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Chelsea Fringe | an enormous pile of…

March 28, 2012

Kew Gardens doesn’t buy compost for its outside spaces.  Ever.  It makes its own, using woody and herbaceous waste created on site.  The compost heap, tucked away near the towering Treetop Walkway, is an open air one that’s surrounded by a moat and sits upon a reinforced concrete slab.  An impressive 6,000 m³ of compost is produced every year, saving Kew £500,000 annually.

I head to Kew to witness the enormous heap on a spring day that could easily be described as tropical.  As keeper of the heap for 12 years, and having worked at Kew for 24, David Barnes is the perfect guide.  He leads me onto to a platform from where we can best admire the shimmering dirt mountain of which he is rightly proud.

The compost heap is busy with staff dropping off clippings, and with a JCB that shifts waste from the master pile into a huge shredder.  This shredding speeds up the composting process – it takes just six to eight weeks for material to transform back into soil that can be used on the gardens as a mulch or soil improver.

David explains that to make a herbaceous mulch, they take one part herbaceous waste and mix it with one part horse manure, add air and water, and mix it together to create a rich soil.  To make a woody top dressing, they take three parts of woody waste, mix it with one part manure, shred it and then pass it through a screener, which picks out any huge bits that need to pass through the shredder again.  Kew gets 50 m³ of manure from the Royal Artillery every week – it’s predominantly hay rather than pooh.  Amazingly, the enormous heap smells quite sweet.

Kew Gardens sits on the Thames floodplain and the soil is pretty terrible, according to David.  It’s sandy and thin, so putting organic matter around plants and trees helps the earth to retain water.  This is a great tip in a drought.  Incidentally, Kew has a duty of care to protect a national collection and the Heritage Act ensures it’s immune to any hose pipe bans.  David says they are, nevertheless, very keen to conserve water on site.  The compost heap has a system for collecting rain water, which they filter and then pump through a sprinkler system to keep the manure suitably damp.

All this decomposing plant matter creates a lot of heat, and with it a lot of condensation.  In the winter, the piles of dirt can look volcanic with clouds of hot air blowing off them.  David digs in and lets me have a feel – we reckon the soil I touch is at least 65°C.  These high temperatures guarantee the Kew compost is sterile.  They have to damp things down to prevent spontaneous combustion – the fire brigade have had to put the heap out twice, although one incident was due to a firework.

Such a warm and sheltered spot is tempting, and ducks and peacocks have been known to nest on the manure.  Peacocks especially like it, as there are lots of insects and no children.  If birds do make a home on the heap, staff work around them.

After a good hour exploring the heap with Dave, I head off to the Treetop Walkway. From up above the canopy, on a stunning steel installation that embarrassingly makes my knees wobble, you get a great bird’s eye view of what has to be one of world’s most enormous compost heaps.

Want to know more?  Kew Gardens are taking part in the Chelsea Fringe and the brilliant David Barnes himself is leading a guided walk around the heap.  This is your chance to get up close and personal with a serious pile of dirt.  Find out more on the Kew website.  This article was originally written for the Chelsea Fringe website.

One Comment leave one →
  1. March 28, 2012 7:43 pm

    Visited Kew a few years back and those were some impressive compost heaps!!

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