Gardening has gone vertical. With more people interested in growing their own food, new growing systems have emerged to take advantage of vertical growing space.
But, do these systems really work?
Living walls are heavy and they dry out easily, which makes them a poor choice for lush, water-loving plants. During hot, dry weather, living walls would need nearly constant irrigation supplemented with a fertilizer if you are growing annual flowers or vegetables.
Living walls are a great showcase for plants with shallow root systems that like hot, dry soil conditions. However, most people who want to build a living wall are trying to grow small amounts of food or colorful annual flowers.
What’s a green thumb to do?
Before you start collecting Pinterest boards with hanging pallet gardens, learn more about green walls and whether or not they will work for you.
What is a Living Wall?
Green wall, living wall, and vertical garden are all used interchangeably in the horticulture world, although they have very subtle differences.
Green wall and living wall are interchangeable and they are both examples of vertical gardens. The purpose of a living wall is to cover a large portion of a wall on or in a building with greenery.
Green walls are primarily found in metro areas or in commercial buildings, although some homeowners create living walls on homes or fences.
Living walls provide many benefits:
- They provide insulation and help cool buildings
- They add greenery
- They help filter city air
- They absorb rainwater, which helps cool the surrounding area and keeps rainwater out of storm drains
One university even developed a brewery waste-water treatment system using the living wall concept (source).
Vertical gardens are gardens grown in vertical structures. This includes living walls and green walls, but it also includes containers mounted on a fence, A-frame hydroponic systems, a tall indoor plant stand with multiple plants, or any other structure that takes advantage of vertical space.
Most homeowners who want to take advantage of vertical growing space will use one or more of the following:
- Hanging pallets
- A-frame planters
- Hanging cloth planters
- Wire frames with mounted containers
There are many more creative interpretations of vertical gardening, but these are the most common.
The problems that come with living walls affect all vertical gardens, so regardless of the structure you’ve chosen, you may run into a few difficulties.
Living Walls & Irrigation
The most difficult part of maintaining a living wall is balanced irrigation.
Flat gardens are easy to irrigate because water is distributed evenly over the entire surface and then drains down into the root zone.
Vertical gardens are difficult to irrigate because water will flow from top to bottom. So, the top layers or containers may get overwatered, and then the bottom containers or layers will be dry and stressed.
If living walls are planted with water-loving plants, the problem can escalate. This is why proper plant selection is important to a successful living wall.
Living Walls & Nutrition
Living walls are heavy, so most structures minimize soil space to reduce weight.
Hanging pallet gardens are the most popular living wall for homes and apartments. The Pinterest pictures are cute, but, in reality, these containers are heavy.
- A standard pallet weighs 30-50 lbs.
- A standard pallet holds 4.5 cubic’ of potting mix, which weighs ~100 lbs. dry
- A standard pallet can hold 40 succulents, which weigh ~5.5 lbs. as new cuttings
So, the average hanging pallet garden weighs about 150 lbs. before it’s watered.
What does this have to do with nutrition?
Pallets are about 4.75” thick, which leaves about 4” of root space for plants. Each inch of depth adds 1.1 cubic’ of potting mix, which weighs about 20 lbs.
The easiest way to reduce weight on a living wall is to minimize the amount of potting mix in the container. This means most living walls and vertical gardens have very shallow containers.
Potting mix is the source of nutrition, so plants in a living wall can become malnourished quickly if there is no supplemental fertilizer.
Low-maintenance plants like succulents have shallow root systems and low nutritional needs, but annual flowers or vegetables will suffer in these containers without weekly fertilizer supplements.
Living Walls & Root Problems
Plants in living walls are prone to root rot, root dehydration, and stunted root growth.
Due to uneven irrigation and shallow containers, plants in living walls either end up with stress from too much water or stress from too little water.
Plants near the top of a living wall are prone to root rot because they receive the most water, and living walls tend to have poor drainage.
When potting mix dries out, it becomes hydrophobic, which is a fancy way of saying it repels water. When this happens, water runs along the outside of the potting mix instead of saturating the root zone.
In a living wall, the bottom third can become hydrophobic because it doesn’t stay consistently moist. Once soil becomes hydrophobic, it is extremely difficult to resaturate.
This dry layer of potting mix at the bottom can prevent proper drainage from the top layers. Instead of a slow and steady stream through the root zone, water pools at the top and resists seeping into the bottom layers.
When moisture builds up in soil, deeper root systems from trees or shrubs can absorb the excess moisture, and earthworms and other insects can burrow holes in the soil which helps with drainage.
In a container, you have to wait for the water to either evaporate or be absorbed by the plant, and if the plant is stressed from overwatering, then it may not be able to soak in excess water.
This can result in root rot, which looks similar to drought stress, but kills plants much faster.
You can mitigate this problem by planting water-loving plants near the top of your living wall.
Plants near the bottom of a living wall are prone to root dehydration because they receive very little water.
Potting mix near the bottom of a living wall can become hydrophobic, and once it completely dries out, it is almost impossible to get it to absorb water in the future.
You can restore hydrophobic soil through careful re-saturation, but this is difficult in a vertical container where water drips out instead of soaking in like in a horizontal planter.
When soil is consistently dry, the roots dehydrate and slowly begin to wither and die.
You can mitigate this problem by planting plants that prefer dry soils near the bottom of your living wall. Succulents are the most popular choice, because they tolerate extremely dry conditions.
Shallow containers and root zones exacerbate irrigation and nutrition problems, and also lead to deep-rooted plants being root bound.
A plant is considered root bound when the roots wrap around themselves so tightly that it cuts off circulation and roots begin to die. Deep-rooted plants, like most annuals and vegetables, struggle in living walls because they cannot form a normal root system.
If the containers are packed too tightly, the plants will fight for space, water, and nutrients, and all of the plants will suffer.
You can alleviate this problem by planting plants with shallow root systems that tolerate poor soils.
How to Plant a Successful Living Wall
Are living walls doomed from the start?
But, living walls planted with vegetables or flowers will be high-maintenance and may have more problems than they are worth.
Living walls are meant to look nice and help insulate a building. They are not meant to be a vertical vegetable garden.
If you want to use vertical space to grow food or flowers, look into A-frame planters or stacked raised beds to give plants ample root space.
If you want an aesthetic living wall, use the following plants:
- Petunias (water hogs)
- Wallflowers (the irony!)
- Dry herbs (rosemary, thyme, oregano)
Install a drip irrigation system in your living wall to maintain even moisture. These can be difficult to troubleshoot once the containers are full, but they do help to keep the soil evenly moist without being oversaturated.
Living walls are a beautiful addition to a fence or wall space to add color and greenery to a landscape, but they can be high-maintenance and they are prone to root problems. Choose the right plants and fertilize regularly to maintain a lush, green vertical space.
Learn more about gardening in small spaces by reading Thriving Yard’s article on apartment gardening.