Gardening: Progression of roof garden science

Native plant best able to survive crazy weather up on the roof

Nodding onion can provide some tasty harvesting on the roof.

Nodding onion can provide some tasty harvesting on the roof.

I recently attended a green roof certification seminar at NATS Nursery in Langley and was impressed by how far this plant science has progressed over the past 20 years.

It brought back memories of my first rooftop garden installation back in 1986, which I thought was quite cutting edge at the time.

After installing a waterproof pigment and irrigation, we brought in bags of Styrofoam chips to serve as a lightweight drainage medium (in lieu of gravel or drain rock). By the time we had finished raking it out, we looked like snowmen because the static electricity caused it to cling all over.

Today, we have much more practical pre-grown modular trays with built-in drainage, engineered soil and select plant species that have been tested for durability.

The LiveRoof system also incorporates temporary soil lifts (which are later removed) and moisture portals, so that the trays lock in from side to side and the soil from each module connects with the one next to it. This allows the plants from different trays to grow into each other, so that you have a continual garden instead of just isolated modules.

During the nursery tour afterwards, we went out to the fields, where the green roof trays (which are produced on a custom basis) and plant plugs are grown. It was here that the multicoloured blocks of various groundcover sedums really impressed with their divergent hues. Whether it was the powder blue foliage of sedum reflexum ‘Blue Magic’, the brilliant crimson rosettes of sedum spathulifolium ‘carnea’, the fine gold carpet of sedum acre ‘aureum’ or the eye-catching olive green of sedum album ‘green ice’ – it was a virtual treasure trove of foliage colour.

Then there were the custom-mixed green roof trays, many of which were an unpredictable mélange (different cultivars will dominate one part of the tray) of the above species, and I have to admit that after seeing these, I was rather envious of the future owner’s roofline view. I left with an understanding that the tough, somewhat invasive nature of these sedum species and cultivars is what makes them such good candidates for green roof gardens – where they can choke out weeds, tolerate drought, endure harsh wind exposures and thrive on rather lean soils.

Of course, not every roofline sits in full sun, and so NATS Nursery has developed tray blends that thrive in the shade or filtered light, calling it the Wild West Shade Mix. It includes such native plants as aquilegia formosa (western columbine), maianthemum dilatatum (false lily-of-the-valley), blechnum spicant (deer fern), dicentra formosa (Pacific bleeding heart), polystichum munitum (sword fern) and fritillaria lanceolata (chocolate lily).

There is also a sun version of this native roof garden blend that can include achillea millefolium (yarrow), eriophyllum lanatum (woolly sunflower), castilleja miniata (indian paintbrush), fragaria vesca (wild strawberry), allium cernuum (nodding onion), camassia quamash (camas lily) – as well as  various species of sisyrinchium, solidago (goldenrod), fescue grass and penstemon.

The reason these native plants seem to work so well in green roofs is that they have adapted to our local weather patterns over thousands of years and are acclimated to our wet springs, often dry summers (when many of the bulbous perennials go dormant) and unpredictable winters.

 

Mike Lascelle is a local nursery manager and gardening author (hebe_acer@hotmail.com).