Planting extensive green roofs
Green space design covers many very different fields, and carrying them out successfully requires the designer to have thorough, up-to-date knowledge. This article offers reliable guidance on designing extensive green roofs.
When designing a green roof, great care is needed both in choosing the technical substructure and in putting together the horticultural layers above it. Where less common or novel solutions are used, the available literature can provide reassurance (although the reliability of much of the information found online is questionable). Of the main types of green roof (extensive, semi-intensive and intensive), it is extensive vegetation that demands the greatest care at the planting design stage, as the growing medium is only between 8 and 15 cm deep. Thanks to its light (max. 150 kg/m²) and thin build-up, this solution can be used in many places, and designers and developers also favour it because its maintenance needs are extremely low (2–3 visits a year), while the benefits of a green roof are already felt with such a thin build-up (water retention, moderating temperature fluctuations, protecting the waterproofing, etc.).
Only plants with good drought and frost tolerance can withstand the conditions on an extensive green roof, and within this narrow group we favour those that spread well but not aggressively. Taking these criteria into account, the list of plants that can be specified narrows down to stonecrops, houseleeks and other xerophytic perennials. Given Hungary’s weather and the experience of recent years, only Sedum and Sempervivum species develop steadily under fully extensive (unirrigated) conditions. Other drought-tolerant perennials do not survive longer dry spells or winter frosts. If required, these plants can also be kept going with 300–400 mm of supplementary irrigation a year, but this raises both installation and maintenance costs.
A special type of extensive green roof also deserves a mention, as it can be an excellent way of blending a building into the landscape. Annual and biennial wildflowers and perennials are sown together; as a consequence, once the spring and summer flowering is over, the low proportion of evergreen plants makes for a more modest picture in autumn and winter (any mowing should only be done after the plants have set seed). Without periodic irrigation, this extensive type struggles to cope with Hungarian conditions.
Once we have chosen the plant mix that best suits the setting or building, we need to decide how and when to plant it (April or October is ideal). The following methods are available:
- spreading cuttings (scattering shoots of 4–8 sedum species; watering is recommended until they have rooted)
- seeding (for wildflower roofs, with periodic watering in hot, dry summers)
- seeding and spreading cuttings can be combined
- hydroseeding (wind and birds do not affect how evenly the seed is sown, and it creates an ideal microclimate for the plants while they root)
- planting pre-grown plug or pot-grown plants (the more developed the plant, the harder it is to acclimatise and the more slowly it grows, so aftercare is needed until full cover is achieved)
- laying ready-made vegetation mats (4–8 different sedums pre-grown on a carrier mat; initial aftercare is needed)
Plants and planting method have to be chosen in the light of the site conditions and the envisaged end result. The client should be made aware that each planting method comes with its own establishment period (until full cover is achieved), and trying to rush it can cause problems in the longer term (weeds, frost sensitivity, clogging). Even within one city there can be areas with markedly different mesoclimates, let alone across an entire country, which is another reason why a mix of several plant species is recommended, leaving it to nature to decide which of the 4–8 species become dominant. With monoculture (pattern-based) plantings, the different growth rates of the plants mean that regular corrective work is needed on top of basic maintenance (feeding, weeding). Sempervivums are best used where their fine structure can be appreciated close up (they spread extremely slowly, so a high number of plants per square metre is needed).
A drainage board with good water retention and drainage capacity, together with filter and protection fleeces of suitable permeability, is at least as important as the right choice of plants. I would particularly stress the importance of the special growing medium, which is the ‘soul’ of the green roof and can under no circumstances be replaced with natural soil!
By following the points above, we already go a long way towards creating a well-functioning green roof; the rest comes down to the contractor’s expertise and the care of whoever maintains it. Do not leave an extensive green roof to its own devices after sowing, especially when it is planted in spring or summer. Immediately after sowing (except with hydroseeding), the cuttings or seeds must be watered in, and during longer hot, dry spells it is also advisable to apply 15–20 mm/m² of water weekly until the plants have rooted. Feeding should be provided once a year with a slow-release (9-month) fertiliser, so as not to increase the organic matter content, which also helps to keep weeds in check. In the first two growing seasons, maintenance is mainly a matter of aftercare (periodic watering, removing invasive species); once 80–90% cover has been reached, the most important task becomes keeping the safety strips and outlets clear, alongside the annual feed and 2–3 rounds of weeding. With mixed sowing, local climatic conditions make every roof unique compared with the initial planting ratios: some species retreat, some may even disappear, while others grow stronger and become dominant.
Sedums and houseleeks recommended for green roofs
They only need watering at the time of planting.
| Scientific name | Common name |
|---|---|
| Sedum album ’Micranthum’ | white stonecrop |
| Sedum album ’Murale’ | white stonecrop |
| Sedum album ’Roseum’ | white stonecrop |
| Sedum kamtschaticum | Kamchatka stonecrop |
| Sedum ellacombianum | Sedum ellacombianum |
| Sedum floriferum ’W. Gold’ | Sedum floriferum |
| Sedum hispanicum | Spanish stonecrop |
| Sedum hybridum ’Immergrünchen’ | hybrid stonecrop |
| Sedum hybridum ’Czar’s Gold’ | hybrid stonecrop |
| Sedum kurilense | Sedum kurilense |
| Sedum laconicum | stonecrop |
| Sedum oreganum | Oregon stonecrop |
| Sedum pachyclados | Sedum pachyclados |
| Sedum pulchellum ’Sea Star’ | widow’s cross |
| Sedum sediforme | pale stonecrop |
| Sedum sediforme ’Silver’ | pale stonecrop |
| Sedum sexangulare | tasteless stonecrop |
| Sedum spurium ’Coccineum’ | Caucasian stonecrop |
| Sedum spurium ’Purpurteppich’ | Caucasian stonecrop |
| Sedum spurium ’Tricolor’ | Caucasian stonecrop |
| Sedum rupestre | reflexed stonecrop |
| Sedum spectabile ’Brillant’ | ice plant |
| Sedum ’Matrona’ | stonecrop |
| Sedum ’Herbstfreude’ | Autumn Joy stonecrop |
| Sempervivum hybrid cultivars* | green, yellow and burgundy houseleeks |
* Houseleeks are protected plants in Hungary and may only be obtained from certified horticultural sources.
Meadow annuals, biennials and perennials recommended for flowering green roofs
With periodic irrigation.
| Scientific name | Common name |
|---|---|
| Allium schoenoprasum | chives |
| Festuca rubra | red fescue |
| Festuca ovina | sheep’s fescue |
| Lolium perenne | perennial ryegrass |
| Poa pratensis | smooth meadow-grass |
| Achillea millefolium | yarrow |
| Anthemis tinctoria | dyer’s chamomile |
| Campanula rotundifolia | harebell |
| Dianthus carthusianorum | Carthusian pink |
| Hieracium pilosella | mouse-ear hawkweed |
| Chrysanthemum vulgare | tansy |
| Linaria vulgaris | common toadflax |
| Salvia pratensis | meadow clary |
| Silene vulgaris | bladder campion |
| Petrorhagia saxifraga | tunic flower |
| Prunella vulgaris | selfheal |
| Papaver rhoeas | common poppy |
| Verbascum phoeniceum | purple mullein |
Perennials recommended for green roofs
With periodic irrigation.
| Scientific name | Common name |
|---|---|
| Alyssum saxatile | gold dust |
| Antennaria dioica | mountain everlasting |
| Armeria maritima | thrift |
| Cerastium tomentosum | snow-in-summer |
| Festuca gautieri | bearskin fescue |
| Festuca pallens | Festuca pallens |
| Delosperma cooperi | Cooper’s ice plant |
| Delosperma nubigenum | yellow ice plant |
| Dianthus plumarius | common pink |
| Euphorbia myrsinites | myrtle spurge |
| Geranium sanguineum | bloody crane’s-bill |
| Lavandula angustifolia | English lavender |
| Origanum vulgare | oregano |