Mosses are heroic little plants. They have carpeted the nooks and crannies of this earth for 300 million years. Mosses are different from other plants in one fundamental way: they lack the plumbing that allows them to move water up from roots and move sugars down from leaves. This lack of vasculature has some major implications for our low-slung friends, namely that they can never grow far from a moisture source. Except in the wettest places on earth, mosses cling tightly to soil, rock and bark so they can sponge up what little water comes their
way via rain, snow or dew.
Simplicity does have its advantages, though. Mosses are incredibly resilient, withering completely when dry and freezing solid when cold, only to revive almost instantly once rain or warmth returns. You can mist a crunchy pad of moss and watch it transform in minutes into a soft, verdant cushion. During the winter, mats of tiny mosses on rocks and logs provide splashes of rich green amidst the brown and gray, for they continue to grow happily whenever the temperatures climb above freezing.
Mosses don’t have roots, though most grow thread-like rhizoids that anchor them in place. The average perennial or shrub invests up to half of its energy into growing roots, not to mention reinforced stems, flowers and seeds! Without these extravagances, the energy needs of mosses are minimal, explaining why they can grow in shade too dense for even most ferns. It’s not cold but ice crystals that threaten most plants in winter, but because mosses have no pipes to burst, so to speak, they can survive unharmed through some of the coldest temperatures on earth. A simplified anatomy also means mosses require far fewer nutrients than their larger competitors, so they thrive on substrates far too nutrient poor for much else.
GARDENING WITH MOSS
The trick to growing mosses is simply to keep them moist and free of larger competition. Because we associate them with age, mosses give any garden a wizened, ancient appeal, and a moss-filled garden has an unmistakably restful quality. In foggy, cool environments such as the Pacific Northwest, boreal Canada or the coast of Maine, shaded ground is almost always carpeted with a spongy green shag of moss. As you travel south, mosses become smaller, less abundant and increasingly restricted to wetter and shadier habitats such as thick forest and swamp. Even so, it’s possible to cultivate mosses in all but the driest parts of North America if you choose wisely.
If you have conditions appropriate for mosses, there will likely be a few growing there already that can be encouraged with some basic care. It’s not really important to identify the species as long as you can tell them apart enough to figure that this one likes bare rock; this other,
thin, dry soil; and a third, rotten logs and stumps. You may be content just to recognize mosses by their two basic types: carpet- and pad-producers. The former creep along and interweave stems to form a blanket while the latter grow vertically in dense tufts or mounds.
Two of the most common mosses in my area are feather moss (Hypnum imponens) and fernleaf moss (Thuidium delicatulum)—both carpeting species that are easily transplanted to rotten stumps, logs and low rocks. These protuberances in the wooded landscape offer mosses an important advantage—they elevate the tiny plants above the leaves that fall from the tree canopy every autumn. The one thing mosses cannot abide is a smothering blanket of dead leaves or other debris. Big species like stair-step moss (Hylocomium splendens) can push up through a scattering of fir needles, but no moss can handle the barrage of maple and oak leaves in deciduous forests. The bare edges of larger rocks are home to such species as broom moss (Dicranum scoparium), a pad-former. This deep-green moss is one of my favorites as it looks good even when frozen or dry.
These simple but incredibly resilient plants require very little from the gardener. The most critical thing is to keep them free of fallen leaves, twigs and weeds. A few passes with a rake, leaf blower or shop vacuum will remove the debris that would otherwise cause moss to brown out. I do this once in spring, once in summer and twice in the fall.
Small-seeded trees germinate readily in moss, and if allowed to grow they are difficult to remove without tearing up the moss. I snip unwanted seedlings or gently hold down the moss as I tease out the interloper.
Even in damp climates, a little extra water will help mosses grow more quickly and luxuriantly. At the Coastal Maine Botanical Gardens, I’ve set up a simple mist system that comes on for 15 minutes every 2 hours from 10 a.m. to 4 p.m. during the warmer months to keep a large planting of common hair cap moss (Polytrichum commune) green and sparkling. The moss would grow fine without irrigation, but it has the tendency to shrivel up and turn brown when dry so the bit of mist keeps it green. I’ve noticed that it grows about three times faster than other nearby hair cap moss. Its close relative, juniper hair cap moss (P. juniperinum) is more drought tolerant and a good choice for dry, acidic soils in partial sun.
Mosses need little if any fertilizer. In fact, fertilizer can burn moss while encouraging weeds. The pH of soil can also help or hinder mosses. Many species thrive in highly acidic soils (pH below 5.0), so by lowering the pH with powdered sulfur you can encourage mosses while discouraging weeds and lawn grasses that generally prefer a sweeter soil. For the most part, mosses will do best in a damp, shady location where even ferns may struggle. Shady areas stay damp longer and the moss has less competition from other plants.
There’s nothing as soothing as an emerald carpet of moss on a damp, misty morning. Mosses combine wonderfully with ferns and evergreen groundcovers in the shady garden and I hope you’ll agree that mosses are one plant that no garden should be without.
William Cullina is the Director of Horticulture and Plant Curator of the Coastal Maine Botanical Gardens in Boothbay, Maine. This article first appeared in the March 2009 issue of Horticulture.