Glengall Wharf Gardens is a haven of nature in a very urban part of South London (the Old Kent Road is only five minutes walk away). It depends entirely on volunteers who have done great things with little or no money. It was indeed a wharf on a long defunct and filled in canal. What remained was an area of concrete and cobbles which is now abundantly ecological.

 The hard standing inherited by the Glengall gardeners meant that they decided to build up rather than dig down. A large area is given over to raised beds and an equally large area to a developing forest garden based on Permaculture principles.

 The forest garden grows on a number of Hugel mounds- basically a giant raised bed where soil has been layered on top of decaying wood. The idea derives from Sepp Holzer's concepts of 'Hugelkultur' (hill culture), a big topic in its own right. At Glengall the mounds have been periodically mulched with wood chip, horse manure and compost.
 It's become a living soil rather different from the heavy clay based soil we have in much of London (though there are other soil types in London too). The Hugel mounds are very free draining and the soil is light, not rich but still fertile.
 [Correction Since writing these lines I have learned that not all the raised areas are Hugels; some are soil and mulch on top of rubble that had accumulated on the site prior to becoming a garden.] 
 The concept of indicator plants is a useful way to get a sense of soil type i.e. plants that suit particular conditions will thrive whereas others will not. It's noticeable that Mediterranean plants self-seed at Glengall very readily -Borage, Calendula, Wild Rocket for example- finding conditions on the mounds that correspond to their natural habitat.

 Numerous trees, shrubs and large perennials have been planted as part of the ongoing design process and many of these have flourished along with the aforementioned self-seeders and other weeds/wildflowers that like the conditions. Some "weeds" are welcome; no harm in having a patch of Stinging Nettles for example. 

 None the less the forest garden needs some attention as there are signs that 'succession' is starting to take hold. Where plants are concerned succession refers to the process whereby bare ground becomes grassy and weedy. The finer weeds and grasses are supplanted by the coarser ones which gives way to scrub which becomes woodland. Nature abhors a vacuum and will always seek to fill it.
 Now there's something to be said for letting nature run wild but it won't result in a food forest in the permacultural sense. Permaculture derives from the concept of a permanent agriculture using natural processes so seeks to address this by creating herbaceous and ground cover layers. These are productive in their own right and spread under the shrubs and trees preventing the ranker weeds from taking hold.

 NB Martin Crawford's forest garden in Devon is an exceptional example of a mature forest garden, 25 years in the making. See entry on 27th. February concerning my recent visit with examples of the seven layered approach.