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Moss rose or plain moss for rock wall?

“In the Park Seed catalog, I saw some beautiful portulaca. When I think of this little spready plant with short, yet succulent leaves and rose-like flowers, I think of it by itself, isolated, disregarding how it might fit into the garden as a whole.” — Jamaica Kincaid, “My Garden [Book],” 1999

Jamaica Kincaid can be forgiven for thinking about portulaca, or any garden plant, apart from the context of an actual garden because that is what seed and plant catalogs are designed to do. The colorful, closeup photographs and rapturous descriptions of flowers and foliage are meant to make eager gardeners forget to consider exactly where that irresistible plant will go and, really, if it belongs in their garden at all.

I checked the 2022 Park Seed catalog and I, too, saw some beautiful portulaca, also known as moss rose. On offer were two seed mixes of P. grandiflora, one called ‘Happy Hour’ — with vibrant red, purple, orange, white and yellow flowers — and another called ‘Sundial’ with the same tropical colors in somewhat more muted shades and double flowers.

I was looking for ideas for a rock retaining wall that was just built on the side of our gravel driveway in Bandon. Portulaca can be planted in the crevices of rock walls or in rock gardens, where the “little spready plant with short, succulent leaves and rose-like flowers” makes a nice addition to hot, dry growing spaces.

However, when I considered the context of my particular rock wall, situated mostly in shade with a few hours of dappled sunlight, it became clear to me that sun-loving P. grandiflora was not a good planting choice after all.

If I can’t have moss rose on my wall, I can at least have some moss. I turned my attention to how to grow this ancient, spore-bearing, nonvascular plant, which includes more than 12,000 species worldwide. The Pacific Northwest has been called the “land of mosses” because so many different kinds of mosses grow happily on rooftops, tree branches, forest floors and rocks.

I learned there are two main groups of mosses: acrocarpous mosses that form clumps and pleurocarpous mosses that have a spreading growth habit. A genus of acrocarpous mosses called Orthotrichum is commonly found in Oregon growing on tree branches. I’m particularly interested in the Racomitrium genus because these acrocarpous mosses grow well on basalt rocks like those in my retaining wall.

Acrocarpous mosses are dense, so they grow more slowly than pleurocarpous mosses. The latter group includes Hypnum mosses and fern-like mosses in the Thuidium and Kindbergia genera. Planting a combination of faster-growing and slower-growing mosses on my rock wall would be ideal.

A common recommendation for growing moss on rocks is to make a moss “milkshake” by blending pieces of a selected species of moss with a cup of buttermilk or yogurt. The mixture is then painted or sponged onto the rock and covered for a few weeks until the moss attaches to the rock’s surface.

Mosses attach to rocks by tiny, hairlike structures called rhizoids. They do not have roots to anchor or to take up water and nutrients from the soil; instead, they get their sustenance only by absorbing moisture and carbon from the air. This absorptive characteristic makes moss one of the best air-cleansing and oxygenating plants in the world. Moss has sometimes been called a “green lung” because of its effectiveness at capturing pollutants and allergens from the atmosphere.

I have to digress a bit here to relate an interesting story about how moss cleaned up Portland. In 2015, the Oregon Department of Environmental Quality discovered alarming concentrations of metals, including cadmium and arsenic, in Portland’s air. They used samples of Orthotrichum lyellii, a moss that grows on trees throughout the city, to pinpoint toxic hotspots and where the emissions were coming from. The study results were used to force the polluting factory to install new exhaust filters on its furnaces to keep the toxins out of the atmosphere. Ubiquitous, unassuming moss to the rescue!

Now, back to my rock wall. There are several problems with painting my wall with moss milkshake, not the least of which is all the effort such a project would entail. Covering a 50-foot wall would be tedious, but to leave it uncovered would likely result in rain washing away the applied moss mixture or deer licking it off. A stray hog keeps wandering down our driveway, and he would probably find the moss milkshake irresistible.

My best bet is to tuck pieces of moss in the wall crevices with a bit of soil and wait to see what happens. I plan to do this in September when (hopefully) it begins to rain a little more in Bandon. Another advantage to growing moss is that it adapts to wet and dry weather conditions. After the wet winter, it will go dormant during the dry summer months for easy maintenance.

Yet another benefit of growing moss on my retaining wall is that it will provide food and shelter for a variety of insects, which will, in turn, attract birds. My mossy rock wall will provide a splendid micro-habitat for wildlife, not to mention a pretty entrance to our property.

As for the moss roses, I’ll leave those for Jamaica Kincaid’s sunny rock garden and the alluring pages of the Park Seed catalog.

Rhonda Nowak is a Rogue Valley gardener, teacher and writer. See literarygardener.com or email her at Rnowak39@gmail.com.