Best Inorganic Media for Orchids? (Proven by Experiment)

Inorganic media LECA and gravel for orchids

AskGardening is reader-supported. We may earn a commission through products purchased using links on this page. Learn more.

More and more people are switching from the conventional moss and bark media to inorganic media to grow orchids, especially in a semi-hydroponics setup, to reduce the hassle of regular repotting, the cost of replacing media, and the risk of root rot due to decomposed media.

Which type of inorganic media is the best for orchids?

The best type of growing media depends on your growing environment (e.g. humidity, temperature, ventilation, etc.), your personal circumstances (e.g. watering habits and schedule), and the needs of the orchid species. Normally, the best is to mix two media types of different water retention capacities and topdressed with a small grade, non-porous media to prevent a top dry layer. 

This article will help you make the decision by analyzing the experiment results of 7 common inorganic media (LECA, Seramis, pumice, lava rocks, kyodama, perlite, Synthic) in terms of the changes they bring to the water pH, their water absorption capability, water retention capability, and wicking efficiency.  The size and quality of the media as a factor will also be reviewed. 

There is a more comprehensive guide than the article on LECA and Seramis if you are interested.

1. Types of Inorganic Media

Inorganic media is by definition “inert”, meaning they do not break down over time and can be reused when disinfected, unlike organic materials such as moss and bark.  It is thus misconceived to view inorganic media as unnatural.  In fact, the majority of the inorganic media are made of naturally occurring raw materials.

There are four major types of inorganic media that people use for growing orchids:

  • baked clay (e.g. LECA, Seramis, Kyodama),
  • volcanic rocks (e.g. pumice, lava rocks, perlite),
  • river rocks (e.g. gravel, pebbles), and
  • man-made materials (e.g. “Synthic” or artificial sphagnum moss, styrofoam peanuts). 

In this article, we will focus on baked clay (LECA, Seramis, Kyodama), volcanic rocks (pumice, lava rocks, perlite) and man-made materials (Synthic).

1.1 LECA

LECA is Lightweight Expandable Clay Aggregate, also known as clay pebbles or hydroton.  They are clay heated to a very high temperature so that they expand and result in a highly porous structure. They come in different sizes, ranging from 4mm to 16mm in diameter.

LECA is a popular choice of media for a setup of hydroponics or semi-hydroponics.  Their characteristics are:

  • Their highly porous structure can retain water as well as providing aeration to the roots which are crucial in water culture. 
  • They have fantastic drainage
  • They are economical and are easily available online (e.g. GROW!T) and even at some IKEA stores.

1.2 Seramis

Seramis is similar to LECA as both are made of expanded clay. 

The difference is that Seramis are clay granules, smaller than LECA and are made of clay from a certain part of Germany.  Seramis is hard to find in the USA. I would recommend small-sized LECA or lava rocks as a replacement.

Seramis uses a special technique that results in even more porosity than LECA, making it much more water retentive than LECA.  For more details, see the comparison between LECA and Seramis.

1.3 Pumice

Pumice is a type of volcanic rocks, with a chemical profile similar to granite and rhyolite.  They are created naturally at high heat during a volcanic eruption. 

Pumice is very lightweight and porous and can be found in different sizes. 

However, pumice is quite expensive for being a limited natural resource which needs to be mined from different places around the world. 

1.4 Lava rocks

Similar to Pumice, lava rocks are produced when they are ejected during a volcanic eruption. Lava rocks are also porous. 

The difference is in their mineral composition.  The chemical equivalents of lava rocks are gabbro and basalt, and relatively speaking are richer in calcium, iron and magnesium than pumice.

1.5 Kyodama

Kyodama is clay granules, about 2-5mm big.  It is very porous that can retain a lot of moisture and provide aeration.  They are commonly used and sold as a supply for planting Bonsai.

1.6 River rocks or pebbles

River rocks (also known as pebbles) have smooth, polished surfaces or are not porous, which makes them not wicking in nature. Since they are not wicking, they do not dry out the air roots that are in contact with them by absorbing moisture from the roots. This makes them perfect for top dressing to lock in moisture in the top layer.

They are also heavier than LECA and are thus perfect to give the pot some weight to stabilize the plant.

They are also of different colors, making them decorative as a top layer.

1.7 Perlite

Perlite is naturally occurring, non-toxic volcanic rock glass exposed to high heat to form expanded, porous balls that can absorb many times its own weight of water. 

Advantages for orchids:

  • Perlite can add moisture and also air to the substrate, creating a good air-water balance for healthy orchid roots.

Disadvantages for orchids:

  • As an orchid substrate, perlite is too lightweight and cannot give structure to the substrate.  They float to the surface when regular flushing is done.
  • Being very small in size, perlite is difficult to clean and disinfect for reuse and are easily crushed into dust.

2. Experiment Aim and Method

A huge thanks to Annabel from Orchid Room on YouTube. She did three experiments on a total of 7 types of inorganic media, 2 types of organic media, and in different sizes.  Different manufacturers of the same media type have also been studied, namely Kaizen Bonsai (KB), Lava-Lite (LL), Orchid Garden (OG)

Her readings have greatly helped me to improve the accuracy of this post. Hop over to her YouTube channel to follow her.

A summary of the experiment aims

In Experiment 1, the aim was to study the pH changes brought by the media, water absorption and retention capacity of LECA, Seramis, a cat litter, medium-sized pumice, perlite and Synthic.

In Experiment 2, the aim was to study the pH changes, water absorption capacity of Sphagnum moss, bark (in small, medium, and large sizes), LECA, pumice (in small, medium, and large sizes), perlite, kyodama, and lava rocks.    

Experiment 3 was a continuation of Experiment 2 in looking at the water retention capacity and the wicking efficiency of the media.

The experiment method

  1. First, all the media have been prewashed to eliminate dust (especially for Seramis, pumice) and dried.  Their initial dry weight was measured.
  2. Then the same amount of water was added to each cup to soak the media for 24 hours. 
  3. To determine the pH changes caused by the media, the water used in soaking was poured out and the pH was measured.
  4. To determine the water absorption capability of the media, the water was then drained (not squeezed) and the media was weighed for checking the change in weight.   
  5. To determine the water retention capability, the media were then left in pots uncovered in room temperature of around 65 F (19 C) for 7 days to evaporate away its moisture.  The media were then weighed again to measure how much water has been lost or how much water is still retained by the substrate.
  6. To determine the wicking efficiency of the media, water was then poured into each pot reaching the same level of about 1 inch deep, like a reservoir in a semi-hydroponics setup.  After 7 days, observation was made to see how evenly the water distributed from the bottom to the top of the media. 

It must be noted that the experiment was performed in a plastic cup with no drainage holes on the bottom or sides, and there was also no plant to suck up the moisture.  These external factors will affect the accuracy of the results in real life conditions. 

3. Comparison of Media Properties: Results from Experiment

3.1 Changes in Water pH

PH changes of the water is important as it can affect how soluble the nutrients are and how easily they can be absorbed by the roots.  Here are the results:

 Initial pH of waterpH after 24-hour soak
Sphagnum Moss8.655.66
Bark, small8.655.88
Bark, medium8.656.31
Bark, large8.656.27
Pumice, small (KB)8.658.33
Pumice, medium (KB)8.658.32
Pumice, medium (LL)8.658.45
Pumice, large (OG)8.658.42
LECA8.658.30
Lava rock8.658.41
Kyodama (BG)8.658.13
Superlite black (KB)8.658.17

  • Organic media:

Pros:  As the findings show, only sphagnum moss and bark are within the optimal pH range (5.6 and 6.5) for nutrient absorption. 

Cons:  But, the pH of the organic media will drop further as they decompose which would be too acidic for orchids and would cause root rot.

The problem with the organic media can be mitigated by frequent repotting before they decompose and using new media, but each repot can be stressful for the orchid. 

  • Inorganic media:

Pros:  Inorganic media do not decompose and thus their pH will not reduce over time like moss and bark.  It is easier to adjust the pH of an inorganic media.

Cons:  All the inorganic media are quite alkaline (8.1 and above) which are much above the optimal range of nutrient uptake. A high pH can prevent the plant’s uptake of nutrients, creating a buildup of nutrients in the form of salts on the media, a problem called efflorescence

Media with higher porosity (e.g. LECA, Seramis, pumice, perlite, lava rocks, etc.) tend to have a higher salt buildup, which can lead to fertilizer burn on the roots.

The high pH problem with the inorganic media can be mitigated by performing pH adjustment.

Therefore, in terms of the pH changes that the media bring, it is better to use inorganic media with a pH adjustment, to avoid frequent repotting to lower the stress to the orchid and also to lower the costs of buying new organic media.

3.2 Water Absorption Capability

The results from experiments 1 and 2 are shown in Table B below.  The results from the two experiments can be put together since the amount of water absorbed by LECA in experiment 1 (26%) and that in experiment 2 (28%) are quite similar.  

 (a): Initial dry media weight, grams(b): Wet weight, soaked & drained, g(b) – (a) = (c): Water absorbed, g(c) ÷ (a): Amount of water absorbed relative to dry media weight, %
Experiment 1
Seramis96.2165.369.172%
LECA83.9105.521.626%
Perlite29.896.967.1225%
Synthic7.5110.1102.61368%
Experiment 2
Sphagnum Moss Besgrow51111062120%
Bark, small49964796%
Bark, medium49924388%
Bark, large71982738%
Pumice, small (KB)1121756356%
Pumice, medium (KB)931475458%
Pumice, medium (LL)1601953522%
Pumice, large (OG)951333840%
LECA1211553428%
Lava rock2502904016%
Kyodama (KB)2092554622%
Superlite black (KB)761184255%
Table B: Water absorption capacity of media after a 24-hour soak

These findings show that there are 4 types of media in terms of the level of water absorbency. 

1. Extraordinary water absorption capacity (over 1300%):  This group of media shows an extraordinary capacity to absorb much more than the others.  They are Sphagnum moss that can absorb 2120% or 21 times its own dry weight of water and Synthic (synthetic Sphagnum moss) that can absorb 1368% or around 14 times its own dry weight of water. 

2. High water absorption capacity (above 70%): Media with high absorption capacity are perlite (225%), small bark (96%), medium bark (88%) and Seramis (72%).

3. Medium water absorption capacity (around 40-69%):  Media with medium absorption capacity are small pumice (56%), medium pumice (58%), Superlite black KB (55%), large pumice (40%), large bark (38%)

4. Low water absorption capacity (under 40%): LECA (28%), Kyodama (22%), medium pumice (Lava-lite) (22%), Lava rocks (16%).

3.3 Water Retention Capability

There are two problems with the water retention results. 

First, the numbers from experiment 1 are incomplete (left blank in Table C below). 

Second, there is a discrepancy in the LECA number where in Experiment 1 the water held was found to be only 30% while it was found to be 47% in Experiment 2. 

Asterisks (*, **) have been added to these media names so that caution can be taken in the analysis.

 (d) Media weight after 7 days, g(b) – (d) = (e): Water loss after 7 days, g(b)-(e)-(a) = (f): Water retained after 7 days, g(f) ÷ (c): Amount of water retained relative to water absorbed
Experiment1
Seramis*   65%
LECA*   30%
Perlite*   65%
Synthic*   70%
Experiment2
Sphagnum Moss84277975%
Bark, small80163166%
Bark, medium74182558%
Bark, large88101763 %
Pumice, small (KB)151243962%
Pumice, medium (KB)126213361%
Pumice, medium (LL)176191645%
Pumice, large (OG)113201847%
LECA**137181647%
Lava rock267231743%
Kyodama (KB)232232350%
Superlite black (KB)95231945%
Table C: Water retention capacity of media after 7 days of evaporation in room temperature

These findings show that there are 2 types of media regarding their water retention capacity.

1. High water retention ability (above 50%):  Sphagnum moss (75%), Synthic* (70%), Small bark (66%), Seramis* (65), Perlite* (65%), Large bark (63%), Small pumice KB (62%), medium pumice (KB) (61%), medium bark (58%)

2. Low water retention ability (below 50%):  Kyodama (KB) (50%), large pumice (OG) (47%), LECA** (47%), medium pumice (LL) (45%), Superlite black (KB) (45%), Lava rocks (43%), LECA* (30%)

3.4 Wicking Efficiency

 Wicking efficiency (high / medium / low)Dry top layer?Molding?
Sphagnum MossHigh, wet to the topNoMold on top layer
Bark, smallMedium, wet almost to the topA little bitNo
Bark, mediumHigh, wet to the topNoMold on top layer
Bark, largeMedium-low, wet almost to the topYesNo
Pumice, small (KB)High, no water leftNoNo
Pumice, medium (KB)highNoNo
Pumice, medium (LL)MédiumA Little bitNo
Pumice, large (OG)Medium, wet to the topA little bitNo
LECAMediumA little bitNo
Lava rocksHigh, wet to the topNoNo
Kyodama (KB)High, wet to the topA little bitNo
Superlite black (KB)Medium, wet to the topYesNo

All the media are generally efficient in wicking water from the reservoir to the top.  

Large bark and LECA are not very efficient in wicking water from the bottom to the top.  As water evaporation is faster than water wicking efficiency, a dry layer is formed at the top. 

It must also be noted that although the moss and bark can wick water up to the top, staying wet throughout the 7 days or too long would accelerate decomposition, thus losing its structure and becoming suffocating for the roots.  Also, a wet, decomposing organic media would become very acidic, causing orchid roots to rot.

3.5 Media Size Matters

The smaller the media size is, the higher the water absorption capacity and the lower the water retention capacity.

As for water absorption (Table B), small bark absorbed 96% of its own weight in water, medium bark 88%, while large bark only 38%.  Similarly, small pumice retained 56% of the water absorbed while large pumice only 40%.

As for water retention (Table C), small pumice retains 62% of the water absorbed, more than what large pumice could retain (47%).

Such a difference in properties may be due to a larger surface area

3.6 Quality matters

As shown in results in Table B, the properties of the same media type and size also depend on the manufacturing quality.

The medium-sized pumice produced by the company Kaizen Bonsai absorbed more water (58%) and retained more water (61%) than the same-sized pumice by Lava-Lite in the water absorbed (22%) and the water retained (45%).   

This may be because the same media produced by different manufacturers may end up with different porosity.  And as we know, the more porous the media is, the more water is absorbed and retained.

an orchid medium topped by lava rocks

4. Best Inorganic Media for Growing Orchids?

Choosing the right media for your orchid involves three steps.

4.1 Know the behavior of the media in your environment

The first step is to understand the water absorption and retention properties of your media of choice and how they behave in your growing environment (e.g. humidity, temperature, ventilation, pot size, pot materials) and personal circumstances (e.g. your watering habits, watering schedule). 

For example, in this study, LECA that has been found to have low water absorption and retention capacities may be considered perfect for a low-temperature, humid environment.  But, a LECA-only environment, especially the top layer, may dry out too fast, damaging young roots and desiccating orchids in a hot, dry environment.  

4.2 Know the needs of your orchids

The second step is to understand the special ventilation and water needs of the orchid species, the orchid’s age and health conditions, and match those with what a media can provide in your environment.

  • Warm growers (e.g. Cattleya, Phalaenopsis) which have thicker roots prefer drying out between watering and require more gas exchange around the roots and less moisture.  The medium for them should not be too small because if the spaces in the middle are small, they can be filled when water is held in between the particles by surface tension, blocking the gas exchange pathways.  Because of that, they may do well in chunky media such as LECA, medium or large-sized pumice.
  • Cool to cold growers (e.g. Miltoniopsis, Oncidiums) which have thinner roots enjoy lots of moisture.  A smaller grade and more water retentive media would be ideal for them because they enjoy less gas exchange but more moisture around the roots.  A small grade substrate will also help stabilize the fine roots in the pot.  Alternatively, a drying substrate like LECA covered by a top layer of lava rocks or small pumice would also be ideal.
  • Young seedlings and rootless or leafless orchids in an ICU setup would need more moisture around the base of the stem and would appreciate sitting on top of a moist media that does not dry out, for example by Sphagnum moss or Synthic (synthetic Sphagnum moss).

4.3 Mixing multiple media types

The third step is to experiment with mixing media with different properties together. The goal is to let each type bring desirable characteristics to the mix and mitigate the shortcomings to suit different orchid needs. 

If you are not sure what to mix, experiment with a 50/50 blend of a more water-retentive media type or a smaller grade (e.g. Seramis, small pumice, small-grade LECA, Synthic) with a less water-retentive media or more chunky media (e.g. large LECA, large pumice, lava rocks) and adjust the ratio according to factors of the environment and the orchid species. 

It is best to put the chunky media at the bottom of the pot, and a smaller grade media in the middle and the top layers.

If there is a problem with a dry top layer, topdress with fine-grade and non-porous media such as small river rocks, horticultural grit, gravel to retain more moisture at the top.

Conclusion

To sum up, based on the results of an experiment, the media with high and medium water absorption and retention capacity are:  Sphagnum moss, Synthic, small bark, Seramis, Perlite, large bark, small and medium pumice.  The media with low water absorption and retention capacity are: Kyodama, large pumice, lava rocks, LECA. 

The smaller the media size is, the higher the water absorption capacity and the lower the water retention capacity.

Quality in the manufacturing process may also affect the porosity of the media, affecting its water absorption and retention properties.

However, the media may behave differently in different growing environments.  The idea is to find a mix that can suit the growing environment, the gardener’s watering habit and circumstances, and the ventilation and moisture needs of the orchid species

Happy growing!

Related

Orchid Nutrition and Fertilizer: A Complete Guide

Self-Watering Pots For Orchids? Pros vs. Cons (+DIY Tips)

Can You Reuse Sphagnum Moss? (Explained)

Photo credits

“Perlite für Zimmerpflanzen” by blumenbiene is licensed under CC BY 2.0

“Airfall pumice (~1065 A.D. eruption of Medicine Lake Volcano’s Glass Mountain eruptive center, northern California, USA)” by James St. John is licensed under CC BY 2.0

“Gravel Texture” by wwarby is licensed under CC BY 2.0

Any questions and comments?  
Join our community of plant lovers
Scroll to Top