Cultivating Orchids & Crafting Terrariums

7 Reasons Pumice is An Excellent
Orchid Potting Medium

When looking for a good potting medium for orchids, there are several different types of choices. Several examples are orchid bark, charcoal, coconut fiber, river rock, Styrofoam peanuts, and so many others. Among those varieties of media, you might have heard of pumice. What is pumice? What does pumice do for my orchid? How is pumice different than perlite?

Pumice is an excellent potting medium for orchids, provided it is combined with other media as well such as orchid bark and charcoal. Being heavier than perlite, pumice will maintain more stability in the orchid pot. Pumice also provides fast drainage for water, absorbs water, and raises airflow inside the orchid pot. Overall, pumice should be added to your formula of potting media.

Pumice
Image Credit:  "Dacite pumice (15 June 1991 eruption of Mt. Pinatubo, Luzon Volcanic Arc, western Luzon Island, northern Philippines) 2" by James St. John is licensed under CC BY 2.0

 What is Pumice?

Pumice is a rock made from the quick cooling down of magma after the volcano erupts.

When the magma cools at such a fast rate, (sometimes after the magma is shot into the air, it cools and forms its solidified state even in the air before it reaches the earth’s soil again) the atoms couldn’t crystallize in time.

The sudden cooling forms a highly porous, foam-like rock—technically it’s glass—which has an extremely rough surface.

This glass stone is filled with pores that were formed by the gas trapped inside the magma with no way to the surface. The fine walls in between these gaps of air are minuscule, making the stone incredibly light-weight for its size.

The air that is trapped inside of the rock makes the pumice float. Over a long period of time, the pumice will eventually sink as water finds its way in between the thin walls, but for the most part, pumice can float for years.

There are several types of volcanic “left-overs” and pumice is one of them. It is a grayish-white, sometimes yellowish-brown color.

This is what makes it different than perlite, which also is a by-product of the volcanic activity, but is pure white. Perlite also is more lightweight, having a negative specific gravity.

They can be used interchangeably, but pumice has an advantage over perlite when it comes to orchid care: pumice will provide taller orchids with longer, dangling roots the stability they need inside the pot. This is just because pumice weighs a tad bit more than perlite.

How is Pumice Used Outside of Orchid Care?

Pumice is used in horticultural centers mainly for bonsai growers and succulent planters.

Outside of the garden and flower niches, pumice is used for the most extraordinary things. Since it has a rough, abrasive texture to it, pumice is mainly used for items that need traction or abrasion. (Source)

For example, the white patches in your blue jeans are made by white-washing the denim with pumice. The “stonewashed” material then lightens the color and roughens the texture.

Many beauty products will use pumice, especially as a light abrasive. Liquid soaps add pumice to the formula to exfoliate the skin. Extensive facial treatments have pumice as one of the main ingredients, since the rough surface provides the exfoliation that is needed.

Even pencil erasers also have a slight percentage of pumice in them. This is to provide the mechanical friction strong enough to get the math mistakes of your kid’s homework. If only life’s mistakes were as easy to erase…

When it snows, pumice provides the extra traction needed to stay on the road. This can be achieved in two ways: either by sprinkling pumice on the driveways, sidewalks, and roads; or by adding pumice directly in the rubber when fabricating tires. The idea behind using pumice in these places is to provide the extra traction that is needed since the rough surface and highly abrasive surface will increase surface tension.

…and to the last example, since I don’t want to bore you with details that aren’t orchid related…

Pumice is also added to the clay in pottery. Sometimes the clay mixture is so heavy that the pot becomes extremely weighty to handle. I say pot, but it is technically anything you are crafting with clay. The pumice is so lightweight that it adds volume without making the clay pot hefty.

How is Pumice Used in Orchid Care?

This is where the article gets interesting—pumice in orchid care. From the characteristic of pumice, you can already guess that the first positive side of using pumice is that pumice provides a higher level of airflow in the pot.

1. Being extremely porous—those pores are called vesicles—the air gets sucked into the pumice easily. The surface area of the pumice is so filled with tiny holes that the roots have contact with mostly air.

2. Since pumice is a non-organic material, it will not break down as orchid bark will. Let me add a huge parenthesis here: there are not so good pumice brands that will break down in 1-2 years. These are what most sellers sell here in the USA and in Canada. The best pumice you can find is from Japan, which will have three ingredients: kanuma, akadama, and satuma. I’ll explain more about that in a little bit.

3. Pumice absorbs water without swelling. Since it is so porous, the microscopic holes will allow both air and water into them.

4. Pumice also allows for fast drainage. When watering orchids, which happens to be the most difficult part for orchid care in the beginning, pumice will allow the water to exit the pot quickly. The bigger the pumice, the better it is for quick drainage. When you combine both number 3 and number 4, you have the perfect orchid potting material. 😊 Something that absorbs water but also allows water to flow through the pot quickly.

5. Pumice is lightweight. This is an enormous positive point because, with just one orchid, your shelf won’t budge. But as time goes by, and orchids become more of an addiction and obsession, that shelf gets loaded quickly. The heavier the orchid pots, the more the shelf bends. Being lightweight, the shelves are safe.

6. Pumice is also heavier than perlite is, which provides stability inside the pot. Some orchids have long, dangling roots, like Cymbidiums and Phragmipediums. They will prefer a pot that is straight and tall, more of a vase-type pot than a regular, short, fat, flower pot. Since these are taller orchids, or top-heavy, they have a huge tendency to fall over.

Some Phalaenopsis orchids like to relax and lay off to the side, tipping the vase to one side. The unbalanced pot then has a tendency to tip and fall. If you have household pets, then they fall over easier. Perlite has the same structure as pumice but is so lightweight that the pot topples easily. Pumice is a tad bit heavier and can provide more support.

TIP: If your orchid has a tendency to fall over, use two methods that have helped me in the past.

1) You can use decorative rocks and lay them on the opposite side of the potting medium. This is especially good if you have a Phalaenopsis orchid that tends to lean. The rocks provide a top counter-weight that supports the orchid from above.

2) Use short, fat candles and prop the orchid pot up against them. Not only does this method make a prettier orchid shelf, it also still allows light and air to circulate. Given that the candles are totally surrounding the pot, and just propped up to one side, the orchid still can receive adequate airflow and light. It’s also an easier method to water since there is no moving and relocating rocks up and down.

7. Pumice doesn’t decompose since it is non-organic. Other potting media such as orchid bark and coco fiber will decompose in time. The material crumbles, crushing the roots, raising the pH in your pot, and critically lowering airflow. Water has a hard time exiting the pot, as it pools up inside.

What are the Cons of Pumice as an Orchid Potting Medium?

Pumice floats. If you water your orchid by submersion, then beware. You’ll have pumice all over the place. I prefer to use pumice mixed in with other potting material for this reason. If you have orchid bark (which also floats to some degree, but not like pumice) sphagnum moss, and charcoal, the pumice will be trapped inside the pot and not overflow with the running water.

Pumice has an awful habit of building up minerals quickly. Remember when I said that pumice is excellent for absorption and for drainage? The water will enter the pot, attach itself to the rough surface of the pumice or get pulled into the air pores, and there it will stay until released.

Depending on the water you use or the fertilizer that you add, the mineral build-up will quickly be absorbed by the pumice. If you’re not sure what is the best water for your orchid, then read this article where I go over rainwater, reverse osmosis water, distilled water, and others. Depending on what kind of orchid you have, this can be critical.

The third and main reason you probably shouldn’t use pumice is that since its rough surface area, the delicate velamen on the roots will attach so firmly that the repot is a frustrating, disheartening, and unnerving experience.

The roots attach themselves so strongly to the rough surface of the pumice that you cannot pull them off in one piece. This painful and thwarting sight of seeing all the roots break off one by one is to the least, discouraging.

This is the main reason why I do not use only pumice as potting media. Always add something else to the media, either orchid bark, charcoal, sphagnum moss—anything.
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Are there Different Types of Pumice?

There are better quality pumices and lower quality ones. In the continental United States, the main pumice sellers are located in Oregon. That whole area thousands of years ago was extremely volcanic and pumice is everywhere (kind of near where Yellowstone National Park is now).

The states of Nevada, Idaho, Arizona, California, New Mexico, and even Kansas (Uhuu!! Go Kansas) have large pumice sellers from old volcanic sites. The main target for these sellers are bonsai growers and a few succulent growers also.

Outside the United Sates, the best pumice is from Japan. In fact, it’s even better than the American one. In Japan, the pumice is a bit different, containing three different elements (not really elements, but as ingredients): kanuma, akadama and satuma.

Kanuma is a hard substance that provides the right stickiness so the roots can adhere to the pumice.

Akadama is a baked product that helps the pumice retain moisture.

The satuma part is what provides the holes and air into the pumice. (Source)

These three materials make the Japanese pumice better in quality than the other brands. As of the time of this article, I have not found a seller that ships to other countries. I’m still looking.

Some Japanese pumice brands that have only one or two of the three materials listed above. These will last around 1-2 years, being of lower quality. These will break down over time. They also are very expensive, which does make me want to bring up the next point.

In all, would I buy Pumice for my Orchid?

Pumice, no doubt, is a marvelous product. The only downside is that I can’t get access to it at a reasonable price. If I lived in Japan, I could, but so far, that isn’t in my foreseeable plans.

Because of the simple fact that by the time I pay shipping and handling, perlite comes out as a cheaper product compared to pumice. It has its pros and cons, too, but since they are so similar, I prefer to go ahead and use perlite. The last time I bought perlite, I bought CZ Garden Supply (Affiliate Link) and so far, I’m happy with it. I have also bought smaller bags from Miracle-Gro (Affilaite Link). Both results where about the same. Miracle-Gro is a bit smaller-sized chunks.

Perlite has almost the exact same qualities as pumice, being the downside is it’s more lightweight.

With all the advantages of using perlite, I can’t pass it up in the potting media I use. I always add charcoal, a tad bit of sphagnum moss, orchid bark, and depending on the kind of orchid other little ingredients to cater to their specific tastes. I wish I had a better way to finish this article, but that’s my honest opinion.

I wish you the best in your orchid care and hope this article provided some answers to your questions. If it did provide value, please comment below. Just a reminder, I do not publish comments when your username is a URL, a link, a seller, or if I think it is spam.

Happy Cultivating!

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