Grow With Rockwool, A Newly Learned Old Trick | gardening tips

I I am lucky to have met a large group of plant science geeks from all over the world through platforms such as Instagram. Thus, recently, when I was offered the opportunity of a botanical road trip with my friend Rogier van Vugt, head of horticulture at Leiden Botanical Garden, about 25 miles south of Amsterdam, to visit tiny niche growers and collectors of rare plants across Europe, I jumped at the chance. Yet, to my surprise, the most amazing fact I learned from the experts we spoke to was not about a top-secret plant cultivar or closely guarded growing technique, but probably the simplest of all ideas: a new approach to an old growing medium.

Enter the private greenhouse of Xavier Garreau de Loubresse from the nursery Select orchids, I was confronted with species I had only ever seen in textbooks: shoal after shoal of rare orchids nestled among exotic aroids. Systematically arranged in neat rows of clear pots, all of these plants were set off by a perfect emerald carpet of velvety moss that completely covered the growing medium. It wasn’t until I picked them up to inspect some of the more delicate flowers that I saw what was beneath the surface.

Filling each jar was what looked like a loose pinch 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 Mediterranean-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 medium has been something pretty standard in horticulture for decades: rockwool.

Cube art: rock wool seedlings. Photography: meeboonstudio/Shutterstock

This material is made by melting basalt rock and spinning the molten mass into a soft, spongy “wool.” Its use is a common practice in hydroponics, where it comes in the form of large bricks like a kind of inert sponge through which water and nutrients can flow. It is used for growing cash crops such as tomatoes. What’s different here, however, is Xavier’s use of tiny 1cm cubes of the same material, dropped into pots instead of compost to create a water-retaining sponge mix with large voids. of air which represent up to a third of the volume of the pot.

This mixture of air and water creates the paradoxical conditions that many hard-to-grow species need to thrive – constant access to an even level of moisture, but simultaneously perfect drainage and continuous airflow. Being inert, rockwool also does not harbor many soil pathogens that can rot plants. It even seems to naturally encourage an incredible living dressing of moss, which Xavier says helps inhibit the growth of mold and bacteria.

What about its environmental impact? Well, the impetus to experiment with it was as a greener alternative to peat or coir because, despite the carbon cost of creating the material, rockwool doesn’t decompose over time like many other organic substances. This means that large or long-lived specimens can stay in the same pot for decades without the need to purchase new equipment or the effort of transplanting. All this from a material composed of natural minerals.

The first thing I did this morning when I got home was to buy a bag to experiment for myself. I’ll let you know how I go about it in a few months.

Follow Jack on Twitter @Botanygeek

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