The Green Roof: lessons from the first five years

This is a visual catalogue of how my green-roof developed over its first five years, following its initial planting in Autumn 2009. My intention had always been to establish a low maintenance bio diverse plant community of drought tolerant garden plants. I now know that this is possible, but looks more like a weedy wild meadow than a garden.

This initial experiment was a success in many ways, as the beautiful photos below show, but by mid-2015 it’s ecological direction was clear: self seeding grasses and annual weeds were destined to win. More than half of the plants varieties I had tried to grow became extinct. Several garden varieties I had not planted turned up and ran riot. The whole area was becoming a seed bank that was infecting the surrounding garden. But I had learned some important lessons and had enjoyed its unfolding beauty along the way.

Phase two will require a complete overhaul based on the lessons learned first time round. It will be more garden-like, less wild, but more manageable long-term. I hope. So this seems like a good time to summarise the evolution of the first phase of this unique gardening eco-system.

In a separate post I will report on how individual plants fared. And I plan a third post in which I will focus on some of the naturalistic plant combinations that, to my eye, were particularly beautiful and only possible when plants are allowed to follow their own ecologically inclinations and mingle naturally.

— 2010 —

After their first winter, the new plants are looking fresh in early spring.

Seed heads of pasque flower, pink thrift and sea campion.

— 2011 —

Spring in the second year. The clumps are filling out.

Overview showing most of the roof garden, looking North with a ladder to the back.

View looking South. Hardly any weeds yet.

Notice the open areas like a gravelly ‘path’, and the mounds of  golden thyme (middle-right)

— 2012 —

The golden thyme has spread to form good dense mounds. Carex Evergold doing well.

The open ‘path’ has developed a nice mossy cover. It’s looking more like a natural environment.

This was the roof garden at its best: Many plants, knitted together into a magnificent tapestry, with low weed numbers easy to control. Notice, however, a seedling of ponytail grass, bottom left.

— 2013 —

From a low angle it looks like a meadow. The chives had appeared on their own and would try and take over.

The pink Aquilegia to the back is self sown. Ragworts and dandelions appeared in number too.

— 2014 —

The weeds begin to take over. Some from the more enthusiastic garden plants, others from wild plants brought on the wind or by birds. Weeding is now a major headache. See the dozens of ponytail grass seedlings.

It’s still very attractive, but there are dozens of self sown grasses and chives. And dandelions.

The hazy sea of tiny leaves are wild Dove’s foot Geranium with tiny pink flowers. Black medic weaves its stems through everything in a tangled mat. I’ve lost control: nature has taken over! Perhaps drought will kill off the weeds and the toughest ornamentals will survive?

— SUMMER DROUGHT + NEGLECT WHILE WE BUILT OUR EXTENSION —

Die-back from six weeks of dry summer weather. What survived? The large Sedums are OK…

…so are the Chives and ponytail grass, and a million weed seeds which germinate with the first rain 🙁

— 2015 —

Iris bulbs push up through dead mounds of thrift. Weed seedlings are everywhere. Grape hyacinths soften the devastation. Between the grasses a tangle of weeds is getting ready to riot.
The ponytail grasses are taking over. Their seeds germinate everywhere. If I don’t cut them back they will produce infinite seedlings.

— Summer 2015 —

It’s still beautiful, but increasingly unmanageable. 
Left to it’s own devices the ponytail grasses would make a turf-roof pushing out everything else.

Lessons learned

1. A green roof is a tough environment for plants and there are going to be many casualties. The ones that cope are likely to become invasive if they self-seed.
2. Ornamental grasses may self-seed. Cutting off the seed heads before they ripen every year might keep this under control, but it is onerous. One plkant setting seed can produce hundreds of seedlings.
3. The huge area of soil is a magnet for weeds which once you lose control are almost impossible to get on top of. Again – let a batch set seed and hundreds of seedlings will pop up.
4. Even so, the chaos that sets in can be very picturesque. The unique environment created by shallow, drought-prone soil favours certain kinds of wildflowers that produce a distinctive habitat. This is fine if you want a bio diverse wildlife garden, and don’t mind the inevitable seeds spreading
5. Irrigation in dry periods is essential to maintain most garden perennials. A simple manual or automatic irrigation system would transform the success of desirable plants. Annual weeds are better adapted to summer drought, than most of the plants one wants to grow.
6. Weeds will take over unless dense mound-forming plants are cultivated and planted close enough together to cover the soil in a few years – thrift and golden thyme worked well, whereas the sedums and low-growing perennials did the opposite, providing perfect shelter for seedlings to sow themselves amongst. One solution would be to cover bare soil around and between plants with weed suppressing fabric.
7. Vigorous self-seeders, like chives and pony tail grass should be avoided, unless you want to let them take over and do their own thing. However, they act as an increasing seed bank, self-seeding into the surrounding garden beds, paving and paths profusely.
8. Ecological plantings are only maintenance-free if you don’t mind them getting more and more wild, weedy and seedy.

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• Construction drawing

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