Moss in the City... Literally
Since this column debuted last summer, I have received the same question repeatedly: How do you grow moss in the city? Some people have even accused me of false advertising. I make no apologies for my name or neighborhood. However, I do apologize for taking so long to write on such an obvious topic.
In many ways moss is an ideal plant: low growing, low maintenance, evergreen, and readily available. Its simple tranquility has been appreciated in Japanese gardens for centuries. Bonsai and rock garden enthusiasts have also used moss in their designs. Everyone appreciates the ethereal scene of dappled light flowing over vibrant carpets of moss as they walk in the woods. But only recently has America begun to accept moss as a garden ornamental.
The lack of flowers surely has limited its popularity. If moss bloomed, like creeping phlox, it would be all the rave. But if it bloomed, it wouldn't be moss. This ancient plant is one of the bryophytes, along with liverworts and hornworts. Mosses have no true flowers, leaves, stems, roots, or even a vascular system to distribute water and nutrients throughout the plant. Root-like structures anchor moss to soil, rocks, or wood (depending on the species), while thin-celled "leaves" absorb nutrients and water directly from the air. Although there are no flowers, reproductive organs and spore capsules can be decorative.
Catering to Moss
Moss grows naturally in areas that are shady, acidic, moist, sheltered, and free from other plant competition. Most types tolerate filtered light and morning or late afternoon sun. Some species can handle deep shade, while others can handle more sun. The pH of the soil should be somewhere between 5.0 and 6.0. Moss will not survive a pH higher than 6.5. Many east coast and west coast gardeners already have acidic soil. Those of us in the middle of the country can use soil acidifiers like aluminum sulfate and liquid sulfur to lower the pH.
Without roots to absorb and store moisture and nutrients, moss needs humidity and surface moisture in order to develop and reproduce. The leaves are thin and lack a waxy cuticle to prevent water loss, so moss does not fair well in exposed areas where winds can quickly dry them out. Humidity levels are typically higher in sheltered locations. Once established, some mosses can tolerate drought and desiccation and then revive once water returns.
Moss does not compete well with other low-growing plants. Weeds, mulches, and leaf litter should be kept off moss. In woodlands, moss is usually found in isolated patches of bare ground or on logs. Ephemeral bulbous plants like toothwort, trout lily, and snowdrop sometimes naturally occur within moss clumps, but as a rule all plants and debris should be cleared away, especially while moss is establishing. If debris and leaf litter are left to accumulate for long periods, they will smother the moss.
As always, plan before you plant. Choose a sheltered spot with northern or eastern exposure. Clear the ground and adjust the pH if necessary. If the soil is loose, firmly tamp it down. Moss likes hard, compact soil.
There are several methods of obtaining moss. Specialty retailers sell moss in sheets or slurries. Laying sheets is similar to laying sod. They are simply placed on the ground and pressed in to establish good soil contact. Slurries — moss pieces blended with acidic liquids — are spread like a paste across the desired areas. You can also collect moss (only from your own property or that of a friend) and transplant it directly, or make it into a slurry with buttermilk, cheap beer, and/or water. Those with extreme patience can prepare the ground and wait for spores to naturally accumulate and grow.
Water transplants frequently and gently for the first three weeks. Until the moss is actively growing, a strong stream from the hose can wash it away. Monitor growth and maintain a watering schedule for the first season. Once established, moss is a tough plant, tolerant of light foot traffic. Moss favors lean soil so don't use compost and fertilizer. Fertilizer may even kill it. Some fertilizers contain ferrous iron, which is used to control the growth of moss in lawns.
The urban landscape is a natural fit for moss. Many buildings cast deep shade and provide shelter from wind. The soils are typically compacted and deficient. These conditions often prove unsuitable for typical ground covers like turf, but they are perfect for moss. Small trees and understory shrubs are the classic companions in moss gardens. But feel free to experiment with moss in all sorts of shade and rock garden designs.
The new frontier for moss will be roof gardens. On rooftops that are too shady for sedums and other succulents, mosses that are tolerant of desiccation and exposure are proving to be adequate substitutes. Rooftop moss gardens can be durable and provide all the benefits of typical green roofs, such as insulation, reduction of storm water runoff, and aesthetics.