Sunday, November 25, 2018

Plants on Walls

plant wall
Plant wall, outdoors, Mexico City
Sticking potted plants with interesting colors and textures of foliage or flowers into niches in a wall is a recent, stylish form of decoration, often called "green walls."

Sometimes it works, but I see it generally as plant abuse.

plant wall
plant wall, indoors, Colorado
Note the gaps 
First, the walls are vertical. To have plants visible from a grid, the plant sticks out horizontally, or as nearly horizontally as the makers can arrange.

But the vast majority of plants, coleus or jade plants or roses, have strong tropisms (hormone-driven responses) that direct them to grow their roots toward the ground and shoots going up. If you turn a flowerpot onto its side, the change in the direction of gravity results in the production of hormones that direct the roots to turn downward, the shoots to go up. (more: link) If the pot is left on its side, righting itself--which is the plant's point of view--leaves the plant's roots growing in a very limited space (the lower half of the pot).

The instructions online for green walls often say tip the pots at 30 degrees. That will work to display the plants and yet leave the plants close to upright. Balancing flowerpots or window boxes on a vertical wall so that the foliage covers the wall is not easy. It has to be built well to be stable, accessible for maintenance and attractive.

pots in plant wall
The flowerpots of a green wall
Well-designed walls can be attained. My concern is for the plants in the wall, the living things.

Plants take up most of their water through their roots. Sitting normally in a pot, water runs down through the pot, to be captured by the roots. In a tilted pot, the water still runs vertically down.  Good watering is more difficult the more the pot is tilted.

pots in a plant wall
Pots in a plant wall. The pots onthe top are not doing so well.
Indoors, Colorado
Plants on walls are likely to be frequently water-stressed. First, the tilt of the pot reduces efficiency. Second getting water to plants at the top is difficult. Outside you can spray the whole thing with a hose. Inside that is often undesirable. Commercial plans build in a watering system. That can make a big difference.

plant wall, Taiwan
Healthy plant wall, outdoors, Taiwan
Plants at the top dry out very rapidly when the ones at the bottom are too wet, with all the run off from the pots above.

Beyond that, the cutting placed in the pot will grow. As it grows, the roots expand to fill the small flowerpot. At that point, the plant needs a lot more water than when it was small. Not just because it is bigger but because it has filled the pot with roots and there is little soil there to store water for later. Plants on a wall will, over time, need more water more often.

So you get effects like the one below. The leaves are reddish and some plants have grown quite  far out from the wall, but they are not doing well. (The living plants are inch plants,  Tradescantia zebrina link or a similar species, spiderwort family, Commelinaceae)
plant wall, Taiwan
Not doing well, but they're not all dead yet.
Outside, Taiwan
But these are not going to revive:
plant wall, Taiwan
Dead plant wall, outside, Taiwan
Even the professionals, who tout all the benefits of a green wall, respond to the question "Are they hard to maintain?" with "The short answer is yes." (link)

And yet,

I have seen some spectacular green walls. For example:

plant wall, Taiwan
Along a driveway, Taiwan
What works? First, a wise choice of plants. Some plants have a more flexible relationship between shoots, roots and gravity, so do not find tilted small pots particularly stressful environments. Vines in particular. Philodendrons, for example, grow well even if their branches hang well below the roots. Tropical epiphytes, plants that grow on trees but are not parasitic, can be good choices because they normally grow from a small mass of roots on a tree branch, an environment rather like a pot on a wall. Online websites recommend succulents because they cope well with drought and have limited roots. Beyond that, it depends on whether the wall is indoors or outdoors, in high humidity or low humidity, full sun or addition to choosing plants that tolerate small pots.

Climate is probably important for success. In humid areas, the plants will generally do better because they will get water from the air or at least will not lose water to the air very fast. And if nature waters the plants frequently, it is less important if humans do not do it well.

Finally, the display is not "set and forget." Of course they need water. But furthermore, individual plants will die or grow too big and need to be replaced.

When walls of plants work, they can be spectacular. My photo is too distant to show it well, but  small-leaved plants form the green background, with ferns and vines for the focal points.
Ferns as bouquets,
plants with tiny and small leaves as the green background.
Outdoors, second story wall, Mexico City

But I cringe whenever I see a plant wall with unhealthy plants.

Comments and corrections welcome.

Kathy Keeler, A Wandering Botanist

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