Why and How to Build

I’ve got to learn to look up more, you know, the way tourists do; natives generally fix their gazes straight ahead to a destination or downwards, in thought. I am reminded of this when a visitor (a “tourist” in this context) walks up my front path, smiles, and tells me, “I like your green roof.” So then I (the “native” in this context) look up and join in the appreciation.Two cats on my green roof

My green roof was born about twenty years ago. After fumbling too many times with packages or keys in the rain at the front door, the time had come build protection from the elements. Rather than cover just the area around the door, this cover would extend over a small patio. And rather than just shingle the roof, why not make it a planted roof, a “green roof?”

So my friend Bill and I built a sturdy, shallowly sloped roof supported on three corners by the walls of my house, and by an 8 x 8 white oak post in its fourth corner.Roof, ledger up The roof had enough slope so plants could be seen from the path, yet not so steep that a hard rain would wash the soil away.Roof completed

To keep the wooden structure dry, it was covered with EPDM roofing material and flashed with copper. A two-inch high lip of copper flashing along the low edge keeps plants and soil from sliding down off the roof. To prevent water from puddling at this lower end, I drilled holes and soldered short lengths of copper tubing at intervals into the lip, figuring that excess water would stream decoratively from each tube during rains. Roof drains(It streamed, but not so decoratively, because some tubes kept getting clogged and the bottom edge of the roof was not exactly horizontal so the flow burden was taken up by only a few tubes.)

How and What to Plant

Next was the gardening part of the roof. To keep the weight down, for low fertility to suppress weeds, and for good moisture retention I made a planting mix of equal parts peat moss and calcined montmorillonite clay (often sold as kitty litter).

The root environment on the roof was going to be harsh for plants. Only a two-inch depth of root run. And, in contrast to the moderated temperatures within soil out in open ground, roots on this shallow roof would experience mad swings in temperature that would closely mirror that of the air. And even though peat moss sucks up and holds moisture, there’s not much peat moss moisture to draw from in 2 inches of rooting.

The plant choice, given the conditions, became obvious: some kind of hardy, succulent plant. I chose hens-and-chicks. After filling enough 24 x 10 inch planting trays with the planting mix, I plugged in hens-and-chicks plants every 4 inches in each direction. Hens-and-chicks in traysAll this planting took place almost a year before the roof was readied for the plants, important so the hen-and-chicks could make enough “chicks” to spread and pretty much cover the planting trays, which otherwise would have left too much planting mix exposed to washing from rainfall.

Up the trays eventually went on sloping roof, laid down like tiles on a tile floor. Everything looked very neat and trim.Green roof, early on

Nature Collaborates

Despite very little intervention from me, the years have brought some changes to that green roof. This was not a garden area that I ever planned to weed, and I stuck to my plan. I figured that a green roof with a weed-free planting mix, sheltered on two sides by walls, and eight to ten feet off the ground would not harbor weeds.

I was wrong. Some weeds have moved in. Well, not weeds per se, because a “weed” is plant in the wrong place, and the roof is welcome to pretty much any plant.

Gazing up on my roof now, I see, in addition to the original hens-and-chicks, plenty of foxtail grass, quite decorative through the year with green shoots in spring and tawny, fuzzy foxtails in fall and winter. Also a single cedar tree about two feet high.Roof with weeds
More weeds on roof

I had a hand in introducing another sedum, Angelina. This sedum has borne small, yellow flowers and, just as decorative, its fleshy leaves turn a deep red color in winter. Angelina started out as a single plant I set in a nearby stone wall; over time a few plants of it appeared up on the roof. Angelina on roof

Angelina floweringShe looked good up there, especially when draped over the front lip. She also multiplied rapidly both on the wall and on the roof. To further encourage her, I grab bunches wherever in excess on the wall and toss them up on the roof to root.

One year I also planted oats left over from cover cropping in the vegetable garden. Oats’ extensive roots would be good to further knit together the rooting mix and lessen chances of rain washing it down. Planting involved nothing more than grabbing handfuls of oat seed, tossing them up on the roof, then waiting for rain.

Writing about my green roof has encouraged me to more frequently look up at it. It does bring a smile. Is that because there’s something anomalous about an aerial garden?

Brooklyn Grange

Urban farm high in the sky at Brooklyn Grange


Not Green Enough

I’m looking up at my green roof, my evergreen roof, and it’s not green enough. Literally. I had expected that by now the roof would be solid green. It’s not.Two cats on my green roof

The green of this roof was supposed to come from the plants growing on it. Because conditions up on the roof are very harsh, the plants I chose were tough ones, hens-and-chicks (Sempervivum spp.). Hens-and-chicks look like little cabbage heads of stubby, succulent leaves. Baby plants push out from around the mother plants, grow, and make more babies, and so on, ad infinitum. Or so I hoped.

The roof only has a couple of inches of “soil” on it and covers a porch, so has no heated space or insulation beneath it. If winter temperatures plummet to 10 degrees below zero, not uncommon here, temperatures within that thin layer of soil also plummet to 10 degrees below zero. If summer temperatures hit 95° in the shade, the soil, which is shaded at one end, also hits 95° — and more in the sunny end. The roof never gets watered, except by natural rainfall.

The hens-and-chicks have established and survived and spread. But not enough. By now, I expected the roof be packed solid with hens-and-chicks, with excess plants spilling decoratively over the front edge. But too much soil still shows. Part of the green problem is that hens-and-chicks are not all that green; the leaves are more pale blue-gray.

Enter Angelina

So I’ve been taking steps to green up the roof.

The first step has been introducing a companion plant for the hens-and-chicks. The plant, which I believe is ‘Angelina’ rocky stonecrop (Sedum rupestre), has been magically appearing here and there in and around my rock walls. Well, not magically. As with other succulents, ‘Angelina’ easily grows into whole new plants wherever any piece of stem or leaf merely drops onto the soil. Over the past few years, whenever I’m so inclined, I grab a few pieces of ‘Angelina’ and toss them up on the roof. They’ve rooted and spread, parading up there as forest-green patches.Sedum Angelina on green roof

I periodically get more serious with ‘Angelina’. “More serious” means filling some cell-type seedling flats with a “soil” of equal parts moist peat and perlite, and poking inch-long pieces of leafy ‘Angelina’ stems into the mix. After a winter in the greenhouse or a sunny window, those cuttings are rooted enough to plug into holes I dibble into the soil on the roof among the hens-and-chicks. The roof is a little more than 100 square feet. Each plant could potentially fill up a square foot in a couple of seasons, so 100 cuttings of this plant would do the trick and take up only a couple of square feet of space in their holding cells.Propagating Angelina plants

Another step to making the rooftop greener is to beef up the “soil.” The soil is actually a mix of equal parts peat and calcined montmorillonite clay (a.k.a. kitty litter, unused). The mix is heavy enough not to blow away, and the peat is relatively resistant to decomposition. Some shovelfuls of this mix periodically tossed up on the roof replace what’s washed away or settled.

The mix is lean in nutrients so, come spring, I could also beef up the rooftop with some fertilizer. Not much, though, because succulents are light feeders and too much fertility would encourage weeds.

Is Green Better Than . . . ?

I don’t get it. Green roofs are so “in” these days, for their green appearance and for their environmental green-ness. Sure, green roofs insulate rooms below from heat and cold. And green roofs capture and evaporate some rainwater rather than let it run down gutter pipes and into sewers or streams. The air above green roofs stays cooler than that above conventional roofs, so heat islands aren’t created.

But are the above good enough reasons to put plants on a roof? After all, good insulation also insulates, a lot better than soil and with a lot less weight. And how much water could a roof of succulent plants — plants known for their low water usage — evaporate?

Much as I love plants, I’d rather see solar panels on roofs. My green roof is for looks (and not sunny enough for solar panels).

And A Bow To The Weeds

My green roof is a testimonial to the tenacity of plants. Despite the leanness of the soil mix and its being initially pretty much weed-free, some weeds have colonized the roof. And they survive, despite the harsh growing conditions up there.Weeds on green roof

The weeds that came in weren’t succulents, but grasses and perennials such as foxtail grass and goldenrod. Every time I look up at the roof, I am awed at how these and other plants not only got there, but how they manage to survive there year after year.

Weeding up there would seem such a travesty — and be very difficult.



My Nanking cherries (Prunus tomentosa) made a lot of people happy this year. Joy was first spread in early April as thousands of pinkish white flowers burst open along the stems, enough to almost completely hide the stems. Passers-by enjoyed the hedge of plants, which run along the driveway; some people even asked about the name of the plant.
In early June, the blossoms morphed into small, red cherries, oodles and oodles of them. Now, the end of June as I write, just a few cherries still cling to the stems. Throughout the month of June, though, friends, strangers, relatives, birds, chipmunks, and creatures unseen feasted on the abundance.
Nanking cherries are admittedly small and somewhat hard to harvest because they cling closely to the stems on short stalks, but these two deficiencies are far offset by the care the plants need. Almost none! Every few years, I whack back some stems that become decrepit or send the plant high or wide out of bounds.. And I usually spread wood chips or leaves beneath the bushes as mulch to suppress weeds and conserve soil moisture. But neither of these minor tasks is absolutely necessary.
Perhaps the most amazing thing about Nanking cherries is that there are no improved varieties. That is, they are all random seedlings, each a genetic individual. Yet they all taste good, ranging in flavor and texture, depending on the individual, from almost sweet to sour cherry. (Compare this to wild apples, which pretty much all taste bad; the good-tasting apples that we have are the result of hundreds of years of selection and breeding.) Nanking cherry fruits are small, as are the wild cherries from which cultivated cherries are derived.
One of my bushes yields cherries that are slightly larger, slightly firmer and, hence, easier to harvest, and slightly sweeter than my other 10 or so bushes. Seeds from that bush didn’t get spit on the ground; I collected them for planting. Nanking cherry bushes bear fruit within a couple of years and repeated selection of plants bearing the best fruits could result in bigger and better fruits. Improvements might also come from widening the genetic input with pollen from a wider range of Nanking cherry individuals and even some related species, such as sweet cherry. The combination of Nanking cherry’s tolerance to winter cold, late spring frosts, and insect and disease pests and sweet cherry’s fruit size, sweeter flavor, and firmness would make a plant that was easy to grow with even tastier fruits.
I’ll report back in years to come. For now, run-of-the-mill Nanking cherry is well worth growing and another perfect fruit for ambulant consumption on the way to the front door or to the mailbox at the end of the driveway.
I have a good excuse for the weediness of one of my gardens: It’s on a roof, not an area I frequently walk past with the opportunity to pull a few weeds. I would have to get a ladder and, because this garden is not to be walked on,  reach in as far as possible. I don’t weed it.
This garden is a “green roof” covering a front porch. Green roofs absorb rainfall and the sun’s heat, insulate whatever is below, and look — well — green and alive. The last reason prompted my roof planting about ten years ago.
Original hens-and-chicks laid on roof
But first I had to build the porch roof. Construction was standard — oak posts and crossbeam with 2 by 8 joists covered by 1 by 6 planks — except for the covering of rubber roofing bonded to copper flashing provided with weep holes at the lower end. Planting was begun the year before with hens and chicks (Sempervivum spp.) in seedling flats filled with a mix of equal parts peat moss and calcined montmorillonite clay (the latter also known as “kitty litter). That spring I snuggled the flats next to each other on the roof. Setting flats on the roof intact would, and did, prevent rainfall from washing the planting mix and plants down the slope, the angle of which was determined mostly by aesthetics. I wanted the top of the roof just visible from the driveway.
The goal was for the hens and chicks to make more chicks, and those chicks to make even more chicks, spreading to make a dense, blue-green mat over the surface and draping over the lower eave. They didn’t spread thoroughly or fast enough.
Angelina now filling in the roof
It takes a tough plant to survive and grow on this roof. The soil mix is only a couple of inches deep so plant roots are exposed to the full brunt of winter cold and summer heat, and the roof gets only natural rainfall. Because hens and chicks weren’t fully up to the task, I started planting other succulents to fill in bare areas amongst the hens and chicks.
Over the years, the most successful of these plants has been Sedum rupestre ‘Angelina’. Not only does ‘Angelina’ survive and grow under the austere conditions, but she also looks pretty year ‘round. Right now, the plant is a trailing mat of fleshy, pointed, pale green leaves up through which push foot-high shoots capped with clusters of yellow flowers. In fall and winter, the leaves take on an amber hue. The plants root very easily to furnish new ones to fill in the few remaining bare spots. I pluck off pieces of shoots and toss them back onto the roof to eventually root and spread. Very convenient. 
It’s hard to imagine how weeds have gotten onto my green roof, let alone survive. Birds and wind, no doubt, got the plants there. The weeds include fleabanes and some grasses. Some people tell me that these “weeds” look pretty up on the roof, so maybe they’re not weeds. The fleabanes, now in bloom, hold their white, daisy-like flowers high above those of ‘Angelina’. And, if nothing else, both weeds . . . whoops, I mean plants . . . hold the soil in place as ‘Angelina’ continues to spread.