WILDING THE GARDEN: the sand mounds
November 2024
Moy Fierheller | Deputy Head Gardener
The month begins in monochrome. An ‘anticyclonic gloom’ holds sway over the country, and uniformly grey cloud hangs constant and dreary over us like a low roof. But dry! Even the ephemeral pond in the Rewilded Garden grasps the opportunity to drain a good 60cm of water, and we tentatively peel off our habitual waterproofs.
Standing seedheads seize the moment to send their offspring into gentle breezes to settle on bare ground. The long pods of butterfly milkweed (Asclepias tuberosa) split open to reveal delicate silky umbrellas like exotic undersea creatures floating into the air. The stands of Chinese silver grass (Miscanthus sinensis ‘Flamingo’) sway their plumes in unison, the dance releasing an emanation of tiny seeds, a frenzy of dust motes. We’ve been ‘grazing’ the clovers and self-heal, thinning them out where they have formed dense mats to allow seed to settle on disturbed ground, keeping up the plant diversity.
Left; Butterfly milkweed seeds dispersing. Right; Chinese silver grass releasing seeds in the wind.
With low winter sun lighting the grasses in the Rewilded Garden into blazing oranges and yellows, we spent an afternoon like children, playing with sand. The intention was to create a series of sand habitats suitable for solitary bees and other burrowing invertebrates. John Little and his partner Fiona, who own a landscaping company that specialise in biodiversity habitats for brownfield sites, green roofs and urban public spaces kindly shared with us some of their Thanet sand. It is a very fine-grained, green-grey sand laid down 58 million years ago in shallow marine conditions stretching along the London basin and into south-east England. Thousands of tons were removed during improvements to the A13 trunk road and John has long been expounding its virtues as perfect insect habitat as well as a recycled growing medium. The fine-grained nature of the sand enables insects to burrow and move freely through the substrate, while it’s free-draining nature prevents tunnels becoming waterlogged or collapsing. It is still able to retain enough moisture to support molluscs and earthworms and is high in mineral content from the river- washed clay.
The existing sand in the Rewilded Garden, either mixed with crushed concrete or used as a layer of mulch is a land-based sharp sand. It was included in Tom Stuart-Smith’s design to provide a free-draining layer to extend the range of habitats and opportunities for wildlife and bring the project together stylistically – with a nod to climate-change resilience. In the aggregate mix, the sand mitigates the potential for dust created during the crushing of the waste concrete to re-aggregate and form an impermeable pan. The larger grain size means that its constituent parts don’t readily hold together and tunnels of burrowing insects such as ground beetles and ground-nesting or solitary bees collapse. By creating four small mounds around the Rewilded Garden we’re aiming to produce a wider range of conditions. We wanted to fashion the sand to mimic the vertical walls created by the exposed root- base of overturned mature trees in the rewilding project. There are around 100 species of mining bee in Britain that prefer to nest in soft vertical surfaces, free of vegetation. The cones were orientated to have a large south-facing area to help warm emerging broods.
One of the newly created Thanet sand mounds in the Rewilded Garden oriented to a south-facing aspect
There are wider issues to consider when using sand in gardens – like everything, it’s a finite resource and sand is not as abundant as you might think. Desert sand is too smooth for use as a binding agent and sea sand contains too much salt. Demand for sand in construction (projected to rise by 45% in the next four decades) is met from rivers, lakes and shorelines. Overexploitation can cause ecological damage to habitats, loss in biodiversity and food production. Illegal operations have been reported in more than 70 countries.
Our thoughts then naturally turn to seeking alternatives and sourcing sustainably. We are addressing some of the compaction issues the wet summer has highlighted in the Rewilded Garden by clearing some of the worst affected areas and breaking the pan with pickaxes. Using a loam base we created a mound and plan to overlay a deep mulch of recycled sand. The plants we dug up to replant atop the new banks plainly showed the effect of a hard pan beneath the planting holes we dug three years ago, the roots executing a sharp right-angled turn beneath their original root ball.
The roots of a common myrtle (Myrtus communis) showing where they have met an impermeable pan created by compaction.
We’ll need several tons to accomplish this project – it’s quite astonishing how little volume there is to a ton – and we set out on a hunt for suppliers of recycled sand. We wanted to try and source them as close to us as possible, with a view to both reducing our carbon footprint and keeping transport costs down. After a morning of telephone calls to sixteen suppliers, it transpired that many of them advertising ‘eco sand’ appeared to have none in store. The ‘sand’ supplied by others was mainly concrete dust with larger pieces of flint and concrete. The fine particles are likely to re-aggregate, and we are back to the risk of impermeable pans forming.
We were sent a sample by one company that was described as reject sand. It turned out to have been dredged from the sea, had a very high salt content which would burn plant roots, and coagulated like demerara sugar. Arriving at the nub of the problem, we discovered that in order to supply a recycled sharp sand, a company needs to be able to wash vast amounts of aggregate. The infrastructure to achieve this can be upwards of £5 million and there simply isn’t the demand in the construction industry.
Some of the sand samples with a standard land-based sharp sand from a builder’s merchant as a comparison
Stumped, we called back John Little, master of alternative horticultural growing mediums (he has successfully used the calcium carbonate waste by-product from Tate and Lyles’ sugar factory to grow a chalk grassland meadow). A company he uses reclaims sharp sand from waste soil by washing and spinning it. The result is a legitimate recycled, washed, sharp sand that will keep an open, free-draining structure and support plant growth and insect habitat. The transport distance is further than we’d like, but it has been a worthwhile lesson for us, and hopefully others, in the extra time needed to pursue sustainable pathways. Perhaps with some wider discussions between horticultural practitioners, growing demand may make the road a little smoother in the future.
The first frost of the year laid down its crystalline blanket over the garden on 20 November. All the silver leaf planting comes into its own, glistening in ethereal grandeur against dark standing stems and iced seedheads. The low mounds of yellow immortelle (Helichrysum orientale) are white puffballs all through the ridges and mounds of sparkling crushed concrete and sand, the leaves of Greek mountain tea (Sideritis scardia) are feathery fronds, lavenders and mulleins pull the eye, and creeping thymes are like snowy carpets. The sky is a mother-of–pearl blush of pink and blue. Perhaps the dark days of winter are worth it, if this is the gateway day.
Photos courtesy of Charlie Harpur, Joshua Chalmers, Moy Fierheller
What we’re reading;
Home page | The Convention on Wetlands, The Convention on WetlandsÂ
Ramsar Thanet Coast and Sandwich Bay uk078D96Â
1001128 https://ukfossils.files.wordpress.com/2007/09/pegwell-bay.pdfÂ
Urgent shifts in building practices are needed to tackle the global sand crisis – Leiden UniversityÂ
Running out of SandÂ
https://www.bbc.co.uk/mediacentre/proginfo/2024/12/the-documentaryÂ
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