Friday, December 10, 2010

I’m looking up at my green roof, and it’s not green enough. Literally. I had expected that by now, 8 years after being planted, the roof would be solid green. It’s not.

The green color of this roof is from plants. Conditions up on the roof are pretty rigorous so the plants I chose for it 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 he

ated 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 to the point of excess plants spilling 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.


I’m taking two steps to green up the roof.

The first step is to introduce another plant. A 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, I have dropped pieces of ‘Angelina’ onto the green roof; they’ve rooted and spread and parade there as forest-green patches.

Now I’m getting more serious with ‘Angelina’. Today I filled some cell-type seedling flats with a “soil” of equal parts moist peat and perlite, and poked inch-long pieces of leafy ‘Angelina’ stems into the mix. After a winter in the greenhouse or a sunny window, those cuttings will be rooted enough to plug into holes I’ll dibble into the soil on the roof among the hens-and-chicks. The roof is a little more than 100 square feet. Each plant should fill up a square foot in a couple of seasons, so I need 100 plants cuttings which take up only a couple of square feet.

Step two to rooftop greenery will be beefing 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 has enough mineral matter and recalcitrant organic matter so that little of it decomposes into thin air. Some shovelfuls of this mix tossed up on the roof will replace what’s washed away or settled. The mix is lean in nutrients so, come spring, I’ll also beef up the rooftop with some fertilizer.


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.

Are these 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).


My green roof is a testimonial to the tenacity of plants. Despite a “soil” that started out free of weed seeds, weeds have colonized the roof. And they survive, despite the rigorous growing conditions up there.

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 that these and other plants not only arrived and grow there, but survive year after year. Weeding up there would seem such a travesty -- and be very difficult.


  1. Pretty cats. Are they doing that litterbox covering-up thing? If so, that might be part of the problem as well.

  2. No, the cats are new and the planting mix isn't loose enough for "littering." They're just playing.

  3. Hey Lee, have you thought of using lava rock to mulch the green roof with? It is porous so not much weight, holds some additional moisture to be released later along with a tiny amount of nutrients, as well as keeping the weeds from germinating. It might be what it takes to put your green roof over the top into complete success - good luck!