The Little Green Roof: a year later

Things didn’t quite go as planned. Nothing bad happened, per se, but this is not the green roof I was expecting. If you recall- my goal was to try to find some native plants to grow on a green roof. I selected species I thought would do well- those that were easy to grow, were diverse, would tolerate hot and dry conditions, and had a low rooting depth and some that would spread via rhizomes to hold the soil. I wanted this green roof to really fill in and be lush and interesting.

Below is a picture from last summer showing the diverse species.

Here was my list of species with the rationale:

Prairie June grass (Koeleria macrantha), bunchgrass, prolific seeder, deepest root depth of the group
Idaho fescue (Festuca idahoensis), bunchgrass, prolific seeder, low rooting depth
Yarrow (Achillea millefolium), forms a mat and spreads with rhizomes, tallest plant (when in flower)
Rosy pussytoes (Anntenaria rosea) - forms a mat and spreads, their light foliage reflects sun
Cutleaf daisy (Erigeron compositus) - these form a low mat and flower for a long time (starting in April)
Lanceleaf stonecrop (Sedum lanceolatum). I used sedum because it is a requirement on any green roof. It is mandatory. And they enforce this. A nice succulent, though.
Elkhorn clarkia (Clarkia pulchella) this is for color- they are annuals, with very shallow roots, but they readily reseed

Here are the results:
All the lanceleaf stonecrop  survived and only one prairie junegrass survived- everything else died. Furthermore, perhaps to add insult to injury, the stonecrop reseeded like all get-out. The yarrow that was there last year seeded, and now there is a ton of yarrow, but no survivorship.

The few clarkia I planted went to seed and are filling in exceptionally well.

So, why did this happen? I have no idea. The sedum survival was no surprise, but the amount of volunteers was a surprise. They essentially carpeted the whole roof.

It's possible the prairie Junegrass needs more rooting depth- for whatever reason. This one was kind of a stretch- it had the greatest rooting depth of the bunch (a little native grass pun). The Idaho fescue was surprising and so were the pussytoes and cutleaf  daisy-m both have a super low rooting depth. In the wild you find them growing on a rock, and they are happy with it.

Perhaps rooting depth/ rooting media depth was not the issue with some of the failures. For example, with yarrow, maybe there were too many organics in the soil, so it just flowered its head off and died last year.

The roof is actually quite beautiful, and a lot more colorful and interesting than the species composition I had envisioned. There is nothing wrong with the roof- it is just not what I expected and I don’t know why. I need another year of data, so I’ll keep you posted.