UNITED STATES—Flowers have a bigger and better agenda than coloring our gardens and homes. They bloom to get pollinated. Their color and fragrance are designed merely to attract pollinators. Less vain but more abundant blooms take advantage of the wind to disperse their pollen. Once pollinated, flowers fade and deteriorate as resources get diverted to the production of seed and fruit to contain it.
Some flowers are on a tight schedule. They bloom in a single brief season. Others have a bloom season that last significantly longer than the individual flowers do. They might bloom continually for a few months, replacing fading older flowers with new flowers; or they might bloom in phases, with each phase blooming simultaneously, and then getting replaced with a subsequent phase.
Fruit trees and many fruiting vegetable plants bloom once annually, and then produce fruit. Tomato, summer squash and bean plants bloom and produce fruit continually. Tomato fruits are best if allowed to ripen on the vine. Beans and summer squash like zucchini are better if harvested while young and tender. Also, the plants are more productive if regularly deprived of premature fruit.
The priority of these plants is to produce seed. Production of seed requires significant resources. Plants that are busy producing seed within maturing beans and zucchini do not put much effort into producing subsequent bloom and fruit. However, if deprived of maturing seed and fruit, these sorts of plants are compelled to divert resources into new bloom, and seed and fruit production.
The same applies to many flowering plants, particularly perennials and flowering annuals. ‘Deadheading’ is the removal of deteriorating flowers to promote continued bloom. It is not practical for plants with profuse small flower, such as sweet alyssum and lobelia. Nor is it necessary for some sterile or nearly sterile plants that do not produce much seed anyway, like busy Lizzy (impatiens).
French marigold, petunia, zinnia, floss flower and cockscomb all bloom better if deadheaded. Rhododendrons do not benefit directly from deadheading, but look better without old floral trusses. Conversely, the potentially picturesque dead flowers of sea holly might be left intact.
Highlight: ‘Big Blue’ sea holly
This one is no fun to handle. It is just as prickly as it looks. Yet, it is the spiny foliage and blooms that make ‘Big Blue’ sea holly, Eryngium X zabelii ‘Big Blue’, so appealing. The knobby blue thistle flowers are centered on prominently flaring grayish blue bracts that look like metallic snowflakes. The intricately lobed grayish foliage contrasts splendidly with just about any darker green foliage.
Bloom begins with summer and continues almost to autumn. The first flowers are solitary on strong stems. As they fade, sideshoots from these original stems continue to bloom with smaller but more abundant blooms. They are excellent cut flowers, fresh or dried. However, cutting the first solitary flowers with long stems removes some of the sideshoots that would otherwise bloom later.
Mature plants can get three feet tall and half as wide. Shade, even part shade, causes irregular growth that can be quite weedy. Although perennial, ‘Big Blue’ sea holly might live only a few years.