Orchids are famous for living on the sides of trees in the wild. But what about indoor orchids? Do they need potting mix, or would they do better without it?
Do orchids need potting mix? Potting mix helps secure the orchid in place, and provides the orchid with water and nutrients. It is possible to keep an orchid without potting mix, but it involves somehow providing it with a steady stream of water, either through a dish or through constant spraying. There is no advantage to keeping an orchid without mix compared to keeping it in a suitable mix like coconut bark or sphagnum moss.
The guide below first explains why store-bought orchids should be kept in mix, even if many wild orchid species don’t grow in soil. It will then list the different kinds of mix you can find for sale, and examine scientific studies on which orchid potting mix is best.
Do Orchids Need Potting Mix?
Orchids can survive without potting mix, but that doesn’t mean it’s a good idea not to use one.
No statement can apply to every single orchid species. The Orchidaceae, as the family orchids belong to is known, is one of the two largest groups of flowering plants in the world. There are currently 28,000 species recognized worldwide, and still many more to be discovered. All of this is to say that it’s pointless to generalize about what ‘all orchids’ like.
What is true, though, is that many common species of household orchid can survive without potting mix. Wild Phalaenopsis orchids, for example, grow on the sides of trees. They don’t take nutrients from the tree itself, like some other plants do. Rather, their roots sit on the surface and absorb moisture from the rain and from the mosses around them. There’s no soil to speak of.
For the purposes of home care, though, I would recommend always using potting mix. The alternative is to spray your orchid’s roots very frequently, which is a time-consuming job that offers no better results than a potting mix. Well-known growers have found ways to keep orchids hydrated without the use of potting medium, but these aren’t recommended as a permanent way to keep orchids.
Do Wild Orchids Grow in Soil?
At the same time, though, other orchids do grow in soil—or at least in the ground. Many species like sandy soils with fast drainage, which offers retained moisture, but can dry out more easily than other soils like peat.
Phalaenopsis orchids are one of many kinds that grow on trees. They produce very fine, very light seeds that are distributed on the breeze. These seeds have to find small cracks in the bark of nearby trees, where they nestle in a mix of fungus and moss that provides them with the moisture and nutrients they need. Cattleyas do the same thing. Orchids that do this are known as ‘epiphytes’, which mean they grow in air (with a little help from the trees they cling to).
There are some orchids, though, that grow in the ground. These are known as terrestrial orchids. Paphiopedilums and some cymbidiums grow in soil in the wild. As stated above, these orchids need free-draining soil so that moisture won’t make their roots rot.
Why Do Orchids Need Potting Mix?
For the purpose of keeping orchids at home, I would always recommend using a potting mix. Keeping orchids without mix would be far less convenient and easy.
The first reason why is that mix holds onto water for the orchid. Orchids need water like any other plant does, as it serves several key functions: it gives leaves and roots volume and structure, and is necessary for photosynthesis, which is how a plant creates the sugars it needs for energy. Plants take in water through their roots from the soil or mix they’re in. It’s possible to keep some plants without soil or mix so long as you keep the roots damp some other way, like by spraying them all the time, but that’s nowehere near as convenient as using a potting mix.
Besides that, soils and potting mixes contain minerals that a plant can use. It takes in these minerals in the same way as it takes in water, through its roots. Some mixes like perlite don’t provide these minerals, but most do: any kind of soil, coconut chips or tree barks all do.
Which Orchid Species Need Potting Mix?
I keep Phalaenopsis orchids, and it would be difficult to keep them if I didn’t have a mix to keep them in. I imagine I could keep them in ceramic, water-absorbing pots like in the video above, but I doubt it would go too well.
Aside from phals, I really think that every indoor orchid species would benefit from being kept in mix, at least compared to being kept without it. That includes cattleyas, cymbidiums, miltonias—anything, really. While there can be a risk of rot, it’s easier to avoid that with your orchid in mix than it is to keep your orchid hydrated out of its mix.
What Kinds of Potting Mix Can You Use for Orchids?
There are many different kinds of potting media that people use for their orchids. Which one a person uses depends largely on how widely available it is where they live, how cheap it is, and how well it seems to perform.
Fir bark is tree bark, collected from fir trees. It first started seeing widespread use in the mid 20th century, in the 1950s and 1960s. Like other kinds of potting mix, it’s sold in different grades, meaning different sizes. A fine-grade bark mix is one with smaller pieces in it, with medium-grade and coarse-grade mixes having larger pieces and the largest pieces respectively. The bark is clearly woody, each chunk being made up of large fibers. While each chunk is solid, it isn’t sharp and hard like other kinds of wood might be, and can break up if you apply lots and lots of pressure to it (unless it’s very dry).
Which grade you pick does make a difference: finer grades hold onto more water and let in less air than coarser grades. Finer-grade mixes stay damp for longer. Which of these you prefer depends on the orchid species you grow, the humidity in the room you keep your orchids in, and how much water they like to have.
If you plan on using fir bark, or any kind of similar woody mix like coconut bark mix, you should soak it before you use it. That’s because it resists water when it’s dried out. You can see this in action when you water your orchid with bark mixes that haven’t been soaked beforehand: they will look wet, but will dry out thoroughly after only a day or two. This is a problem because each piece of bark is still dry on the inside when you water your orchid, meaning that they won’t offer your orchid’s roots much water. You’ll therefore have to water your plant again sooner than normal.
Another problem with fir bark is that it eventually decomposes. It’s obviously made from an organic material, and over time, organic materials break down and degrade. That’s especially the case if there’s bacteria, fungus or water around, as there very well may be in your orchid’s pot. Most people find that it takes about 18 months to two years for fir bark to break down, at which point it has to be replaced. This isn’t a major issue because repotting an orchid is easy enough, but it’s something you have to be aware of.
Sphagnum Moss/Peat Moss
There are a few kinds of moss that people use to pot orchids, the most common being sphagnum moss. The two chief kinds are German peat moss and Canadian peat moss, sphagnum moss being another name for the latter. The kind you typically see in stores is sphagnum moss, and is usually clearly labelled.
Sphagnum moss can be milled into a finer consistency, or it can be bought essentially untouched, organic, in large chunks. It can either be added to soil mix to help it hold water, or in our case, used as the entire potting medium. You can find sphagnum moss very easily through garden centers or online.
To be clear, peat moss isn’t the same as peat. Peat is probably the worst thing imaginable for potting orchids. It’s a kind of soil that contains lots of clay, so it’s claggy, thick, and retains water very well but doesn’t allow for air circulation. If you potted your orchids in peat, they would quickly rot. You want peat moss, which is, well, moss, which grows in bogs; peat is a kind of soil that forms from decayed plant matter like peat moss.
Perlite and Vermiculite
Perlite, believe it or not, is a kind of volcanic glass. It’s heated to 1,600 degrees F. (or 871 degrees Centigrade), which is twice as hot as the surface of Mercury. But that’s not the end of perlite’s weirdness: when it reaches this temperature, it pops like popcorn, expanding outwards to 13 times its former size.
That’s why perlite is so lightweight, and so able to hold onto moisture. It’s filled full of tiny air pockets, like a bread that’s risen in the oven. Water can fill these gaps, and is then released slowly. That’s why perlite is a common additive to soils, as it makes them better able to retain moisture without making them denser. You may have also heard of vermiculite, which is a very similar substance, but is slightly more dense; perlite drains more readily.
The only problem with perlite and vermiculite is that they don’t contain any nutrients. Soils and barks are organic substances, so have minerals in them that the orchid can make use of. Perlite and vermiculite, while they are natural materials, don’t contain minerals that a plant can absorb. This can be remedied by ensuring that you feed your orchid lots of orchid food. Plus, this isn’t typically a problem, as perlite and vermiculite are better used as additives to a potting mix rather than the entirety of the mix.
Which Potting Mix Is Best?
Some people swear by one kind of potting mix or another, and it’s true that most mixes bar soil are suitable for an orchid. But as for which is best, that’s a matter of debate.
Since orchids are such a widely studied kind of plant, there have been a few scientific studies that look specifically at which potting medium they prefer. The sections beloow look at some of these studies, which potting mixes they recommend, and why.
Which Potting Mix Should I Use for My Orchid?
One particular scientific paper, published in the journal Data in Brief, did exactly that. The scientists behind the study grew two different kinds of Dendrobium in five different growing media: tree fern fibers, coconut fibers, sphagnum moss, asplenium root, and calliandra humus (soil made from calliandras). They assessed how good each growing medium is by measuring the plants’ height, leaf lengths, leaf widths, number of leaves, and number of shoots—all good ways to measure the health of an orchid. The scientists had six of each kind of plant, which isn’t a large sample size, but can certainly offer suggestive results that are worth taking note of. They bought the plants, repotted them into their respective mixes, and measured them several months later.
They found that there were considerable differences in how well each growing medium performed. Here are some tables which show the data they collected:
This table shows the average height of the plants in each potting mix. The plants grown in coconut chips and tree fern mix did the worst, while those in sphagnum moss did relatively well. Kaliandra humus was a good choice for the hybrid species, but only an average choice for the sylvanum.
This table looked at the average leaf lengths of the plants. The results are basically the same: sphagnum moss and kaliandra humud are the top performers, while coconut chips aren’t so great.
Here are the final three tables, which show basically the same results. Coconut bark fares badly, which is surprising, considering how popular it is!
They also found greater differences between each growing medium depending on the species of orchid. For the purpose of the experiment, they measured two Dendrobium genotypes: Dendrobium sylvanum and a Dendrobium nindii x Dendrobium stratiotes hybrid. They found that the hybrid plants were more greatly affected by the medium they were potted in. So for example, while the Sylvanum had an average of 10 leaves in coconut chips and about 12 in the sphagnum moss, the hybrid had an average of around 10 leaves in coconut bark but more than 14 in moss. This suggests that you may find greater or lesser differences than those seen here depending on the species of orchid you keep.
Why Are Some Potting Media Better Than Others?
This is a question that the study doesn’t address in detail, but there are obvious reasons why some mixes are better than others. Some are more porous, allowing more air to circulate around the roots. Others hold onto more water than others. Here’s a table from the paper that lists these differences:
What’s interesting is that there’s no pattern that jumps out very clearly from these data. But the media that perform well tend to hold lots of water and allow lots of air around the roots. This may be slightly surprising, since so many orchids live outside of any kind of medium, meaning that coconut chips would, if anything, be more similar to the orchid’s natural environment. On the other hand, the sphagnum moss you use to pot an orchid is quite similar to the moss and other plants that grow around an orchid’s roots on the side of a tree.
Can You Pot Orchids in Regular Soil?
I would recommend against potting orchids in regular potting soil, i.e. the kind you would use to repot other house plants. This kind of soil holds onto water too well to be good for an orchid. While orchids benefit from some water retention in their potting mixes—which is why sphagnum moss works well—heavy soils can hold onto much more water than moss.
This is backed up both by experience and scientific study. Another study we found, this one in Horticultural Plant Journal, looked at a few more potting mixes than the last one. The scientists again looked at how well Dendrobium plantlets thrived when potted into all sorts of mixes, from soil to crushed brick, styrofoam, sand and more. They measured how well the plants did based on how many of them survived, because this study was looking at how best to grow Dendrobiums for maximum profit (and the more that survive, the more money you make). They looked at dozens and dozens of potting mixes, and found the survival rates for some to be drastically different to others:
- Vermiculite: 95% survival rate
- Sphagnum moss: 100% survival rate
- Charcoal and broken rock: 100% survival rate
- Coconut fiber: 100% survival rate
- Sand, soil, brick pieces and charcoal pieces: 60% survival rate
- Compost, coir pith, bricks and charcoal pieces: 56% survival rate
- Brick, charcoal and decaying litter: 71% survival rate
Matters are complicated somewhat by the fact that the plants were often kept in different conditions, e.g. in greenhouses, in plant growth chambers, and at different levels of temperature and shade. But there was a clear correlation between denser media like compost and sand, and lower survival rates.
All of this is to ignore the fact that any book or experienced grower would tell you not to use regular potting mix. In my experience, most shop-bought orchids come in thicker soil, and their roots are typically white and somewhat rotten. That’s what happens when they stay wet for too long, which in turn is caused by thick media and/or overwatering.