it detracts from the good looks of the garden. The Mexican Tulip Poppy shown here is gorgeous when in bloom, but should be deadheaded as soon as its blossoms have faded.
Second, even though we say it's dead, it's actually very much alive and continues with its growth toward seed production. This process pulls plant energy that would otherwise be available for new foliage and flower production into the seed head.
Third, removal of spent flowers helps to quickly redirect plant energy to side shoots for smooth and speedy transfer to new growth.
To make this rerouting most efficient, always cut back to just above the first side bud that is already beginning to grow. If there is no active side bud below the bloom, cut back either to a side branch or immediately above a leaf node where a latent bud will be likely to push out new growth. Make a clean cut with a sharp-bladed knife, since ragged cuts take much longer to heal and are likely sites for entry of rot and disease. These rules for cutting apply to the removal of cutting flowers as well.
Occasionally it becomes necessary to cut back growth in order to keep a plant from becoming leggy or from drowning out neighboring plants. Cutting back should be approached in the same way as removing dead flower heads. Always cut back to a side growth shoot or branch that is headed in the direction you want future growth to go. This way you can steer and control growth as you see fit.
Annuals have earned their reputation as beautiful elements of any garden. With close attention from you, and the regular use of these tips, you will be able to cultivate annuals this year and for many years to come.