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Saturday
Dec222012

Tank-scaping the Bamboo Garden

We have been putting some finishing touches on our suburban permaculture wildlife haven, The Bamboo Garden, with some hydrological micro-engineering strategies for not-so-level sites.

The site is not steep, but has enough of a slope for it to be a constant theme all through our development of the garden (since 1998). Historically, rain would just run-off downhill and exit the property in a hurry, leaving bare compacted dirt in heavy traffic areas. Grass struggled to establish in thin soil and was non-existent in the shade of the (weed) trees. Where it did grow, it was in giant lanky clumps growing up between piles of coarse prunings (ours) which caught some of the escaping soil and held on. This inspired the first of our slope interventions.

Cheap and dirty

In the beginning we had just a few large trees (some natives plus local invasive woody weeds) so the large-scale structure of the site was more-or-less set. We pared back woody weeds such as Chinese elm (Celtis sp) and other well-known Brisbane exotics, until we had the bones of a native forest garden: a massive Silky Oak (Grevillea robusta), sizeable mature White Paperbark (Melaleuca leucadendra), Bottle-brushes (Callistemon viminalis), Tea-tree (Melaleuca alternifolia), and our most beloved feature tree and nitrogen fixer, a beautiful Forest She-oak (Allocasuarina cunninghamiana).

We removed and chipped the woody weeds, and used the woodchips along with the larger logs/branches to create cross-slope mounds designed to catch and settle soil out of the run-off water. Into them we planted a huge number of native species from seed, seedlings and cuttings, begged, borrowed and (occasionally) thieved from various local and regional sources.  We planted in clumps, or groves according to a method (championed by Jackie French and others) with which I had previously had a lot of success.  A certain amount of luck augmented our thoughtful placement of species, in establishing mutually supportive groups of different plants (although these are known by some permaculturists as “guilds”, I prefer the term pact to avoid confusion with the specific ecological concept of a guild) on and around the swale-mounds.

The mounds shifted shape over years of adding material (sourced from a local tree-lopper) and re-design by Armillaria (a tree-killing fungus), scrub turkeys, extreme weather events, and fickle humans, until they came to define the areas which are currently our major garden “rooms”.

As these piles of coarse mulch and woodchips rotted down, they turned into a rich black soil which accumulated in the establishing root-net of the native food-forest we were nurturing. Rainforest species such as Silver Quandong (Elaeocarpus grandis) grew buttressing roots which helped our fight to slow run-off and have it deposit its load of silt where it was most needed. (A form of positive feedback encourages cross-slope roots in natural forests to grow larger and more prominent, creating a system of intersecting sills and swales which slow water and trap organic matter) But unfortunately we still lost a lot of the water dumped in larger rain events, and a fair bit of our hard-won soil went along with it. We had to come up with a way of infiltrating into the soil, and storing, every drop of water that arrived on the property, in all but the very largest storms.

Getting technical

The previous owners’ “landscaping” attempts had created some raised beds with sleepers, but used plastic weed matting (a crime against nature) to effectively impose a decade long drought upon the soil underneath it, while harbouring an impressive array of native and exotic volunteers (the “weeds” they so feared) in the accumulated litter on top. One or two established plants (including a large cycad) which had managed to penetrate through the matting, were rescued and replanted when we originally dug up the old mess and began rehabilitating the soil. We eventually re-used those old sleepers to box up the areas heavily trafficked by people and animals, but rather than putting in hard paving, we opted to go for a drainage layer of river stones (i.e. gravel - graded to about 20mm in diameter).

Gravel is often used in landscaping, for paths and drains. Gravel paths are less psychologically severe than concrete or paving, but the main ecological benefit is in its permeability. For this reason it is also used as a bedding or drainage layer for water features, rubble drains and other systems. To these uses we must add the less-obvious one of storage. Water not only drains freely through gravel, but when flow stops it is retained in the small gaps (capillaries) between rounded pebbles in a gravel deposit. In this way, we can not only slow water enough for it to infiltrate into the soil surface below, but also build up a modest mass of water above this boundary, without it running back off downhill again.

Excavating the sub-surface water storagesSo, back in the garden, we not only used the permeable surface of the gravel to slow the flow of water on site, but also the entire gravel deposit acts as a kind of capacitor or surge tank to gobble up excess flow while the soil below is slowly absorbing it. To increase the effect, we excavated hollows in the original soil surface (softly softly) between the larger roots of our precious trees. This took the greatest amount of time, care and highly skilled labour with a heavy crowbar, spade, rake and bare feet (Megapode style), but eventually we had crafted a series of connected ponds which zigzagged across the slope and nestled amongst the feet of our most water-hungry plants.

Storm gods testing the systemA timely downpour tested the system (and the nerve of the designer) and found that it worked perfectly. It was like an urban keyline system, in miniature.

And then we filled it all in again! But this time with the gravel… Water retained in the (now sub-surface) miniature impoundments was held wrapped around the stones. Despite the fact that the dams were now filled mainly with gravel (at least 80% stone : 20% space), we estimate that the overall surge capacity of the system has at least doubled. Another positive feature is the effect on microclimate in the garden - the individual stones have a high thermal mass, as well as being insulated from each other by the capillary cavities between them (the stones only touch their neighbours at a few points). This combines to make the gravel an effective solid mulch for the soil beneath, buffering extremes of temperature as well as slowing evaporation and run-off.

Backfilling completeIn effect we have made our property far more drought-resistant by leading a greater proportion of the water falling on it, into the soil. We have also gone a long way towards making sure that every crumb of humus we create stays on our property, instead of becoming a problem for neighbouring people and ecosystems, particularly local waterways like Oxley Creek. Last, but definitely not least, (fulfilling the permaculture principle of multiple functions, literally in spades) we have a permaculture wildlife garden that’s not just functional, it’s actually neat! People enjoy the space so much more, and they are asking more questions too!

All that makes this old activist much happier. =)

Notes:

Shoestring permaculture is my speciality, partly because I am a tight-fist, but also because I believe that necessity is the mother of invention. Over a decade and a bit, we have spent very little ($wise) in encouraging a significant and diverse assemblage of useful and beautiful species to live in, and visit our garden. (See the list of plants and fungi we have recorded in the garden here)

A breakdown of our approximate costs would look something like this:

  • Materials (sleepers, woodchips, mulch, gravel, pots etc.) less than $3000
  • Stock (all plants, trees, seeds and propagules) less than $1000
  • Water tanks (13kL plus pump and plumbing) $10000 approx
  • Leafguard for gutters (essential for forest-lovers) $10000 approx (fitted)
  • Labour (done for love) $0 - the labour just for the latest excavation, boxing-up and gravelling was about 36 hours
  • Design (done for love) $0 - equivalent retail value $500-$1000

Species List: Plants and Fungi.

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Reader Comments (2)

Beeautiful bamboo garden. It hadn't occurred to me before what damage plastic weed matting does to the soil. :)

July 5, 2013 | Unregistered CommenterHelen

Thanks Helen!
Yep, weed-mat plus pine bark (as per industry standards) is a total disaster... Almost as bad as concrete, though not quite as much of a pain to get rid of!

July 7, 2013 | Registered CommenterAiki Designs

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