Herbaceous plants are plants that, by definition, have non-woody stems. Their above-ground growth largely or totally dies back in winter in the temperate zone, but they may have underground plant parts (roots, bulbs, etc.) that survive.
Technically, all annual plants are herbaceous, because an annual is a non-woody plant. Annuals take it a step further and die altogether at the end of their lone growing season, both above the ground and below it.
Biennials, likewise, lack woody stems, therefore they can be characterized as herbaceous. However, biennials such as foxglove (Digitalis) and silver dollar plant (Lunaria) maintain live, low-growing foliage above-ground during the winter (known as "basal leaves"). Thus the question of whether a plant is herbaceous or not hinges on the presence or absence of woody stems, not on winter die-back.
Herbaceous Plants That Are Perennials
Nevertheless, when people speak of "herbaceous" plants, they usually limit the discussion to perennials. While some are evergreen, for Northerners, "perennials" and "herbaceous perennials" are almost synonymous. These are non-woody plants that die back to around ground-level once cold temperatures return. They do, however, survive the winter, thanks to their underground plant parts. This group includes some of the most cherished plants in the landscape. The following shortlist mentions several examples:
- Columbine (Aquilegia)
- Montauk daisies (Nipponanthemum nipponicum)
- Larkspur (Delphinium)
- Hardy mums (Chrysanthemum)
- Peonies (Paeonia lactiflora)
- Garden phlox (Phlox paniculata)
- Stonecrop (Sedum)
- Oriental poppies (Papaver orientale)
- Leopard plant (Ligularia)
- Rodgers flower (Rodgersia)
- Mayapple (Podophyllum peltatum)
Even under the general classification, "herbaceous perennial," there are a number of sub-classes, based on how a plant stores nutrients underground during the winter (nutrients upon which it will draw when the weather warms up enough for it to produce vegetation once again). Everybody immediately thinks of the "roots" that survive underground during winter, but some perennials have other kinds of specialized plant parts that patiently wait out the winter below the surface.
For example, there are spring bulb plants that store nutrients in a bulb, then burst upon the scene in springtime with leaves, stems, and early-spring flowers. Daffodil flowers (Narcissus) are a well-known example. Other plants, such as Dahlia flowers, differ in two ways from the spring bulbs:
- Their plant part that holds nutrients underground is classified as a "tuber," which resembles a bulb.
- Being from tropical countries, they need to be overwintered indoors. They can then be brought back outdoors when warmer temperatures return; they do not bloom until summer.
Still, other herbaceous plants have "corms" to act as underground nutrition reservoirs. The enormous snake lily (Amorphophallus konjac) emerges from a corm in spring and famously proceeds to stink up the joint with its unusual flowers designed to attract bugs that feed on animal carcasses.
Although gardeners may cringe at dignifying them with such a name, even some weeds are herbaceous perennials. An example is Japanese knotweed (Polygonum cuspidatum), which uses underground rhizomes to store its plant food. Eradicating such weeds typically requires more effort than getting rid of annual weeds.
What Are Underground Rhizomes?
A rhizome is a plant stem that runs underground and sends out both roots and shoots (stems that grow foliage and flowers). Rhizomes typically grow horizontally right below the soil surface.
Herbaceous Plants in a Northern Winter Landscape
Annuals are dead by the time winter arrives, and the basal leaves of biennials do not stand tall enough to be able to poke their heads out through the snow cover in the North. But do not give up altogether on deriving winter interest from herbaceous plants.
Herbaceous perennials may die back to ground level, but that does not mean that they necessarily disappear. Some turn brown and hang around (if you let them). Sometimes, their above-ground growth remains attractive, despite being dead.
That is why gardeners often struggle with the question, To cut, or not to cut? The basic answer is that, as long as the disease is absent, it is perfectly acceptable to refrain from cutting down the plants until spring comes. In fact, leaving the above-ground vegetation may even afford a bit of insulation to help the plant survive winter.
Here are some herbaceous perennials some gardeners choose not to cut back in fall, to take advantage of their display value in the winter landscape (and the specific reasons why):
- Joe-Pye weed (Eupatorium; adds architectural interest)
- Coneflowers (Echinacea; wild birds eat the seeds)
- Maiden grass (Miscanthus; the straw color its leaves assume in winter is pretty)
However, remember to supplement herbaceous plants in your winter landscape design with evergreen trees and shrubs, since the latter offer more winter interest in the landscape.