I am trying to select plants for a green roof on Lopez Island. I would like to use some native plants, and not have to do too much weeding. Can you help me?
The Miller Library has a booklist on green roofs which includes weblinks. With respect to weeds, as a former professional gardener I don’t believe there is a “maintenance-free” garden. However you may be able to come up with a freely seeding grass (Festuca ovina var. glauca comes to mind) that would do well and look good with other plants.
Regarding Pacific Northwest native plants (i.e., grasses), I recommend using a native plant book (such as Pojar and MacKinnon’s Plants of the Pacific Northwest Coast) and searching for those which suit the conditions of the site. You may also find King County’s native plant
guide useful, as it is searchable by site requirements. Washington Native Plant Society offers a similar plant selection guide. There are also native species of Sedum.
The book Green Roof–a Case Study by Christian Werthmann (Princeton Architectural Press, 2007) evaluates green roof plants and planting techniques, and includes observations regarding soil depth, the factors affecting desiccation (i.e., metal edging and the planting medium/size of the plants used), and density considerations. Here are some of the conclusions of the case study:
- p.69-insects and other invertebrates need >6″ of soil to survive the cold;
drought tolerance increases with soil depth; probability of seed germination also increases with soil depth - p.76-erosion control (bird repellant?) made from a degradable straw mat (such a mat may be useful for preventing birds from pulling up the plugs)
- Plants:
p.95-sweet fern Comptonia peregrine-not too vigorous, suffered from weed infestations
p.96-prickly pear Opuntia fragilis-did well
Phlox subulata, Silene caroliniana-did well
Sedum telephioides, S. lanceolatum, S. stenopetalum-suffered from heat, drought, and birds-these were replaced with European sedums (S. album, S. reflexum, S. spurium, S. cauticola, S. sexangulare, S. floriferum, Sempervivum tectorum, Orostachys boehmeri) and not planted as plugs but as larger plants that started in the green roof substrate (reducing transplant shock)
p.100-mixed prairie perennials with sedums (in 6″ soil), which filled in when perennials died-the architects note that there is no record of prairie plants doing well on green roofs, therefore the backup with succulents-they also note that metal roof edges speed up desiccationPerennials included ice plant (Delosperma nubigenum; my note: be careful about this, as it’s a noxious weed in California and if it spreads by seed, it might become a problem on Lopez); flameflower (Talinum calycinum); Eragrostis spectabilis; Allium schoenoprasum; Allium cernuum; Bouteloua gracilis; Elymus virginicus; Achillea millefolium; Tradescantia bracteata; Solidago nemoralis; Artemisia ludoviciana; Coreopsis verticillata; Asclepias tuberosa; Rudbeckia hirtap.105-in 3″ soil with nearby metal (and thus an overheating issue), they used proven sedum species; heat from metal can be reduced by using aluminum (proved cooler than wood)
p. 108-conclusions: use a combination of indigenous plants with “successful green roof performers”-thin soil and high light “even overtaxed the hardiest succulents”-soil depth and temperature extremes made a big difference; even a slight difference in soil depth in a harsh environment can mean the difference between success and failure-continuous care and maintenance in the beginning is important for success-plugs grown in peat moss were less successful, since completely dried-out peat moss repels water-dense plantings (and/or rapidly spreading plants) important because bare soil gets hotter than soil covered by a plant.