The Green Roof – Phase 3

NOTE: THIS IS A FOLLOW-UP POST TO
RENOVATING THE GREEN ROOF (2023)
(you might like to read it first)

My Green Roof was created in 2010 and extensively replanted in 2016. By 2023 the more thuggish plants were taking over, while others were dying out. Worse, weeds had become uncontrollable. It was then (2023) that I undertook a complete renovation, including replacing the timber parts of the green roof with rot-proof equivalents, as well as redesigned the planting area, replacing open soil and gravel (weed traps) with bare weed fabric. The aim was to create a planting that was far lower maintenance: mainly shrubs, with a minimum of exposed soil where weeds might germinate. 18 months later, I can confirm that the approach is working. The young shrubs are growing well, and I have settled on a mainly shrub-based planting. Below is a planting list, starting with plants at the end closest to my house, working back to the northern end which is furthest away:

Planting Key

1. Shrubby veronica Hebe ‘Emerald gem’ (3 off)
2. Whipcord hebe Hebe ochracea ‘James Stirling’ (3 off)
3. Japanese sedge Carex oshimensis ‘Evergold’ (4 off)
4. Cornish heath Erica vagans ‘Yellow John’ (3 off)
5. Rockrose, Helianthemum ‘Henfield Brilliant’ (2 off)
6. Blue leadwood, Ceratostigma plumbaginoides (3 off)
7. Lydian broom, Genista ‘Lydia’ (2 off)
8. Japanese barberry, Berberis thunbergii ‘Atropurpurea Nana’ (3 off)
9. Shrubby cinquefoil Potentilla fruticosa ‘Sunset’ (3 off)

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Hebe ‘Emerald gem’

A small, dense evergreen shrub that forms a neat dome to 1ft+ (30-45cm) across. I added these recently as the planting was looking too yellow. The adjacent paving stones help hold moisture to give them a cool root run. I’m concerned they might resent cold winter winds as it is very exposed up here… We shall see.

Hebe ochracea ‘James Stirling‘

A very dense, slow-growing evergreen shrub that forms a conifer-like mound to 15in (40cm) across. Fully hardy and more wind tolerant than most hebes. ‘James Stirling’ is a peculiar colour: an ochre brown, espresso-brown, golden-yellow… as I say, peculiar.

Although it does produce small white flowers, ornamentally, these are fairly insignificant. The beautiful texture and (did I mention?) peculiar colour are its main attractions.

Stepping Stones

In several places across the new roof planting I have used small paving stones, no more than 1ft (30cm) wide. They serve two purposes. First, they help keep the double-layer polypropylene weed fabric from blowing away in storms.

Their second function is to help trap moisture, creating cool root runs for adjacent shrubs. This seems to work, and along with my hose-pipe and sprinkler, I have so far had no plant losses despite this year’s hot dry spring.

One downside is that weeds and moss tend to grow along the edges of the slabs, their roots seeking out the moist shade. They are easy to remove, however, as the slabs can simply be lifted and the weeds wiped away. Compared to nightmare weed seeding that happened in my previous gravel mulches, this is so much less work.

Carex oshimensis ‘Evergold’

When I refurbished the green roof in 2023, the only plant I kept was a large clump of Japanese sedge which had survived since the original planting in 2010.

This evergreen sedge is surprisingly tough and drought tolerant. Last year I planted three small plants (9cm pots) which have already formed healthy clumps 1ft (30cm) across.

It is the only perennial (non-shrub) I have allowed on the roof, as it imparts a more naturalistic quality than shrubs alone.

Erica vagans ‘Yellow John’

The Cornish heath is a lime tolerant heather, so easier to grow in average soil. Once established it should be drought tolerant, but I did let it dry out a bit too much, which has caused my three plants to put on less growth than I had hoped. Eventual spread 18in (45cm)

Helianthemum ‘Henfield Brilliant’

I had two clumps of this beautiful sub-shrub growing on my green roof for many years, so decided to use it again in the new scheme.

The pretty orange-red flowers do, indeed, look brilliant throughout May and early June. The blue-grey foliage forms low spreading mounds over 2ft (60cm) across, but only half as high.

Burn a hole

I use a kitchen butane torch to burn holes through the twin layer of weed fabric. This seals the edges and prevents fraying. I then dig out a small hole in the growing medium just big enough for the root ball

Weed Collar

Most of the plants I have chosen emerge from the ground on a single stem. To stop weeds growing in the bare soil of the planting hole, I make a collar from an off-cut of the weed fabric, then position this on the soil, around the stem, tucking the edges under the main weed mat. This significantly reduces opportunity for weeds to get established.

Ceratostigma plumbaginoides

There are two common leadworts sold in garden centres. C. willmottianum, which forms a medium shrub (over 60cm/2ft tall — too big for this roof), and Ceratostigma plumbaginoides, which is much more suitable size at 12-16in (30-40cm), and has a woody, spreading herbaceous habit.

Most of the plants on my green-roof are spring and early summer flowering. However, blue leadwood flowers much later (August-October), and produces plenty of beautiful blue flowers. Its autumn foliage is a deep red too, which gives it a double role. The main downside is that its foliage emerges quite late in spring, and it needs some open soil, so is more prone to harbouring weeds.

Genista ‘Lydia’

A very beautiful shrub with thin stems in an overlapping herringbone arrangement, bearing tiny leaves. In May, it is covered in thousands of bright yellow pea flowers, creating a dazzling spectacle. Mine have grown quite quickly, but hopefully will not exceed their expected maximum size of 1.5ft x 3ft (45 x 90cm)

Berberis thunbergii ‘Atropurpurea Nana’

A slow growing and compact form of the purple berberis. I grew this many years ago as a dwarf hedge at the edge of a Victorian front garden. Part of that hedge remained when I checked thirty years later.

In this roof planting, I wanted some dark foliage to offset the preponderance of yellow. First I tried Euphorbia ‘Miner’s Merlot’, but their herbaceous structure jarred in an otherwise shrubby design. Online, I found it hard to track down the dwarf form of Berberis thunbergii ‘Atropurpurea’ — the ‘nana’ bit — at a reasonable cost. By chance, I found a few plants in B&Q very cheap. I am only planting very small plants (9cm to 1L containers) so that the holes I have to make through the mats can be kept as small as possible.

Potentilla fruticosa ‘Sunset’

I have planted three of these shrubs as a backdrop at the back of my green roof. They have grown quickly and almost formed a hedge. According to the RHS, they grow up to 1.5-3.5ft (0.5-1m) in 5 to 10 years. Mine are over half way there in 18 months!

The longest flowering shrub in this list with orange-yellow flowers from May to September. Deciduous and easy to prune to size. So far, they appear extremely happy with their unconventional planting location.

Sprinkler

To keep the roof irrigated (especially in the years while the plants are getting established) I use a single lawn sprinkler in the centre of the roof, connected to a hose. Both sprinkler and hose remain in place all year round. I simply plug it in to my main garden hose using a double ended connector. When unplugged, it readily drains down, protecting it from frost damage in the winter.

As you can see, I have wrapped electricians tape around the sprinklers outer jet holes, restricting the water jets to just the middle section. This has two effects: firstly, it increases the pressure of each jet, so they can easily reach the far ends of the roof. Secondly, by narrowing the spray pattern there is little overspill beyond the width of my roof, conserving water.

In summer, I put the sprinkler on for 10-20 minutes once every three-four days in the hottest, driest weather. I skip this if we have heavy rain. More difficult to judge are damp days which may be moist, but actually have too little rainfall. I leave a plant-pot tray out so i can estimate how much rain has fallen. Watering in damp-grey weather is sometimes necessary, although counterintuitive. The other unexpected drying factor is wind, which can dramatically increase watering needs, even if there is little sun.

Overall, this simple watering system has helped my new planting to establish very quickly.

How it looked in 2003 prior to renovation and replanting

The aim of the renovation in 2003, and the replanting in 2004/5 was to create a significantly lower-maintenance, yet ornamental green-roof. Although it is still early days, I think I have succeeded. Weeds are far fewer and easier to remove. The planting requires almost no attention other than watering, and the sprinkler set-up makes this a breeze. The shrubs provide a wide range of textures, colours and flowering crescendos throughout the year.

Hopefully this planting will continue to fill out without too many problems. I expect some judicious pruning will be needed, to keep the balance. Another issue will be if too many dead leaves and spent flowers accumulate under the canopy of the shrubs, as this will become a new soil for weeds. My plan is to use a workshop vacuum to clean out under each shrub once per year, probably in late autumn. Stay posted, and I’ll let you know how it all goes.

How it looks now – June 2025

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