This is fun, as long as it’s in someone else’s botanic garden... It’s an Azolla filiculoides lawn atop the Ornamental Lake in Melbourne’s Royal Botanic Gardens.
Not many lawns are made of ferns but then not many ferns look like this.
And it’s not often you have to put up signs telling visitors something is not really a lawn, just so they don’t fall into it.
Quite right too. Although to me it looks more like a pub carpet. I’m sure I remember one like this in Naughton’s, the pub nearest to the Botany School at The University of Melbourne (but sadly, I think, no more).
There is a lovely irony in the bottom part of this sign. The lake is full of nutrients such as nitrate and phosphate, entering as run-off from gardens and lawns, or from the rear end of ducks and other wildlife. The Royal Botanic Gardens have built lots of fancy constructions in the lakes to remove nutrients and, as the sign says, the azolla might just help as well.
The azolla plants require various chemicals to create new little azollas, and given the size of the carpet plenty of this has been going on. But each azolla also carries around a little bonus organism called Anabaena (perhaps these days classified as Trichormus or Nostoc), a blue-green algae (or cyanobacterium) that fixes atmospheric nitrogen into nitrate. So azolla probably produces as much of this nutrient as it consumes. Phosphorus is usually what limits growth and I’m sure there is plenty of this in the Ornamental Lake thanks to the ducks. The azolla could well make a dent on that nutrient.
The sign finishes by saying that by removing nutrients and shading the water the azolla lawn/carpet can help reduce the growth of ‘nuisance blue-green algae blooms’. Quite true but the word ‘nuisance’ is critical. This is not only a lawn of floating fern, but a cryptic lawn of blue-green algae – together, a lumpy, reddish-green, nitrate-producing factory.
I do wonder what the nineteenth century Director William Guilfoyle would have thought of all this? Not the algal connection but the lawn growing on his lake. His beautiful landscape design of 1873 was built around sweeping views through to water, not to a carpet of azolla and algae.
Still the azolla is only temporary and will surely fade away in winter. In the meantime, Guilfoyle’s memory is kept alive elsewhere in the botanic garden by a charming redevelopment of a once hidden water reservoir to what is called Guilfoyle’s Volcano. The landscape is based on Guilfoyle’s original designs for this area, inspired by him seeing a volcano in the New Hebrides.
I couldn’t see any obvious blue-green algae in the volcano reservoir but there were a couple of cycads in the plantings and as we all know, they have blue-green algae in their roots.
Images: taken during a short visit to Melbourne last week.