Sunday, January 30, 2011

Extreme Lightweight, Nature Irrigated, Native Species Green Roofs with Polypodium polypodioides

Humankind many times has had the tendency to over-engineer, over-design and improve upon what Nature has long before already perfected.

Eons before we humans came up with idea of putting plants on roofs to attract wildlife and cool and insulate our abodes and structures, Nature was already putting plants on roofs for the same and many more purposes.

Not to be stymied by the lack of structural support many of our roofs offer, Nature creates some of the lightest weight, nature irrigated native species green roofs ever - and Polypodium polypodioides is one example.

One of the most oft-heard concerns is green roof weight.  Many intensive green roofs can weigh over fifty pounds per square foot (ten Kg/Sq.M) when saturated with rainwater.  Extensive roofs, too can be quite heavy.  Some extensive green roofs, though much thinner with respect to soils depth range from fifteen to thirty pounds (three to six Kg per square meter).

Polypodium polypodioides

Polypodium polypodioides

A natural Polypodium polypodioides covered roof system can weigh less than three to four pounds per square foot (less than one Kg per sq. meter)!

Polypodium polypodioides possess photosynthetic metabolisms and so CO2 is sequestered and O2 is produced.

Polypodium polypodioides provides much needed native forage and communal habitat for many native wildlife species.

Polypodium polypodioides is beautiful, aesthetic and visually attractive (beautiful photo of resurrection fern here).

Polypodium polypodioides

Polypodium polypodioides loves to absorb stormwater, removing contaminants and helping keep waterways, rivers and lakes clean.

The adaptive and interesting root architecture of Polypodium spp. is discussed in a recent blog post here.

In fact, Resurrection fern,  Polypodium polypodioides, was the first fern in space - going up on a 1997 Space Shuttle Mission to see if the roots would absorb water in a space capsule!

Polydodium Green Roof

As with any green roof plant, Polypodium has certain considerations to be taken into account during green roof design.  As with most plants, during long periods of drought the plant appears brown and dry.  Proper integration with other species, such as evergreen CAM plants is an easy solution.

Any dry organic matter may also present a fire issue.  Tall, prairie-type grasses used on green roofs produce much more dry-leaf matter than Polypodium.  Again, proper integration with other species, such as evergreen CAM plants is an easy solution.

Perspective of native plants is one where we can either;
  • look at a native plant, consider it a weed and find problems with it, or
  • look at a native plant, consider biodiversity and wildlife value and listen to Nature.
One of my favorite follows on Twitter is @jardsustentable - a wonderful perspective about listening to plants and working with Nature rather than trying to mechanically change Nature.  Check out JardSustentable's tweets here! 

Following JardSustentable's Twitter advice about working with Nature is an excellent approach to take when designing a natural irrigated and native species green roof.

Look up into trees.  Look up at the roofs of buildings.  Look up into rain gutters.  Watch what plants grow in rock out-croppings.  Examine highway bridges, building walls and bulkheads.  Look to Nature to see what she recommends for local plants for growing vertically.

Look Up for Potential Green Roof Native Species (Polypodium in Juniperus spp.)

So consider integrating resurrection fern into the next appropriate green roof project.  With an over weight of less than one Kg per meter square, the plant can provide a lush tropically green look with no additional irrigation - though a recycled-rainwater micro-irrigation system would work wonderfully also!

As always, comments and emails are appreciated and happy Green Roofing!


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