I am lucky to have met a big group of plant science geeks from all over the world through platforms such as Instagram. So, recently, when I was offered the opportunity of a botanical road trip with my mate Rogier van Vugt, head of horticulture at Leiden Botanic Garden, about 25 miles south of Amsterdam, to visit tiny, niche growers and rare plant collectors across Europe, I jumped at the chance. Yet, to my surprise, the most amazing fact I learned from the experts we met was not about some top-secret plant cultivar or a closely guarded growing technique, but probably the simplest of all ideas: a new take on an old growing media.
Stepping into the private glasshouse of Xavier Garreau de Loubresse from the nursery Select Orchids, I was faced with species I had only ever seen in textbooks: bench after bench of rare orchids nestled in among exotic aroids. Systematically set out in neat rows of clear pots, all these plants were set off by a perfect emerald carpet of velvety moss that entirely covered the growing media. It was only when I picked them up to inspect some of the most delicate flowers did I see what was beneath the surface.
Filling each pot was what looked like a loose sprinkling of packing peanuts. As I looked around, it became clear that everything in the room was grown in the same mix, from huge jungle specimens to tiny Med-climate-loving pelargoniums, all looking very happy. Finally, Xavier, known as a unique thinker, put me out of my misery with a cheeky smile, and revealed that this media was something that’s been pretty standard in horticulture for decades: rock wool.
Cube art: seedlings in rock wool. Photograph: meeboonstudio/Shutterstock
This material is made by melting down basalt rock and spinning the molten mass into a soft, spongy “wool”. Its use is standard practice in hydroponics, where it comes in big bricks as a sort of inert sponge that water and nutrients can be dripped through. It’s used for growing commercial crops such as tomatoes. What is different here, however, is Xavier’s use of tiny 1cm cubes of the same material, tumbled into pots in place of compost to create a mix of water-retaining sponges with large air gaps that make up to a third of the pot’s volume.
This mix of air and water creates the paradoxical conditions many tricky-to-grow species need in order to thrive – consistent access to an even level of moisture, yet simultaneously perfect drainage and continual air flow. Being inert, rock wool also doesn’t harbour many of the soil-dwelling pathogens that can rot plants. It even seems to naturally encourage an incredible living top dressing of moss, which Xavier claims helps inhibit mould and bacteria growth.
What about its environmental impact? Well, the impetus to experiment with it was as a more ecological alternative to peat or coir because, despite the carbon cost of creating the material, rock wool doesn’t break down over time like many other organic substances. This means large or long-lived specimens can stay in the same pot for decades without the need to buy in new material, or the effort of transplanting. All of this from a material made of natural minerals.
The first thing I did this morning when I arrived back home was to buy bag to experiment for myself. I’ll update you on how I get on with it in a couple of months’ time.
Follow James on Twitter @Botanygeek