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When
I was researching my previous post, on living walls, I discovered that an inorganic substance, stone wool, is
used as a growing substrate for some wall systems. Stone wool? I'd never heard
of such a thing. The more I looked into this topic, the more interesting
substrates I found. These all represent advances in the horticultural use of
existing materials, some of them organic like coconut coir, some, like rock
wool, inorganic. Improvements in substrate materials have no doubt contributed
to the increase in living walls; the same improvements are a boon to
horticulture in general.
What's a substrate?
Simply put, a horticultural substrate is a substance in
which a plant's roots grow. Garden soil is a substrate. Vermiculite is a
substrate. Plants do not need potting soil, or even organic matter, for growth.
They do need something that holds roots firmly enough to enable the plant to
grow up and that allows water and nutrients to reach the roots.
Why look for different substrates?
In the 1960s, a couple of factors impelled the commercial
horticulture industry to seek out new types of plant growth substrates. The
growing environmental movement awakened concerns about increasingly intensive
use of farmland, which exhausts the land. These concerns led, among other
actions, to the banning of methyl bromide, a soil disinfectant commonly used to
prepare soil for planting. Finding new
methods for crop culture would take pressure off of existing resources.
Stone wool
I'd never heard of stone wool, and I was baffled that
something made out of basalt would be an acceptable plant substrate. Although
mineral wool insulation has been around for a long time, a mistake - a bad
batch of insulation - led to its use as a plant growth substrate. A Danish
insulation manufacturer, the ROCKWOOL group, tossed the defective stone wool
out on factory property. Lo and behold, sometime later staff noted that small
plants were growing in the insulation batts.
The company worked with university researchers
to perfect this stone wool for
use in horticulture. A subsidiary of ROCKWOOL,
GRODAN, manufactures the horticultural version of Rockwool©. Plant
propagators use cubes of the material for starting seeds or rooting cuttings.
It offers several advantages for green walls and more general plant-growing applications:
- The medium is consistent in makeup from batch to batch, and it's easily sterilized.
- Stone wool is hydrophilic, which enables efficient
circulation and recirculation water and nutrients, but it does not rot like
organic substances.
- Plant roots have plenty of room to grow between fibers.
- Since stone wool substrate does not lock up or release any
substances, growers can provide precise amounts of water and nutrients.
- Stone wool is lightweight.
Coconut coir
Coconut or coco coir is the fibrous material found
between the hard interior shell and the outer coat of a coconut. We're familiar
with many of the products made of coir, whose production goes back as far as the 11th century AD: doormats, ropes, sacking, twine, among
others. During
the 1990s, coir's advantages as a horticultural substrate attracted commercial
growers, and it's popular today, particularly for hydroponics and mushroom
cultivation. At least one company sandwiches coir mats into panels for vertical
gardening. In addition to sheets and loose chopped fibers, horticultural-use
coir is formed into bricks and cubes.
Coir offers several advantages as a growth substrate, not
least of which is the attractive natural tan-to-brown color of coir sheets and
bricks. Other advantages include that
- Coir can hold up to five times its weight in water and
release it slowly, yet it drains well and prevents water pooling and rot.
- Coir is very slightly acidic, with good pH for plant growth.
- Roots move easily into the pores between coir fibers.
- Depending on its origin, coir can contain usable amounts of
plant nutrients, such as phosphorus.
- Coir has some anti-fungal properties and inhibits some other pathogens.
- This medium is long-lasting and can be reused for three to
four years.
- Coir is a 100 percent renewable resource.
A problem recently noted with some coir products is the
potential for inadvertent importation of exotic weed species. Ironically, gardeners can use coir mats to inhibit weed growth.
Felt systems
Patrick Blanc, the genius behind contemporary vertical
gardens, started out using synthetic felt, irrigating plants with
nutrient-laden water. Fast-forward to
the 21st century for the introduction of an improved type of felt
for vertical gardening. This new felt is made from 100 percent recycled plastic
bottles, so it's environmentally friendly and durable.
At least two companies market pockets made from
recycled-plastic felt, with two slightly-different pocket designs and
configurations. Both designs make use of the felt's capacity to wick water
throughout the pocket. Both designs also incorporate at least a minimum amount
of another growth substrate, typically a soil-less mixture. Plant roots can
penetrate the felt.
 
Florafelt Vertical Garden Planter Woollypocket Planter
These pockets make small-scale vertical gardening easy,
indoors or outside. They can be hung on drywall without causing water
damage, and gardeners can swap out and rearrange plants easily. Home gardeners
that lack access to a plot of ground can use these pouches to grow vegetables and herbs -
perhaps not squashes, but definitely lettuce and other smallish plants -- indoors or on a deck or patio.
Resources
http://www.ambius.com/
http://www.florafelt.com/
http://www.grodan.com/
http://www.woollypocket.com/
https://en.wikipedia.org/wiki/Coir
https://en.wikipedia.org/wiki/Mineral_wool
Photo credits: Rockwool/Wikimedia Commons; Shroomery.org; Florafelt; Woollypocket
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