We dead head plants (if we can find the time) for a number of reasons. One is too keep the plant from setting seed. A large amount of a plant’s energy that might be better used in producing strong roots and foliage can be channeled into the seed making process. If a plant is a prolific seeder that becomes invasive (thus requiring much weeding) dead-heading can stop the problem before it starts. Removing spent blossoms also keeps your garden tidy.
The major reason for dead-heading is probably to force the plant to rebloom. A plants main purpose in life is to produce seed and if you prevent it from doing so it will continue to make the effort through new blossoms. Not all plants will rebloom, so the only reason to dead-head them is for the sake of tidiness or to prevent seed formation. These non-rebloomers include: astilbe, bee balm, clematis, dianthus, hibiscus, hydrangea, impatiens, lilac, lily, morning glory, nasturtium, peony, penstemen, salvia, trumpet vine, verbena and vinca. If you are going to tidy up the lilacs do it as soon as they are done blooming because the buds for next year will form shortly thereafter and you don’t want to remove these.
Perennials that will rebloom if dead-headed include: chrysanthemum, coreopsis, coral bells, daisies, delphinium, foxglove, gaillardia, hollyhock, lavender, lupine, pansies, roses and salvia. Most annuals will continue to bloom much better if dead-headed because they will not be setting seed.
Different plants need to be dead-headed in different ways to maximize bloom. On shrubby plants like roses cut the spent flowers and stem off to the first set of 5 leaves. This does not guarantee reblooming in all types of roses but this is advised for those like the Carpet Roses that is bred to bloom all summer. Plants that have many small flowers like tickseed coreopsis or lobelia can be sheered. You can remove just the spent flowers with a scissors or take the plant off all the way to the ground if the foliage is unsightly.
Bulbs that produce only a single stalk are not going to rebloom but the stalk should be cut back to the ground to prevent seed formation. The bulb will grow bigger and stronger for next year’s bloom. Many plants such as columbine, bleeding hearts, coral bells and poppies also produce their flowers on a single stalk. When the flowers on the stem have all bloomed the stem should be removed to the ground.
Plants that produce a terminal set of flowers at the end of a branch will continue to produce flowers on side shoots if the spent flowers are removed. The branch should be clipped to just above the set of leaves where the new buds will appear. The entire stalk should be removed once flowering is complete. These plants include chrysanthemum, lupines and delphiniums. Plants that produce short flower stems off the main stem such as marigolds should be pinched back to the main stem or the first set of leaves if it is a slightly longer stem.
Many plants such as the hardy geraniums become unsightly after they bloom. Cutting them to the ground removes the leggy foliage and stimulates rebloom. Biennials should also be sheered to the ground after blooming. This will add to their life span and make them behave more like a perennial.