The presence of rubble in new build gardens used to be seen as a negative, but there is now an emerging trend to make it the star of the show. So-called 'hardcore gardening' involves employing builder's waste as substrate: including sand, bricks, cement, plaster, ceramics, and more. Gardening fashions can be irritating, but this one has no end of advantages and, with climate change underway, its ethos seems to be the most obvious way forward.
What are the advantages of gardening on rubble?
Gardening on rubble enables a garden to cope with both drought and flooding: two weather extremes that are predicted to worsen. In summer, the rocky hardcore acts as a mulch, conserving moisture, and its extreme drainage suits plants that hail from stony sun-baked places, such as the garrigue of the south of France. These types of plants can be tricky in traditional British gardens because they loathe our wet winters, but the rubble's exceptional drainage means their roots never sit waterlogged, so they can survive unscathed; its warmth also helps to prevent frost damage.
Dry plants, such as viper's bugloss and lamb's ear, bloom more in the challenging conditions of a hardcore border.
There is a lot less weeding to be done because the weeds dislike the low fertility and lack of soil; where the rubble is particularly deep, the gardener might have very little to no weeds. In these challenging conditions, plants grow tougher (so there is less need to tackle pests and diseases) and shorter (so staking is rarely required). Watering may be necessary until the plants establish and in extreme heatwave periods of drought, but, other than that, it won't require any water. And, the most vital part, the enemy (slugs) dislikes sharp aggregates.
Recycling building waste is, obviously, better for the environment than letting skipfuls of it go into landfill. Taking aggregates from the earth is rarely sustainable (gravel strip-mined from the seabed kills marine life, for example) or ethical (in some countries, children may be used to mine sandstone, for instance).
Purchasing aggregates to construct a gravel garden can be incredibly pricey, so using your own building waste or purchasing recycled construction waste (hardcore) is cost-effective. Most rubble-friendly plants (such as poppies) favour poor soil, so the expense of plant food and organic matter is significantly reduced.
How can plants grow without soil?
Traditionally, we garden on soil and ladle on the compost. This certainly suits most edible crops and hungry ornamentals (such as roses). But research shows that a huge number of plants perform better with minimal compost mulching, and pioneering gardeners who cover their soil in a dense layer of mineral aggregates prove the point. In Sweden, Peter Korn has created a dreamy garden in a dense layer of sand atop alkaline soil. As well having mycorrhizal fungi, which aid nutrient and water absorption, sand conserves moisture well. In Wiltshire, Derry Watkins successfully grows in a 20-centimetre layer of gravel atop soil; the plant's roots simply find their way down to the soil. In Canada, planting designer Ben O'Brien has had success growing plants in crushed concrete and sand.
How do you create a border with builder's waste?
Assess what sort of soil you have. Is it sandy and therefore very free-draining, or is it dense clay that is like stone in summer drought and dotted with puddles in a winter deluge? Do you already have hardcore in the earth, if you're in a relatively new home? Has your soil been enriched with organic matter (such as compost) year after year?
The vast majority of plants that enjoy the conditions of a dry garden are sun worshippers, so ideally place your new hardcore border in full sun, such as a south or west-facing spot. A lot of these plants are incredibly tough, so they can cope with a degree of exposure to winds.
The boring part of creating a hardcore garden is to first remove the area's perennial weeds, in order to make your life much easier in the future. Blocking their light with old carpet or thick black pond liner is the most eco-sound method over a large area and should take 1 to 2 months; in a small area, you could use a flame gun.
If you don't have any building waste, purchase recycled crushed aggregates from a local supplier, ensuring that the size is suitable for growing plants in (for example, between 1 and 4 centimetres). If you do have builder's waste on your property (such as concrete, stone, tiles, plaster, sharp sand, and bricks), hire a crusher to reduce it to rubble.
Sharp sand (also known as horticultural sand) is great, but don't use builder's sand because it can become waterlogged, and, of course, never put cement powder into the ground with sand and grit because you'll end up with a patio, rather than a flower border.
Create your aggregate layers according to your soil type and feel free to experiment because this is a very new form of horticulture that is still being trialled by experts. There is no winning formula yet, but it seems that there needs to be happy balance between drainage and fertility, so on rich, dense soil, remove the top layer of soil and add a lot of rubble (over 7 centimetres deep) and on very poor ground, add a little compost before topping with rubble. See the section below on 'the best aggregate depths for a rubble garden' for more guidance.
You could create an interesting landscape – as well as microclimates – by mounding up more recycled aggregates in some parts of the garden; if so, more moisture-tolerant species ought to be planted in the resulting ravines. Alternatively, in a flat bed, create height by planting verbascums and other giants here and there.
Select drought-loving plants that thrive in gravel gardens and ensure you have interest throughout the year. For instance, you might plant tulip and alliums bulbs for spring and early summer, followed by summer gems (such as Oenothera lindheimeri 'Whirling Butterflies' and Dianthus cruentus), and evergreens (such as Sesleria autumnalis) for winter. Extreme gardening expert Peter Korn has found that richly-grown plants struggled to establish when he introduced them to his sand beds; where possible, purchase plants grown at nurseries (such as Biocentric) that slow-grow plants without fertiliser and, where possible, grow things from seed and from bulbs and in bare-root form. Rubble borders are slow to get going, so be free with the annual seeds (such as white campion and pale-lemon tufted California poppy) to colour in the temporary gaps. Some building waste – especially concrete – can be very alkaline; however, plants don't seem to be as susceptible to pH as they are in soil; growing a hardcore garden atop very acid soil could potentially alter the soil's pH, but this has not been trialled.
Drought-tolerant plants grown in the harsh conditions of a rubble garden grow slower (but live longer), so you can plant more densely that you normally would. To discourage weeds, the gardeners at Beth Chatto's Gravel Garden saturate the plant and remove the upper compost from the pot that the plant has been growing in before planting low and then levelling to ground level by adding gravel.
Water the plants in hot, dry weather, for one to two months, until they are established. Remove weeds if they appear. Carrying out the Chelsea chop at the end of May, the team at Beth Chatto has found, aids in plants' drought tolerance.
What are the best aggregate depths for a rubble garden?
The decision of how deep a layer of hardcore or other mineral aggregates to put down will depend on the ground you start out with. But, in general, the deeper the rubble mulch, the slower the plants' establishment will be, but the more minimal the weeding. As stated above, this is a very new form of gardening, still in its development, so feel free to experiment. Here are seven examples of how it could be done:
In the Special Plants' Gravel Garden in Wiltshire, nursery owner Derry Watkins laid down a 20-centimetre layer of gravel (which hardcore gardeners can switch for builder's rubble) after removing her clay topsoil; she never has to weed.
In Beth Chatto's Gravel Garden in Essex, bonfire waste and compost were added to the former car park's layers of shallow poor soil and compacted gravel and sand, after it had been broken up with a subsoiler and plough and then lightly flattened. After a year to allow weeds to come up, it was covered in a 5-centimetre layer of gravel. When refreshing beds, they are double dug and enriched with a 2-centimetre layer of low-nutrient council compost. Weeding is required, as is extensive editing of plants allowed to self-seed. It is the most labour-intensive part of the Beth Chatto gardens, which contain several types of garden, including an experimental new Sand Bank formed of a 15 to 30-centimetre layer of recycled sand and gravel over soil, in which weeding is minimal.
At RHS Hyde Hall in Essex, the Dry Garden's clay soil was covered with a 30-centimetre mix of soil and sandy grit and crushed concrete rubble and then topped with a 5-centimetre layer of pebbles. Whilst the Sky Meadow is sown into a 10-centimetre layer of sand. Both areas require a lot of weeding.
In the Growing Grit trials in Canada, Ben O'Brien has discovered that mixing crushed concrete and sand with compost led to more weeds than beds of pure aggregates, whilst beds mulched with 15 centimetres of nothing but crushed concrete had no weeds at all yet showed the least plant growth.
In Essex, John Little grows in crushed 10 to 50-centimetre deep construction waste, industrial waste (such as chalk), and sand laid above clay soil. Some weeding is required.
At the Horniman Museum's Grasslands Garden in London, 10 centimetres of topsoil was removed, the rich clay underneath was rotavated, then 10 centimetres of gravel was laid on top. Professor James Hitchmough, who designed the garden, has found that a layer of gravel at least 7.5-centimetres deep is required for suppressing weed growth. Very minimal weeding is required in the Grasslands Garden.
In Bristol, garden designer Ed O'Brien laid a 30-centimetre mix of sand and crushed old plaster left from his kitchen extension over the existing lawn in his front garden to create a beautiful rubble garden. It was slow to establish, but weeding is very minimal.
Which plants grow best on rubble?
Drought-tough plants that hail from the stony sun-baked regions of the Mediterranean, as well as California and South Africa, fare well in the extreme drainage of a hardcore or gravel garden. Here are just a handful of the best:
FOR WILDLIFE: bee-magnet Nepeta grandiflora 'Bramdean', butterfly-friendly Achillea 'Fanal', orange butterfly weed, and goldfinches' favourite: teasel
FOR COLOUR: hot-pink Lychnis coronaria, bold-purple Salvia 'Amistad', the bright-scarlet poppy Papaver commutatum 'Ladybird', and lime-gold Euphorbia myrsinites
FOR TEXTURE: the billowing grass Stipa tenuissima, felty Phlomis italica, lacy bronze fennel Foeniculum vulgare 'Smoky', and papery clary sage
FOR SHAPE: firework-like Allium schubertii, towering yellow Verbascum olympicum, thistle-like cardoon, and the orange candles of foxtail lily 'Cleopatra'
FOR SEE-THROUGH LAYERS: claret thimbles of Eryngium pandanifolium 'Physic Purple', violet sea lavender, long-flowering Verbena bonariensis, and airy Althaea cannabina
Which rubble gardens can I visit for inspiration?
Knepp, West Sussex The exciting new garden at the 3,500-acre rewilding project is well worth seeing and includes fabulous purple Aster amellus 'Violet Queen' and wispy Stipa lessingiana growing on crushed concrete from demolished buildings on the estate, mixed with sand.
Beth created the Gravel Garden on a former car park as an experiment to see what would grow. Anemone pavonina, Hylotelephium 'Matrona', and other resilient plants thrive, despite the garden never being watered and situated in one of the driest parts of the UK.
In Sheffield city centre, Pictorial Meadows founder Nigel Dunnett has created a no-water garden on 10-centimetre crushed quarry sandstone substrate that helps to prevent flooding and allows a wide range of plants to thrive, from bearded irises to Russian sage.
Dan Pearson's gorgeous revamped ruins garden, Delos, was created by stripping the dense clay soil and adding gravel, crushed brick, and poor topsoil, in which Bupleurum fruticosum, Centaurea bella and other drought-tolerant plants bloom.
Plants that hail from North America's prairies and South Africa's grasslands, such as Echinacea tennesseensis 'Rocky Top' and Eryngium yuccifolium thrive in the Grasslands Garden, while new dry borders formed with concrete aggregates from old paths in the museum's gardens are the perfect home for the gorgeous glaucous foliage of sea kale, which grows on shingle beaches in the wild.
