Super soil Secrets from the experts at Kew. Plus, growing flowers and family
This month I was lucky enough to sneak in on a behind-the-scenes visit to the newly restored Temperate House at Kew Gardens, a building that blew my mind as a kid with its sheer size, its wonderful plants and its fascinating history. But stepping into this cathedral of glass this time, still smelling of wet paint, I couldn’t help but be entranced by, well, the soil. Yes, weird. I know. But here’s why…
Anyone who has grown in containers will quickly learn that potting compost has a tendency to collapse after a season or two, as the plant-based fibres it is made up of start to break down. This means the costly, and frankly backbreaking, exercise of replacing it can be almost an annual chore. Yet no matter how massive the expanses of glass at Kew, the plants grown in these great greenhouses are essentially still container plants. With the beds as little as 40cm deep, even for towering trees, the horticulturists at the Temperate House face similar issues to most container gardeners, just on an enormous scale.
So back in the early 2000s the plant geeks at Kew began experimenting with a potting mix that would survive the test of time, filling different beds at the Temperate House with various formulations before planting them up. A decade later, as they began to remove the planting from the house to prepare for its planned restoration, one simple mix was found to have basically zero breakdown. This blend should work just as well as a general mix in the containers of my tiny outdoor space as it does in the houses at Kew, and the best bit is it’s pretty foolproof to put together.
It’s simply equal parts loam (ie regular garden soil), grit, sand and composted bark. The relatively low content of organic matter (just 25%) means this formula breaks down far more slowly than traditional mixes which can verge on 100% plant-based material. Yet it’s still rich enough in these natural fibres to retain water and support a healthy bacterial community. This might seem extremely low, if you are used to traditional peat or compost-based mixes, but is actually much more similar to most fertile soils found in nature than those mixes are.
Also, note Kew’s use of composted bark as the organic material here, not peat: it’s a far more sustainable alternative. The addition of loam helps mimic natural soils further, ensuring a far broader spectrum of the essential minerals necessary for healthy plant growth, which greatly reduces our reliance on fertilisers, either natural or synthetic. Sand and grit, at a really high level (I was surprised), help retain an open, free-draining structure and provide enough weight to anchor the roots of large trees. All in all, it’s pretty perfect for raised beds, tubs and troughs.
However, if you, like me, are growing things on balconies or roof terraces, this mix can be prohibitively heavy, both for you to lug up flights of stairs, and potentially even from a structural point of view. So I have made a lightweight tweak for us skywardliving gardeners. Just replace the grit with expanded clay pellets (also sold as hydroleca) and the sand with perlite and you are good to go. Two perfect growing mixes to last you years at a time, based on the best the plant geeks at Kew could conjure up.