Blue Oat Grass (photo credit: Annette MaCoy, Master Gardener)
The plants gardeners call “herbaceous perennials” (plants like peonies and hostas) are a mainstay of the ornamental garden, offering a vast array of beautiful flowers and foliage from spring through fall. As winter approaches, most of these herbaceous perennials go dormant for the cold months – their leaves fall and stems die back, with the crown and roots overwintering, ready to emerge again in spring.
But there are, somewhat surprisingly, many “evergreen perennials” that retain their foliage over winter. Of course, the extent to which perennials remain evergreen may vary from year to year, depending on winter weather conditions and siting in the garden; some may stay only semi-evergreen during particularly harsh winters.
Evergreen leaves have a variety of survival adaptations. Some are stiff or leathery, reinforced with threads of lignin, to keep them upright; some are small, or needle-like, or glossy and waxed, or gray and hairy, to minimize water loss; many stay close to the ground, taking advantage of less wind, slightly warmer temperatures, and last year’s plant debris to insulate them. Plants that have adapted to being evergreen in winter are also often adapted to surviving summer drought – it is all about minimizing water loss from the leaves when there is not water available to replace it.
The worst enemy for the survival of evergreen plants is desiccation or winter burn – frozen ground combined with winter sun and cold winds means that leaves lose water that frozen roots are unable to resupply. Most of our temperate climate perennials would love to have a blanket of snow in place from December through March; snow acts as a great insulator, covering the soil, protecting the roots, and sheltering the foliage from strong winter winds and sun. Lacking a layer of insulating snow, the plants’ best friend is a layer of mulch, covering the soil and protecting the roots from a deep freeze, supplied by the gardener.
Their second worst enemy is poor drainage during the winter months, which can lead to root rots and death if the soil is saturated and frozen. For evergreen perennials susceptible to root rots, careful siting to ensure good drainage and a mulch of pea gravel can help with this problem.
Hellebore (Helleborus species) is the classic example of an evergreen perennial. It keeps the past season’s foliage over winter, a source of readily accessible nutrients and carbohydrates for the new season’s flower and foliage stems, which often begin to emerge as early as January or February in this area. As spring commences, the old foliage dies down, having given its all to the new foliage and flowers. Unless the old foliage is diseased, it is best to leave it until it has naturally died down, being careful not to damage new stems as you prune away the old.
Christmas fern (Polystichum acrostichoides) is a very adaptable native fern and a welcome sight in winter, often found on dry, rocky slopes in Pennsylvania woods. Although the dark green fronds may become flattened by snow over winter, they often still look presentable in spring, as the new fronds expand. Wait until the old fronds have completely withered before removing them, as cutting them back too early can decrease the plant’s vigor.
Although the tan seedheads of many ornamental grasses can add interest to the winter garden, there are some grasses whose foliage remains verdant over winter. Tufted hair grass (Deschampsia caespitosa) is one, forming a 12-inch mounded clump of fine, stiff, dark green leaves interspersed with old straw-colored foliage and spent flowering stems. Another is blue oat grass (Helictrotrichon sempervirens), growing as a dense mound of upright, stiff, silvery blue blades to about 2 feet in height. The cultivar ‘Saphirsprudel’ (Sapphire Fountain) has vibrant steel-blue foliage.
Grass-like plants and sedges offer many choices for winter groundcovers. Lily turf (Liriope species) has mounds of green, leathery, strap-like foliage, about a foot in height, as well as selections with gold, variegated gold, and variegated white leaves. Liriope muscari forms dense clumps; Liriope spicata spreads by underground stems. Black mondo grass (Ophiopogon planiscapus ‘Nigrescens’) is closely related to liriope, with dense tufts of intriguing, 8 to 12-inch purplish-black foliage that slowly spread to form a colony. Pair it with snowdrops for a late winter scene of black and white.
Sedges (Carex species) look very much like grasses, but can be distinguished by their V-shaped blades forming triangular “stems” in cross-section. The many species are adapted to a wide range of habitats, from wet swamps to dry shade, and many remain verdant over winter. Seersucker or plantain sedge (Carex plantaginea) is native to the eastern United States, with 1-inch wide, bright green, crinkly pleated foliage; it prefers moist, fertile soil in sun to part shade and does well in woodland settings. Carex morrowii ‘Ice Dance’ is a variety of Japanese sedge with leathery strap-like evergreen foliage sporting margins of creamy white; its spread by rhizomes can be rapid, and it seems to tolerate just about any garden situation. Another Japanese sedge, Carex oshimensis, is represented in the trade by ‘Evergold,’ with tussocks of glossy, creamy white banded evergreen foliage, and newer cultivars such as ‘Everillo,’ which forms a tousled mop of fine-textured, chartreuse to golden yellow 12-inch foliage.
Spurges (Euphorbia species) are another group of perennials with some excellent evergreen species. All spurges produce a white sap when a stem or leaf is broken; caution is advised when handling, as this sap can cause a skin rash for some people. One of the best is Robb’s spurge (Euphorbia amygdaloides var. robbiae), with 24-inch stems, the upper half clothed in tight spirals of dark green, glossy, paddle-shaped leaves. This plant does well in dry shade to part sun and can spread rapidly; the stems are biennial, dying back in the spring after flowering (and should then be pruned out), but fresh stems emerge to fill in and maintain a dense ground cover.
Adam’s Needle (Yucca filamentosa), with its stiff, sharp-tipped, sword-like foliage, looks like it belongs in the desert, but it is an amazingly tough and resilient evergreen perennial for colder and wetter regions, suitable to sun or part sun and well-drained soil. The species has gray-green foliage, but the cultivars such as ‘Color Guard,’ ‘Bright Edge,’ and ‘Golden Sword,’ offer variegated green and yellow leaves that provide much greater impact in the winter garden as well as year-round.
Yucca filamentosa (photo credit: Annette MaCoy, Master Gardener)
There are so many other evergreen perennials that I do not have space to do justice to them all; here is a list of possibilities to fill the winter garden: bugleweed (Ajuga); coralbells (Heuchera); pigsqueak (Bergenia); carnation, pinks (Dianthus); groundcover stonecrops such as ‘Angelina;’ creeping phlox (Phlox stolonifera); moss pink (Phlox subulata); liverleaf (Hepatica); hardy ginger (Asarum species, Hexastylis); hardy cyclamen (Cyclamen species); candytuft (Iberis sempervirens); ice plant (Delosperma); spotted dead nettle (Lamium maculatum); and hens and chicks (Sempervivum species). There are also many herbs and subshrubs, such as thyme, germander, lavender, and sage, that will remain evergreen during the winter.
Given a changing climate, and with milder plant hardiness zones creeping northward, it seems that our winters will provide less chances for a persistent blanket of snow and more opportunities to enjoy the variety of color, form, and texture that evergreen perennials can add to the ground layer of the winter garden.
