Tuesday, April 27, 2010

Real Gardeners Plant Biennials

I once heard that the gardening terms most misunderstood by beginning gardeners are annual and perennial. So let's review: an annual is a plant which grows, blooms, and dies within one growing season. Or, as the cynics put it, "a plant which should have bloomed before it died." A perennial is a plant which grows and blooms year after year.

Perennials may be herbaceous (soft), dying to the ground but resprouting. Or sometimes we call flowering shrubs such as lavenders perennials, and they are classed as woody perennials. Terms in horticulture often derive from how we use the plant, so there can be overlap between categories such as shrub and perennial.

So an annual flower blooms right away. Perennials bloom year after year. But what are biennials? These are plants which grow one season, bloom the second, and then die. They require patience and planning. They require replanting, although many reseed (some prolifically!). Hence they are plants for devoted gardeners.

So why do we grow them?

The long period of growth while the plant prepares to bloom allows it to store energy and develop large, complex flower structures (the structure that holds a number of flowers is called the inflorescence). In some cases they develop a large root to store water and energy, helping to support rapid elongation of a flower spike. When they finally bloom, biennials do it in a BIG way.

The common characteristics of biennials are:
o late season growth at ground level;
o continued growth in spring;
o late spring and summer bloom, often with very large, showy flowers on tall spikes;
o significant seed production;
o seedlings emerging again in late summer and fall.

This cycle has unique adaptive features. The low growth habit during the initial phase allows the plant to survive frequent mowing or grazing. The later bloom season means there is little competition from winter and spring grasses. Production of large amounts of seed has obvious advantages - for the plant.

Unfortunately, one of the most noxious weeds in California has found this biennial growth cycle very adaptive. Yellow starthistle sprouts in fall, forming a tight clump (rosette) of foliage at ground level. The plant forms a tap root with winter rains, which can grow deeper than the competing roots of nearby weeds and grasses. Leaves on the rosette grow steadily through the winter and spring, with the plant nestled at the base of tall grasses. Then, as the grasses dry off, get grazed, or get mowed to prevent fire hazard, the starthistle rapidly sends up bright yellow thistle flowers and goes to seed.
Starthistle is very prickly and irritating, is toxic to horses, and now is estimated to have spread to over 15 million acres in California (up from 8 million in the mid-1980's, according to the Weed Research and Information Center at UC Davis). A field of starthistle is basically impenetrable on foot or horseback. On the plus side, the flowers are pretty and bees like it.

Many biennials require a cold period (vernalization) before they initiate flower buds. So they are planted in fall for blooms the following spring or summer. As with starthistle, during the initial season they develop a rosette (a tight complex of buds and leaves at ground level, densely packed along a short stem). Once they've had their period of chilling, longer days initiate flower development. The flower structure (inflorescence) elongates and produces dozens, sometimes hundreds, of flowers. Those flowers may set prodigious quantities of seed. Let's just say that your patience is rewarded.

Warren Roberts, retired Superintendent of the U.C. Davis Arboretum, credits an unusual biennial for sparking his interest in plants. He remembers the Tower-of-jewels in a neighbor's yard from early childhood. Echium wildpretii (native name tajinaste rojo) certainly follows the biennial pattern. In the first season you get a mound of silvery foliage, resembling Cousin Itt from the Addams Family. Then in late spring the plant begins to stretch upward, and upward, and upward, ultimately to 7 to 8 feet or more! It was just such a flower spike poking above the neighbor's fence that attracted young Warren's attention, intriguing him with its Dr. Seuss-like qualities and launching his long career in horticulture.

All along the Tower-of-jewels flower spike are hundreds of small, showy flowers in a vivid hue of pinkish-purple. Hummingbirds and bees love all members of the genus Echium. After the long season of bloom thousands of seeds are produced. Even with a low germination rate, you are pretty likely to have seedlings. Tower-of-Jewels is native to Tenerife, one of the Canary Islands, which is a mild-winter area. We sometimes get frost damage on young Echium plants here, but they pull through and bloom.

Biennials we grow for their flowers include some very old garden favorites.
o Canterbury bells (Campanula medium) are planted in partial shade, producing lavender or white cup-and-saucer flowers in late spring. The big, fleshy plants flop over easily. Stake or cage them!
o Dollar plant (Lunaria annua) can grow in full sun to considerable shade. The pretty purple flowers are followed by thin, papery seed capsules that are shaped like coins. The seed capsules are often dried for flower arrangements. I have this plant re-seeding around my property, but not to the point of being invasive. As I was photographing it at night (for better contrast) I discovered that the flowers have a sweet scent in the evening.
o Evening primrose (Oenothera species) is best known for the pink-flowered Mexican species that is used as a ground cover. But the genus has many biennial species. Mostly adapted to desert areas, their flowers open in the evening for moth pollination. Some are quite invasive reseeders. My mother planted O. biennis in her cactus garden in San Diego, and then spent many years digging out seedlings.
o Foxgloves (Digitalis species) and Hollyhocks (Alcea rosea) are probably the best-known biennials. Foxgloves can grow in considerable shade, as well as sun. Although there are now annual varieties of foxglove such as 'Foxy' which bloom the first year from seed, most are biennial or short-lived perennials. Hollyhocks love sun but can tolerate some shade. Both bloom in late spring and well into the summer, with flowers continuing to open along the spikes for many weeks. If you aren't too fastidious about your garden, and don't mind the spent bloom spikes falling over into the border, both will reseed freely.
o Sweet William (Dianthus barbatus) is perhaps one of the most fragrant biennials, possibly one of the most fragrant garden flowers ever. The genus Dianthus includes carnations and pinks, and the scent of Sweet William is similar but even more pungent. Largely superceded by modern Dianthus cultivars, Sweet William is still grown by old-fashioned, patient gardeners like me. It comes in dwarf (6 inch) and tall (12 inch) forms. Hard to find in garden centers, the seed is easy to save and plant from year to year.

There are many edible plants whose biennial habit we exploit, growing for part of the lifecycle and consuming the root or tight rosette that the plant was developing for its flowers. Beets and carrots develop roots in fall and winter, which we eat before the plant has a chance to flower. Cabbage and Brussels sprouts, celery, lettuce, and parsley form heads which, if not harvested, would bolt and flower the next spring (if that happens, leave the blossoms; they attract beneficial insects). If you plant parsley, expect it to flower and go to seed in the second year, and then die. So gardeners generally replant it each year to have a steady supply.

In addition to Yellow starthistle, another invasive biennial is worth noting. Dames Rocket (Hesperis matronalis) is an attractive plant that was introduced by the pilgrims in the 1600's (I guess that would make it simultaneously an heirloom and an invasive!). Each plant sets very large quantities of seed. It is often included in "wildflower" seed mixes, and has spread to nearly every state. It resembles Money plant (above) which is not considered invasive and would be a better choice for gardeners.

Plant biennials for future enjoyment! These old garden favorites have many advantages that reward your patience.

Written for the Davis Enterprise, April 22, 2010

1 comment:

  1. Lots of good information here. I usually buy biennials that are already in bloom. I guess that would make them an annual in my garden!