Plant communities on my green roof

Any garden, as it matures, contains a community of plants: a dynamic mix of species of different ages. Some short-term residents come and go in a year or two, crowded out, swamped or unable to thrive. Others find their niche and establish themselves as permanent residents.

I have always been interested in plant communities. I love the way that species of different habits interact in time and space, forming an ever-changing living tapestry.

My green-roof is an ideal opportunity to witness this kind of communal interaction. As a single large patch (3 m x 10 m) with only 3 in (8 cm) of soil and exposed to full sun, plants here experience very strong selective pressures. Which survive, thrive or decline and die is partly in the hands of the gods, although I undertake some judicious weeding to stop it becoming totally wild. The product is a kind of naturalistic meadow, with various micro-habitats each with its own natural history.

Some of the original plants are still present (Sedum spectabile, Polypodium vulgare, Pulsatilla vulgaris, Festuca Glauca, Euphorbia myrsinitese and small bulbs). Some of the thugs have been removed entirely to stop them taking over (Stipa tennuissima) while other more recent introductions have fitted in nicely (creeping thymes, orange helianthemum)

Views along the roof, from each end

These photos taken in June 2020, show the green roof at its peak when viewed from each end. You can see the variety of different plant communities in different places, each with its own ‘feel’.

There is no such thing as a stable plant community, at least not one that remains static year after year. Plants, by their nature, are continually changing, expanding and contracting in size depending on their fortunes, the weather, predators (slugs), self seeding and through competition with adjacent plants. However, there is the possibility of a kind of dynamic stability and my green roof has reached that point: things are still changing, but the changes are slower and more balanced (or at least that is how it seems). The plants have found a kind of equilibrium with each other. Plants that were previously invasive, such as chives (Allium schoenoprasum) that used to seed about uncontrollably, are finding fewer places to germinate so are less of a problem.

The dynamic equilibrium that my green roof has achieved includes layers of plants from diverse groups: sub-shrubs, perennials and annuals; those that spread by stolons, underground roots, bulbs, surface runners and those that depend on seeding. Some have found their own way into the mix from seeds from afar – brought in by birds perhaps – including Aquilegias and even an Eleagnus shrub! Of course it still needs human interaction. I have to weed it several times per year, but the weeds are increasingly benign and like the chives, find fewer sites to colonise.

Spot the species

(Click here to view full size (5 MB) image)

This photo shows a section of the green roof where many different species have formed a beautiful tapestry of interlocking colour and texture. See how many of the following species you can find:

  • Creeping thyme (Thymus serpyllum)
  • Woolly thyme (Thymus pseudolanuginosus)
  • Golden large thyme (Thymus citriodorus ‘Aureus’)
  • Maiden Pink (Dianthus deltoids ‘Flashing Lights’)
  • Thrift (Armeria maritima)
  • Stonecrop (Sedum spurium ‘Dragon’s Blood’)
  • Golden stonecrop (S. rupestre ‘Angelina’)
  • Orpine (Sedum/Hylotelephium telephium)
  • Columbine (Aquilegia vulgaris)
  • Grape hyacinth (Liriope muscarii)
  • Compact marjoram (Origanum vulgare)
  • Fleabane (Erigeron glaucous ‘Sea Breeze’)
  • Common mallow (Malva sylvestris)
  • Pasqueflower (Pulsatilla vulgaris)

Dense carpet of interlocking species

This kind of planting is so rare in the gardening world – you almost never see it in any garden or website. Even naturalistic prairie and alpine gardens keep plants in separate clumps to a large extent.

Some of the plants above are mat-forming and are spreading horizontally. Others have self-seeded (e.g. Dianthus and Thrift). Some are old straggly shrubs (the golden Thyme). Others are clump-forming perennials just hanging on in there (the larger sedums) How this rich tapestry changes year on year is anyone’s guess, but it has reached a kind of dynamic stability for the last couple of years. The newest addition is the sprinkling of red Dianthus deltoids ‘Flashing Lights’ which have sprouted from self-sown seed this.

The Old Log
(June 2020)

At the far (north east) end of the roof a drift-wood log provides a reference point. To the top of the photo a reliable clump of Schmidt’s wormwood grows (Artemisia schmidtiana ‘Nana’, 8yo). In the fork of the log is common polypody fern (Polypodium vulgare, 9yo). The large pink fleabane (Erigeron glaucous ‘Sea Breeze’, 4yo) spreads under the gravel and pops up new clumps here and there. The small white/pink daisies are Mexican fleabane: and all are self-seeded (Erigeron karvinskianus) – a little too successfully, I fear I’ll have to remove them soon. Tucked in at the bottom-right is a clump of sedge (Carex ‘Evergold’, 10yo) which has surprised me by its persistence in this harsh environment.

A fascinating scene
(June 2020)

At the near (south west) end of the roof this scene was in full swing in June. The main carpeting flowers come from two creeping thymes: Thymus pseudolanuginosus (purple) and T. serpyllum ‘Albus’ (white). In the foreground common Polypody shows that not all ferns require damp shade (this one loves dry sun!) In the background a nice red colony of sedum Dragon’s Blood. The best bit, though are the two little islands of interest centre and centre-right. which include the straw-coloured seed pods of grape hyacinths – weirdly interesting. In the central island a small self sown seedling of Myrtle spurge (Euphorbia myrsinites) is just visible, whereas to the right, a pretty clump of succulent sedum is growing. Back-left is a large clump of Pasqueflower foliage. I cut back their seedheads as they self-sow everywhere otherwise!

An unusual meadow
(May 2020)

In this area a strange little meadow of plants has established themselves. The dominant character is that bright yellow leaves of Golden Creeping Jenny (Lysimacha nummularia ‘Aurea’) It has survived on this roof for ten years! Joining it here are little tufts of self-sown Sea Thrift (Armeria maritima) and young seedlings of Mexican fleabane (Erigeron karvinskianus). Various dwarf sedums creep around between them. The grassy foliage is a weed of giant sedge (Carex pendula ). When we moved here there were a couple of Giant sedges in the rough parts of the garden. I removed them over ten years ago, but we continue to get seedlings popping up, even on the roof it seems!

▵ Silver plants
(May 2020)

Right in the middle of this shot is a little colony of Snow in Summer (Cerastium tomentosum) which has survived for at least 5 years, holding its own and giving a bright silver patch of foliage every year. But only a few flowers! To its left some shrubby variegated thyme (Thymus x citriodorus ‘Silver Queen’ ) which has grown old, leggy and beautifully ragged.

▵ Intimate landscapes
(June 2020)

This colourful area includes the purple-flowered Woolly Thyme (Thymus pseudolanuginosus) with tufts of bright orange Golden Stonecrop (Sedum rupestre ‘Angelina’). Look closely and you will see two bumble bees. The little tufts of crimped-edged shoots are from Wall Germander (Teucrium chamaedrys) which is slowly spreading by underground shoots that pop up well over a foot (30cm) away from the main plant. It has pretty lilac-pink flowers in August, but its tenacity threatens to change the look of the roof long term.

Some intimate plant interactions

▵ Mountain everlasting AKA Pussy toes
(June 2020)

Mountain everlasting (Antennaria dioica — variety ‘minima’ perhaps?) is a pretty little alpine which forms tough carpets of tight silver foliage close to the ground – no more than 1 inch (2 cm) tall in this exposed location. In May it puts up 4 in tall (10cm) flowering stems of everlasting flowers.

It has survived for many years, fighting a turf-war for ground space with creeping thyme and sedums. Somehow it has not been swamped, although last year I had to weed out lots of Mexican fleabane seedlings that had self-sown right in the middle of it.

▵ An interesting combination
(June 2020)

This is a very unexpected combination of plants that has become quite comfortably intertwined at the far (North-East) end of the green roof.

In the foreground Golden Thyme (a shrubby variety) is managing to co-exist with the vigorous Erigeron glaucous ‘Sea Breeze’ which spreads underground by runners. There is weed suppressing membrane under the gravel, so it can’t pop up everywhere, giving the Thyme a chance.

Behind is a ten year old colony of common polypody fern (Polypodium vulgare). If you look closely, you can just see a clump of Thrift (Armeria maritima) peeping out to its left. This is one of the longest lived thrifts on the whole roof, despite it being always in the shade of the fern!

▵ Trailing Ice Plant growing through an old variegated Thyme bush
(June 2020)

Lampranthus spectabilis is a hardy succulent that forms colonies in open areas. It usually does not compete well with taller plants. Here, however, it has taken up residence in the open centre of the old, twiggy heart of a Thymus citriodorus ‘Silver Queen’.

These are the kinds of accidental delights I love.

▵ Weeds that refuse to be evicted
(June 2020)

Chives appeared spontaneously on the green-roof within three years of its creation (thanks to birds no doubt). They self-seed prolifically, so I have always looked on them as weeds and I am sure if I had left them they would have ended up dominating the roof. Even so, I quite like the few that get through my ruthless weeding. Now the roof planting has matured, can I let them be? or will they try and take over again?

2 thoughts on “Plant communities on my green roof”

    • It is possible to make a sloping green roof, but you need to find a way to stop the growing medium from washing down. A series of horizontal slats or a grid might work.


Leave a Comment