A perennial plant that freezes back each fall is called a herbaceous perennial. Examples are garden peonies, balloon-flowers and Japanese anemones. Not all herbaceous perennials stay green until frost; some die back soon after flowering, as Oriental poppies and Virginia bluebells. Other nonwoody perennials may be evergreen, at least in certain climates; the hellebores, Shasta daisies, certain day-lilies and statice often maintain at least a rosette of green foliage through most of the winter.
Begonias, pelargoniums (garden geraniums), shrimp plant and several other "annual" bedding plants are quite perennial, perhaps even semi-woody, in their tropical, native habitats. Snapdragons and petunias frequently go on for two or more years in the Pacific Northwest.
We have to think of perennial plants in two ways: how they behave in our gardens and how they behave in the place where they grow natively. Perennials are the backbone of the garden. In a rock garden, almost every plant is a perennial. Most of the flowers in the wild garden are perennial. Our lilies, daffodils, hyacinths, tulips, crocuses and similar dependable bulbs, corms and tubers, are, of course, perennials. So are the more tender sorts, as dahlias, gladiolus, acidanthera, cannas and crocosmia; obviously, these go on year after year. But they cannot stand frost. It is hard to think of a home garden without perennial flowers. Perennials mean home gardening.
A careful selection of perennials provides flowers month after month. Very early in the spring the low-growing perennials begin to bloom. Hellebores often bloom in the snow; in the rock garden arabis species, the earliest dianthuses, some primroses and candytufts come out in March or early April. At the same time, marsh-marigold and skunk-cabbage blossom in the bog garden and a few miniature irises bloom in the border. In most climates the greatest showing of perennials comes through May, June and July. Autumn is climaxed by displays of chrysanthemums, Michaelmas daisies, the artemisias and Japanese anemones.
When working up a landscape plan, contrive various habitats. The perennial border gets full sun and requires a well-drained site. Two or three closely planted shade trees, closed in toward the east, south and west by low-growing, trees such as dogwood, redbud or black-haw, provide a site for the woodland and woods wild-flower garden.
If you are lucky enough to have a low place where the ground is soggy throughout the year, you may make a bog garden, with or without a pond for aquatic perennials. A rock garden is a wonderful thing provided you have the time to maintain it; quite a few rock-garden perennials thrive in a properly laid-up dry wall and the maintenance is very light. By all means, contrive growing sites for perennials.
Perennial plants have strong root systems. Going on, year after year, the roots of perennials grow outward toward moisture and nutrients. Some perennial plants develop at the ground line a mass of stem-root tissue, more or less well defined, called a crown. Delphinium crowns, for example, are somewhat woody, producing thick, very tender shoots above and rather weak but longish roots below. Summer phlox and hardy aster crowns become extremely woody with age; so woody, in fact, that movement of water and minerals from roots to shoots is retarded, and bloom becomes poor. The crowns of primulas and forget-me-nots remain soft.
Other perennials do not have well-organized crowns, but thickened, fairly woody main roots. Garden peonies, old-fashioned bleeding-heart and false indigo roots are intertwined and tangled, thick, becoming woody with age; these produce strong buds (eyes) near the soil surface that grow into flowering shoots. Smaller, fibrous roots extend outward from the thickened roots, and these absorb water and nutrients.
Some perennials produce more or less thickened, fleshy stems that creep horizontally just at the ground line. Iris rhizomes are typical. When a creeping rootstock is soft and fleshy, it is subject to decay. Plants with fleshy rhizomes need very well-drained soil. Rootstocks and rhizomes of aquatic plants usually are tough, sometimes woody. Cat-tails, sweet flag, water-willow, pickerelweed and the aquatic irises all have these ropy or woody creeping stems, with a mass of fibrous roots beneath.
Border perennial stems usually rise straight up from the crown or from the roots. Sturdy, well-spaced stems produce masses of large-sized, long-lasting flowers. On older clumps, when the leafy shoots are half-developed, clip out (at the base) all weak stems; it usually pays to remove half of the remaining stems on perennial clumps older than three years. For strong bloom and healthy plants lift and divide border perennials every fourth or fifth year. Some perennials resent disturbance, however; peonies, hostas, the gasplant and bleeding-heart make little or no bloom for two or three years after being lifted. Woodland and aquatic perennial stems usually are not thinned.
Perennials bloom in many ways; delphiniums, lupines and holly-hocks produce flowers on a strong vertical stem. While most of the perennials with flowers in spikes bloom from the bottom upward, a few, notably the Liatris species, bloom from the top downward. Other perennials bloom with flowers in close-set panicles or clusters, as summer phlox; still others bear flowers in looser clusters, as coral-bells, or in very open sprays, as columbine. A few perennials bloom on unbranched stems, or with branching limited to second-crop flowers that originate low on the stem of the primary flower, as Shasta daisy. Remove flower heads of perennials as quickly as blooms fade, to prevent seed formation, which saps the strength of the plant.
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