What is perlite? How to use perlite when growing indoors

Gardners, growers and farmers have been familiarizing themselves and experimenting with perlite for quite a while now, but have you ever wondered what is perlite, and what is its application in indoor growing. Check out this article for more information on how to use perlite when growing indoors!

perlite in soil

What is perlite?

Perlite is a growing medium used all across the globe for a variety of purposes. It is a sterile and extremely light material, which is why people compare it to styrofoam and similar materials - but perlite is actually quite different from styrofoam and doesn’t share almost any physical and chemical properties with styrofoam.

Perlite usually comes in 4 different types and sizes:

  • Fine grade
  • Medium grade
  • Coarse grade
  • Super-coarse grade

Aside from being different in size, these types of perlite also have different properties and water holding capabilities. Fine grade perlite is smallest in size, and it is able to hold the largest amount of water among the 4 types, while super-coarse grade perlite has the lowest capacity for holding water and creates a lot of drainage.

Coarse perlite is considered to have the best balance of drainage and water holding capacity, making it the favorite among gardeners and farmers. It is considered to be an all-purpose medium.

Medium grade perlite is best used for mixing with soil and it often comes in premade potting mixes with other components, such as soil or coco. It can hold about 46% of the water, and some growers are known to use it as a standalone medium.

What is perlite made of?

Perlite is made from expanded volcanic glass rocks, which is melted at around 1000°C. At that temperature, the glass rock is transformed due to extreme heat, and it breaks down into small granular pieces. This process happens when the water trapped inside the rocks is vaporized, which causes the rock to expand up to 10x compared to its original volume, and finally bursts into shiny white pearls.

This process makes the perlite completely sterile and pH neutral. Combine that with its natural tendency to absorb water and create drainage - and you get a perfect medium for mixing with soil. So, if you ever wondered what is perlite made of - now you know!

What is perlite used for?

Now that we know what is perlite made of, let’s go over what is perlite used for. We’ve already mentioned that perlite is mainly used for creating drainage in potting mixes. However, that’s not its only use.

Many growers use perlite as a sole growing medium when transplanting cuttings and seedlings. This is done mainly using fine-grade perlite, which has great water-holding capabilities.

Another reason why you may want to grow your seedlings and clones in perlite is that they are significantly easier to move into pots once they start rooting properly. Growing them in soil from the start will make it so that your plants root into soil quite firmly, making them harder to transp
lant later on. The last thing you want to do is hurt your plant’s roots.

What are the disadvantages of perlite?

The most commonly mentioned disadvantage of perlite is the fact that it can often be fine that some parts of your perlite will just be white dust. This dust can create respiratory problems as well as eye irritation. You can avoid this by dampening your perlite before pouring it into your pots and soil.

The other main disadvantage, some may consider it an advantage though, is that perlite is not biodegradable. Once you put it in soil, it’s there to stay, and there isn’t an easy way to get it out aside from washing off all the soil with a hose or similar. This makes it quite a poor medium for growing outdoors, as you wouldn’t want perlite staying in your garden longer than necessary.

Perlite substitutes

There are many products out there that provide the same function as perlite, however, many would say that none of them are actually better than perlite. If you find yourself in a situation where you simply can’t buy perlite, and you have to have some sort of growing medium to provide drainage - here are a couple perlite substitutes that will do just fine!

Perlite vs vermiculite

Vermiculite is very similar to perlite. It is a hydrous phyllosilicate mineral that undergoes rapid expansion when heated, just like perlite. Vermiculite is very good at holding water, but it provides little to no drainage when compared to perlite. It is very well suited for plants that require a lot of water retention, as well as for plants that are resilient to root rot.

perlite vs vermiculite
source: gardenbetty.com
Keep in mind that vermiculite also has an adverse effect - it tends to slightly raise the pH values of your growing medium even though its natural pH value is around 7.0 so you might want to add a pH adjuster to your shopping list as well.

PBH rice hulls

PBH rice hulls - short for parboiled rice hulls are a product most commonly used as a top-dressing material for both indoor and outdoor plants, but it is most commonly used with nursery plants. It is cheaper than perlite and vermiculite and doesn’t produce any dust, unlike perlite.

In the process of making them - PBH rice hulls are boiled in order to sterilize them completely. Unlike perlite, rice hulls decompose naturally, however, this happens over a longer time period (1-2 years) so you definitely won’t run across that issue in your time growing since most strains take only 3-4 months from seed to harvest.