It is a fun DIY project to assemble a garden of tiny succulents inside a terrarium. The challenging part though is to figure out what soil to use in a non-draining, non-breathable glass container without drainage holes. Even more difficult is to create a terrarium that lasts not only a few months, but many years to come.
How to choose the best growing medium for succulents, especially in a terrarium that can last for many seasons?
The best substrate for a succulent terrarium should be mostly inorganic (75%) (pumice/lava rock/gravel/coarse sand/perlite) with a low portion of organic matter (25%) (potting soil/compost/worm castings).
It should provide excellent drainage and aeration, should be resistant to decay, have a neutral pH, and can hold a plant from tipping over.
We will go through the factors in selecting the right substrates, analyze the characteristics of 10 commonly used substrates and find out the best soil mix formula for terrariums.
Let’s dig it in.
Quick Summary
This is the recipe for the best soil mix for succulents in terrariums:
| For top dressing | Gravel |
| Soil mix (25% organic matter) | Potting soil Compost Worm casting |
| Soil mix (75% inorganic matter) | Pumice Lava rocks Gravel LECA clay pebbles Coarse sand Perlite |
1. Factors for substrates in terrarium vs. pot
1.1 Drainage and aeration
The first thing you should consider for a substrate is high drainage and aeration to compensate for the lack of drainage holes in a semi-enclosed terrarium.
This is because succulents are more prone to root rot and fungal infection in a semi-closed enclosure with trapped humidity and limited airflow.
Substrates with a larger particle size drain better and dry out faster than those with a smaller particle size because there are more air pockets around the roots of the plant.
1.2 Water retention
Even though we want a substrate that drains well or dries out quickly, it should still have some capacity to hold moisture for a short time so that it does not need to be watered frequently.
Water retention is related to how porous (how coarse) a substrate is. The more porous (coarse) a substrate is, the more water it retains.
For example, gravel (also known as river rocks or pebbles) which have a very smooth surface or very few visible pores are considered very low on porosity, and therefore retains very little water.
Volcanic rocks such as pumice, lava rocks, and LECA clay balls all have a coarse, porous surface and can thus hold a certain amount of moisture.
Compost and potting soil are considered to be much more water retentive because of smaller particle sizes.
1.3 Resistance to decay
Resistance to decomposition is another important factor.
For a terrarium, we want inert substrates that are rather stable and do not break down rapidly. Being resistant to decay, they also give structure to the substrate.
When substrates decompose, the problem is that they will have a higher capacity to hold moisture and thus attract pests, develop fungi and diseases. Inorganic substrates such as sand, perlite, pumice are therefore more suitable than organic substrates such as sphagnum moss, bark, compost.
1.4 Nutrients
Succulents have low nutrition requirements as they have many resources stored in their leaves.
The existence of organic matter such as compost in a terrarium is not necessary and is even not favorable because it can quickly decompose and will retain water and attract pests in a semi-closed environment.
In fact, it is even argued that terrarium plants do not need to be fertilized to prevent them from growing too big.
1.5 pH
Succulents prefer a neutral or slightly acidic environment. But to prevent rapid decomposition in the closed environment of terrariums, it is better to use a substrate mix that has a neutral pH.
2. Organic Substrates
2.1 Potting soil
Potting soil is one of the most often used substrates because it is cheap and is water retentive, preventing moisture from evaporating too quickly.
However, potting soil alone is harmful to succulents, especially for succulents in terrariums. This is because most commercially available potting soil contains pieces of peat moss which retains water to keep the soil moist when wet, but becomes hydrophobic, or water-repellant, when dried.
When the peat in the potting soil dries out, the whole container of soil shrinks and becomes compacted into a hard block, making it very difficult for water to penetrate into the soil near the roots of the succulents.
Since succulents prefer a dry, well-drained substrate rather than one that is constantly moist, the potting soil in a terrarium will end up being either constantly too moist for the succulents or too compacted and water-repellant.
It is acceptable to use potting soil but mix it with a large portion of inorganic media.
2.2 Compost or worm casting
Some people add compost or worm castings into the soil mix for nutrient release.
They are certainly great additions to the garden for improving the soil and creating a better ecosystem of micro-organisms. But the priority for a semi-enclosed environment such as a terrarium is not soil improvement, but to prevent a soggy, waterlogged substrate.
What’s more, succulents generally have low nutrient needs because they have large reserves stored in their thick leaves.
If you want to fertilize, a small quantity of compost or worm casting is sufficient for terrariums. An alternative is using a liquid fertilizer with minerals in bioavailable forms for immediate uptake.
2.2 Moss
Of all substrates moss is the most water retentive, holding over 21 times its own dry weight of water according to an experiment on spaghnum moss.
It is fine to put one or two pieces of dried moss on top layer for decoration.
But, many people like top dressing or covering the space between the succulents using reindeer moss, sheet moss, or lichen, to make it look like the floor of a rainforest. While these make a terrarium look very pretty, lush and green, they can actually harm your succulents.
Remember the natural habitat of most succulents is not in a semi-arid environment with soil that absorbs water well but dries out quickly in-between the rain.
Top-dressing with moss, on the other hand, retains moisture in the top layer, preventing water evaporation and air circulation around the roots of succulents.
2.4 Bark
Given its larger size, bark is a popular recommendation to improve soil aeration.
But there is no real benefit of using bark in a terrarium, other than the purpose of decoration on the top layer.
Similar to the problem with compost, bark is an organic substance that decomposes with time.
For the purpose of improving aeration, there are many other more stable, decay-resistant substrates such as pumice, gravel, LECA clay balls that can do the job.
2.5 Coconut coir
Some people recommend coconut coir to aerate the soil and bring oxygen to the roots of the plant.
But, coconut coir which are fibers harvested from the coconut husks can break down fairly quickly and are acidic with a pH of 5.2 to 6.8. Some coconut fibers may even have a high salt concentration which may have a negative effect on the terrarium substrate.
There are, in fact, many other inorganic alternatives such as perlite that can provide aeration, have a neutral pH and do not break down like coconut coir.
3. Inorganic substrates
3.1 Volcanic rocks (Pumice, Lava rocks)
Volcanic rocks are created naturally during a volcanic eruption. They come in different types, i.e. pumice, lava rocks. They are very porous, allowing them to drain water quickly and providing much aeration in the substrate.
They are also heavier than other inorganic substrates such as perlite and sand and do not get washed away during watering.
The pumice from Superfly Bonsai is good as it is sifted with minimal dust. Their 7mm particle size is also perfect for small succulents in a terrarium. Their 2.5 dry quart bag will also go a long way.
3.2 Gravel (pebbles, river rocks or polished rocks)
Gravel (also known as pebbles, river rocks) has smooth surfaces due to river erosion and is not porous like volcanic rocks. Because of this, they are also more water retentive than volcanic rocks but they also drain well and can provide aeration around the roots.
Gravel can be used to mix in with other soil types in the substrate. It can also be used for top dressing because of its beautiful smooth, glossy surfaces and decorative colors, and for retaining more water in the top layer.
Look for gravel with a small diameter for substrate mix, around an eighth of an inch (around 4mm). This slightly larger gravel of 4-6mm particle size is perfect for top dressing and the reservoir.
3.3 Coarse sand
Coarse sand is better than beach sand or fine-textured sand which tends to clump with moisture.
Coarse sand also has very good drainage.
3.4 LECA or Clay pebbles
LECA (short for “Lightweight Expanded Clay Aggregate”), also known as hydrotons, is manufactured using clay under high temperatures to create a product that is highly porous.
The benefit of LECA is that they are much more economic than volcanic rocks which can be quite expensive, especially when used in large quantities.
While LECA is ideal for orchids in a hydroponics setup, it is not suitable to be used as a substrate for succulents because of their big particle size which usually measures between 8 and 16 mm, which is too big for small succulents in a terrarium.
Another problem with using LECA is that they are very light-weighted and cannot hold down a top-heavy succulent from tipping over.
However, LECA has an important place in a succulent terrarium in creating a “false bottom” or a water reservoir where water drains and drops through a non-biodegradable mesh from the substrate above. A reservoir is thus crucial for the long-term success of a closed terrarium—-Water evaporates from the reservoir, condenses on the walls of the terrarium and drops back down to water the plant.
3.5 Perlite
Perlite is an expanded form of crushed silicon rocks, with a 2-3mm small particle size. It can easily be crushed into powder and is very light-weighted.
Perlite has excellent water drainage and aeration properties similar to that of volcanic rocks like pumice and lava rocks. It is also more water retentive than pumice.
Mixing perlite with pumice and gravel will make a good terrarium substrate for succulents.
4. Soil mix recipe for succulent terrariums
For a terrarium that can last several seasons, the majority (75%) of the soil mix should be inorganic to ensure fast drainage and to prevent decomposition, fungus and root rot.
You can mix two or three of these types: pumice, lava rocks, gravel, coarse sand, clay pebbles, perlite, for different water capacities and particle size.
Organic substrates should only be used in small quantities, around 25% of the soil mix for some level of water retention and slow nutrient release.
The organic substrates most suitable for this purpose are potting soil, compost, and worm casting.
You can put the compost or worm castings on the top layer above a gritty substrate for it to go down and fill the space as you water, and add them slowly to the top layer in case the substrate dries out too fast in your environment. This approach would allow you to adjust your soil mix according to the needs of your plant and the environment. Don’t worry – succulents can grow in 100% grit without soil.
Bark and moss should be only used sparingly in the top layer for decorative purposes.
Obviously, other factors crucial for the long-term success of a terrarium would be applying the right amount of water and a correct watering technique using a mist bottle or syringe. But that would be a topic for another article.
5. Can I use succulent soil mix for terrariums?
Mixing your own soil mix allows you to adapt the recipe according to the needs of your plant and the circumstances of your environment. But, you can also get a succulent and cactus soil mix that will generally work in various environments.
Check the label of the soil mix, it should have:
– no peat
– a majority of inorganic substrates and a low portion of inorganic substrates
Conclusion
No substrates are essentially good or bad on their own. It all depends on the needs of the plant, the circumstances (temperature, humidity level, airflow) of the environment, and the watering technique.
Generally, the best substrate for succulents in a terrarium is mostly inorganic (pumice/lava rock/gravel/coarse sand/perlite) with a low portion of organic matter (potting soil/compost/worm casting). Bark and moss should be used sparingly and only in the top layer for decoration.
Such a mix provides excellent drainage and aeration but does not dry out too fast, is resistant to decay, has a neutral pH, and lowers the risk of rotting and fungal infection due to trapped humidity and limited airflow inside a terrarium.
Happy growing!
References
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DelPrince, J. & Bachman, G. (n.d.) How to Design a Closed-System Terrarium. Mississippi State University.
Griffiths, H. & Males, J. (2017). Succulent Plants. Current Biology. 27(17). 890-896.
Kluepfel, M. & Lippert, B. (2012). Changing the pH of Your Soil.
Mottram, R. (1986). Nutrition in Succulent Plants. British Cactus Journal. 4(1). 9-13.
Nimmo, J. (2004). Porosity and Pore size Distribution. Encyclopedia of Soils in the Environment. 3. 295-303.
Olorunfemi, I.E., Ogunrinde, T.A. & Fasinmirin, J.T. (2014). Soil Hydrophobicity: An Overview. Journal of Scientific Research and Reports. 3(8). pp 1003-1037.
Prisa, D. (2019). Quality improvement of cacti and succulents with alternative substrates.
Tjosvold,S. (2019). Soil Mixes Part 2: Water and Air Porosity. University of California: Agriculture and Natural Resources.
Tumer, A., Karacaoglu, E., Namli, A., Keten, A., Farasat, S., Akcan, R., Sert, O., & Odabasi, A. (2013). Effects of different types of soil on decomposition: An experimental study. Legal Medicine. 15. 149-156.
University of Illinois. (n.d.). Choosing a container for Planting.
Photo credits
“Succulent terrarium” by Lubica Vinicenko is licensed under CC BY 2.0
“Teardrop Terrarium Lifestyle Image” by urbanbotanist is licensed under CC BY 2.0
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