The Green Roof: lessons from the Last Seven years

This was how my green roof garden looked in 2022.
The planting was getting tired and difficult to maintain.
I knew it was time to replant it… But what should I change?

Back in 2015, I wrote a similar post to this one, “The Green Roof: Lessons from the first five years” about the successes and failures of my roof planting since its creation in 2010. Mistakes were made — including the introduction of vigorous self-seeding grasses, along with neglecting it over a hot summer which killed off most of the more showy ornamentals and left it looking like an unkempt verge. Taking stick, I set about overhauling the roof in 2016 with the aim of correcting those failings. Many of the plants were removed, and a weed proof membrane was added to reduce weeds. You can read all about it here: Refurbishing the Green Roof Planting (2016)

It has been seven years since then, and in many ways Phase 2 of the Green Roof has been far more successful. However, since 2021 things have increasingly been getting out of hand again, such that by summer 2023, I realised a serious course-correction was needed. The reasons were different, but at the same time painfully familiar: over-zealous species were going to take over and there were increasing weed problems. I thought I had selected plants more carefully this time round, but I still made some bad choices. In this article I will consider the lessons I’ve learned in Phase 2 and lay out my plans for Phase 3.

My three biggest Planting mistakes

Creeping thyme
Tserpyllum & pseudolanuginosus

Beautiful in flower, and forming a low growing ground cover. It spread quickly over the newly laid gravel mulch. Each year, new shoots grow back and forth, building up a network of fine stems just an inch or two tall. In among this tangled mat, seeds easily germinate, sheltered by the microclimate. When I came to remove it, I even found grape hyacinth bulbs happily growing in it, suspended above the gravel. Shoots and roots of creeping thyme also easily penetrate the gravel, helping retain dust and dirt, quickly turning the gravel mulch into new soil for weed seeds.

Mexican fleabane
Erigeron karvinskianus

At first sight, this appears to be an ideal plant for a roof garden: delicate, fine stems, drought tolerant, flowers all year round. The problem is it is just too well adapted to a green roof environment. It seeds prolifically, and in the second year popped up all over the roof. Hand weeding was very difficult, as they grow everywhere: in the crown of other plants, under shrubs and in every available crack and corner. They are difficult to hand-pull, as the base often remains attached to the ground and regrows. Their seed has also fallen into my garden below, giving me additional weeding problems.

Wall germander
Teucrium chamaedrys

Another plant that on paper looks like it is ideal for a green roof: Drought tolerant, evergreen, pink flowers in summer, loved by bees, just 8in high (20cm) and mat forming. That last characteristic conceals the problem: Wall germander spreads by sending out shoots just below the surface. On the green roof, it found its perfect home, sending out hundreds of shoots under the gravel, popping up metres away in just one season. It was clear that this plant would take over the entire roof if left alone. This plant led to my decision to strip the roof and start again.

Along with these planting woes (and the increasing weeding woes) I realised that some of the timber structure of the green roof was in need of repair. Together, these prompted me to undertake a much more dramatic overhaul and renovation than I did in 2016, which would involve removing practically all of the plants to create a truly blank slate. I will cover those repairs and the replanting (phase 3) in later posts, but here I want to reflect on how phase 2 went.

In my 2015 post, The Green Roof: Lessons from the first five years, I said “Phase two will require a complete overhaul… It will be more garden-like, less wild, but more manageable long-term. I hope.”

Clearly, that didn’t work out, but the sentiment for phase 3 must surely be the same. This time round, however, I’ve hopefully learned my lesson and won’t make the same mistake again. I will not use a gravel mulch, nor grow any plant that spreads by runners, stolons or underground shoots. When I replant this roof it will only have a few, well-spaced plants with lots of bare ground between them for easy weeding.

△ Above, is a sneak-peak of the latest refurbishment (as of Dec 2023). Quite a change, don’t you agree? Not only have all but two of the plants been removed, but the roof edging, planting surround and weed matting have been completely replaced. I’ll replant it in the spring.

— 2017 —

July 2017 (one year after the 2016 revamp)

What stands out to me in this shot are the large sedums (S. spectabile ‘brilliant’) which were strong and healthy in 2017. They had survived here since the original planting, impressing me with their hardiness and vigour. Unfortunately, this year was probably their peak, as they went in decline from this point forward, virtually disappearing by 2023. Not sure why. I suspect vine weevils. But it might also be that they objected to the increased irrigation. Or perhaps they disliked the competition from the weeds that grew at their feet.

A majority of the plants at this time were growing in well-defined clumps with little ground-cover in between them. The gravel is still visible and the weeds few and far between. The magenta Lampranthus roseus (bottom right) was doing well, but would die back over the next few years. The blue grasses look nice, but

If I could freeze time, this is how I’d want the roof to remain. I like how each clump is distinct, with bare gravel between each plant. Adding ground cover plants, such as thyme, to fill in these spaces was a mistake. If there was a bit more space between them, less weeding would be needed, and could potentially be done using a carefully directed herbicide.

However, many of the plants in this scene require bare soil at their base (e.g. the large sedums, ferns and bulbs), which means weeds will always grow in the crowns. This is not the case with plants like sea thrift or the shrubby thymes (‘Silver queen’ and golden Thymus citriodorus) which have a single central stem, so require only a very small planting hole minimising exposed soil.

— 2018 —

Here’s an unusual view of the green roof, taken in May 2018. Just two years after it’s revamp it was looking very tidy with nice distinct healthy clumps of plants.

The green domes in the bottom left are Hebe ‘Emerald Green Globe’ which I think were ideal plants, except they died during a dry period. My mistake. I have since rigged up a sprinkler on a timer for the dry season, so might try them again when I revamp the roof.

The purple patch of thyme looks beautiful at this stage. Little did I know how it would spread so fast and cover all the open space in just four years.

I really liked the clumps of mauve fleabane (Erigeron glauca ‘Sea breeze’) in their early years. But I soon learned that they have some nasty habits. They send out shoots at soil level (in this case, under the root membrane) and pop up several feet away. Inevitably, they emerged where they found a hole in the membrane — typically, right in the centre of another plant.

In the far back-right corner you can see a small green shrub. It’s an Eleagnus that self-sowed on to the green roof. It would go on to get 6ft tall, despite my knocking it back with heavy pruning each year. When I came to remove it in 2023, I found it had 1 inch thick roots spreading out in a 2m radius in the three inches of soil under the weed mat.

— 2019 —

June 2019

Here’s a good view of one of the bad-boy plants. In the centre of the picture, just below the purple fleabane daisies, is a large clump of wall germander (Teucrium chamaedrys — the one with purple pink spikes.) This was planted just a few years ago from a little pot and has grown )and spread) rapidly since. You can see several other smaller clumps in front of it, to the left and down. There are even see some shoots in amongst the Pasqueflowers, to the right and the sedum to the left. The gravel and mulch mat appear to have provided the perfect environment for its rapid colonisation.

I ended up spraying this patch with herbicide in 2002, which was effective on the main clump that was easy to spray, but little of the poison was translocated any distance, so the new shoots that had sprouted in hundreds of locations up to two metres away continued to grow. These were so widely dispersed and mixed in with other plants that it would have been impossible to control them chemically.

— 2020 —

June 2020

By 2020, the large sedums were in significant decline. Several clumps had died out, and the remainder were considerably smaller. The large area of purple and white flowers are creeping thymes: Very beautiful, but they have now spread everywhere. After flowering they are covered in dried dead flowers that make them look untidy.

Although the Thyme only grows 2 inches tall, it displaces small succulent plants such as the trailing ice plant (Lampranthus spectabilis) and many small sedums (e.g. S. spurium ‘Red carpet’, bottom left), probably because its mat of tangled stems increases humidity close to the surface which the succulents dislike. Despite my best efforts to weed them out in previous years, you can see there are many of the pink and white daisies of Mexican fleabane dotted across the scene. These are hard to remove, especially from the dense colonies of fern (Polypodium vulgare, bottom right). Each daisy allowed to go to seed can produce a dozen new plants, so unless every single one is removed, they inevitably increase each year.

— 2021 —

May 2021

Following the pasqueflowers in April, the May display is dominated by the orange-red flowers of (aptly named) Helianthemum ‘Henfield Brilliant’ (top-left), a plant that has been very low maintenance and very reliable. I can see me replanting this after the makeover. Its growth habit is ideal: A sub-shrub that grows from a single stem and spreads relatively tidily above ground with dense silvery foliage. I think it’s a keeper. Note also yellow grassy clump Carex oshimensis ‘Evergold’, towards the back of the roof. This sedge has thrived on my roof since 2006, requiring very little maintenance, and has never self-seeded. Another keeper.

On the other hand…

June 2021

The central ‘alpine meadow’ of my green roof produces beautiful tapestries of interweaving perennials. In the photo above, you can see the kind of effect, which is delightful from May-July. Although I love this kind of ‘natural habitat’ of ornamental plants, like all ecosystems it is transitional. Over time, certain plants will come to dominate and others will be outcompeted. The main problem with the mix above, is that it is an ideal situation for seeds to germinate, protected by the low cover of thyme. The result is nice when you get pretty aquilegias turning up uninvited, but much less welcome when fine-bladed grasses crash the party (You can see them scattered throughout the scene). These grasses — some weed species, some self-sown ornamentals — are almost impossible to remove, and given a few more years they will take over and start producing seed themselves.

Another competitive thug keen on world domination is the wall Germander. The original clump was planted a good 6ft away from this part of the roof, but if you look closely at the top right next to the orange shoots of sedum ‘Angelina’, you can see a dozen bright green shoots of Germander that have spread by runners. They have no interest in stopping and will displace the sedums in another few years. Again, they are almost impossible to selectively remove.

— 2022 —

May 2022 — Starting to look a bit tired

A year later, and the scene roof garden is looking less happy. Some of the sparkle has gone. The planting is increasingly messy with many previously showy alpine plants (Sedums, Euphorbia and trailing ice plant) in retreat. The large perennials (Sedum spectabile and Erigeron ‘Sea Breeze’) are looking tatty and lack vigour. The carpet of thyme, which was a star in its younger days, has lost some of its lustre. Partly by accumulating more dead ‘under wood’ but also from invasive grass seedlings sprouting in its thatch. At the edges, it has stated spilling out of the planting bed and is colonising the gutters. Not good for the drainage.

Adding to the unloved-look, is the deteriorating edging. The white edge timbers are starting to rot and need painting, while the timber border to the planting, that separate the soil from the pebble gutters, is breaking down. Which is why it is time for radical change and refurbishment…

Lessons learned in phase 2

What does this mean going forwards into phase 3 of my green roof? I’m not getting any younger, so a major goal is to reduce maintenance considerably.

  1. Self-seeding ornamental grasses and wild grasses have become a major headache, so I will not have any ornamental grasses on the roof in future.
  2. Self-seeding perennials should also be excluded.
  3. Weeding is difficult when seedlings germinate within the mat of ground cover plants (like thymes and small sedums) and hand-weeding is laborious, so no small alpines and creeping ground-cover plants.
  4. Small shrubs and larger mat-forming perennials that spread out across the surface from a central point (e.g. helianthemum) are easy to maintain. I will use more of these next time.
  5. For similar reasons, gravel mulch will not be used on top of the weed-barrier mat. This may lead to the weed barrier deteriorating quicker due to UK exposure. This can be mitigated by growing shrubs and plants that block light from reaching the mat. Heavy-duty woven polypropylene can last up to 8 years, and I can patch exposed areas with a second layer in a few years time to mitigate against this.
  6. The automatic sprinkler system I set up in 2020 made dry spells easy to cope with. It may have led to the decline of succulents who prefer dryer conditions. As it benefits larger drought-prone shrubs, I will use it again in phase 3.

More About the Green Roof

The Green Roof Construction

  • Sub-deck structure
  • Butyl liner installation
  • Drainage detailing
  • Growing medium & planting
  • Construction drawing

The Workshop construction

Learn how the building under the Green Roof was designed and constructed.


2 thoughts on “The Green Roof: lessons from the Last Seven years”

  1. a very interesting post. thank you very much. i am feeling that you are struggllng a little bit too much against the nature of plants. a normal roof garden should be cultivated a little bit more extensively, because most people do not want to go up to the roof so often. but i agree fully with you, that we can do much more for a pretty roof greening than it is common. i try to find out also step by step, what is going and what not.
    i have started with bulbs https://www.rolandsstaudengarten.de/garten-blog-2023/#Dachgarten.
    .

    Reply
    • Thanks for leaving a comment. Your roof garden is looking splendid! I planted bulbs on mine back in 2010. The crocus died out quite quickly. Iris reticulata did very well, flowering up until 2020, but not multiplying much. The best by a long shot have been grape hyacinth which have increased without becoming invasive. They die back prettily too. I love their seed cases. Iris reticulata has tall leaves that remain all summer and look a bit weedy.

      I don’t think I ‘struggle’ with my roof – I see the whole process as a creative one. I enjoy watching the plant communities change over time. I’ve had many beautiful years. The question is, how to stop ANY green roof reverting to weed infested turf? I don’t think it is possible. My plan now is to grow just a dozen clumps of well-spaced ‘shrubby’ plants and see how that experiment unfolds. Hopefully, we can all learn from each other’s trials and experiences.

      Reply

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