Drainage for Cannabis Plants: Why is it Important and How To Improve it

We discuss the ways to prevent the soil in your cannabis garden from not drying out or, vice versa, drying too fast
06 September 2021
7 min read
Drainage for Cannabis Plants: Why is it Important and How To Improve it

Contents:
  • 1. What to put in the bottom of indoor planter for drainage
  • 2. Perfect mediums for cannabis growing
  • 2. a. Natural topsoil vs soilless mixes
  • 2. b. Coco coir
  • 3. Choosing a grow pot for better drainage
  • 4. Cannabis soil conditioners/amendments
  • 4. a. Perlite
  • 4. b. Vermiculite
  • 4. c. Other cannabis soil conditioners
  • 5. Drainage for cannabis plants is a fairly easy concept

It is a rare kind of plant that likes its roots to sit in stagnant water. Shoreline plants make an exception, but cannabis is definitely not one of them. It’s not that weed plants don’t like water. On the contrary: cannabis grows very fast and thus is quite a heavy drinker. But its roots also need a lot of oxygen, and that’s the reason drainage for cannabis plants is so important.

To improve the drainage for cannabis plants in your indoor garden, you should start with a right growing medium, then choose a type of container that provides the optimal aeration, not forget about drainage holes, and finally, amend your medium with soil conditioners, such as coco coir, perlite, and vermiculite.

But first, let’s briefly touch upon such a controversial subject as using a drainage layer for pots.

What to Put in the Bottom of Indoor Planter for Drainage

To this day, many horticulturalists have no doubt that putting terracotta shards, coarse sand, expanded clay pebbles, or something similar in the bottom of a planter is a good idea. They call it a drainage layer for pots, thinking that it can actually help remove the excess water from the medium. In fact, it doesn’t. Experiments have shown that the water drainage remains exactly the same — with such a layer or without it.

 

Drainage for Cannabis Plants: Why is it Important and How To Improve it - Drawings of Potted Plants, one without a drainage layer, the other with a layer of gravel in the bottom

Gravel or any other such 'drainage layer' don't actually improve the drainage.

 

This seems to contradict common sense, but there is actually a very good explanation for this phenomenon. When you water your potted plant, gravity pulls the water down, and some of it will leak through the drainage holes. But not all. There is a force that works in the opposite direction to gravity, meaning upwards. It’s called capillary action, or wicking ability, and makes water climb up between the particles in the medium.

When gravity and capillary action have balanced each other out, the water level settles, and that’s the end of it. It doesn’t matter what sits just beneath the medium — the bottom of the pot or the ‘drainage layer’. The only thing that water retention of the medium depends on is the coarseness of the medium.

You can also think of it as aeration, meaning the presence of air pockets between medium particles. The coarser the medium, the less water it can hold, the better its drainage. And vice versa.

Drainage for Cannabis Plants: Why is it Important and How To Improve it - Drawing comparing water holding capabilities of a fine-grained medium and the same medium but with coarser grains mixed in

Finer-grained medium can hold more water than the same medium but with coarser grain.


And with this myth out of the way, let’s talk about what’s really important.

Perfect Mediums for Cannabis Growing

Since cannabis roots like to drink and breathe, the best growing methods are hydroponics and aeroponics — because they provide both water and oxygen at the same time. In hydroponic systems such as DWC (deep water culture), roots constantly float in a nutrient solution but it’s also constantly oxygenated with a water pump. In aeroponics, the situation is exactly reversed — roots are hanging in the air but get constantly misted with a nutrient solution.

With other growing mediums, such as soil or coco coir, you’ll have to take extra measures to ensure good drainage. Let’s start with soil.

Natural Topsoil vs Soilless Mixes

For an indoor grow, never use topsoil from your garden. Besides the risk of infestation with pathogens and pests, topsoil tends to be too compacted and to retain too much water. It’s because there is too much clay and silt in it. Your weed plants will simply suffocate in such a poorly drained medium.

In theory, you can try and use some amendments to make topsoil airier, but it’s best to skip natural soil altogether and use a soilless mix. You can find those in any gardening center. They are sold in bags and look very much like soil, but, in fact, they consist mostly of peat moss. Soilless mixes have a lot of coarse particles and fibers which makes them much airier. Some contain amendments like perlite (see more below) which create even more air pockets and make the drainage even better.

Coco Coir

Coco coir is a mass of fibers made from coconuts. It’s one of the most efficient growing mediums for cannabis. Another great advantage over soil is that coco is reusable. However, coco in itself is a medium with rather poor drainage characteristics. To make it perfect for marijuana cultivation, you need to use a mix of coco coir and perlite.


Drainage for Cannabis Plants: Why is it Important and How To Improve it - A gardener wearing gloves adds a soil-perlite mix into a growing medium

The ideal coco/perlite ratio for the purposes of aeration and drainage is around 70-30.


If you look at ready-made mixes available in grow shops or browse through coco grows online, you’ll find that the coco-to-perlite ratio in those mixes is mostly 70/30. 

 

Example Brands of Coco/Perlite Mixes
BrandCoco CoirPerlite
Mother Earth70%30%
Platinum90%10%
Envelor70%30%
Terra Aquatica75%25%
Hydro Crunch70%30%
Mr. Stacky80%20%
Growdog70%30%

Just a few brands of coco growing mediums available online and their coco/perlite ratios.


However, with frequent fertigation, especially in automatic drip irrigation setups, you can raise this ratio to 50/50.

Choosing a Grow Pot for Better Drainage

You can further improve drainage for weed plants by choosing a proper planter. A plastic container may seem like a good idea to a novice gardener, but its walls are not permeable to air.

You’ll get much better results with smart pots, or grow bags. Their sides are made from non-woven breathable material and thus provide the root structure with all the oxygen they need. The same can be said of Air-Pots®. You can find out more about different types of containers for growing cannabis in a separate post.

And whatever pot you end up choosing, don’t forget that the excess water should be able to drain freely. So, for plastic or terracotta containers, make drainage holes in the bottom. And, obviously, you’ll catch the overflow in drainage saucers. Don’t forget to empty them regularly. Your containers should never sit in stagnant water.

You can also think of it as aeration, meaning the presence of air pockets between medium particles. The coarser the medium, the less water it can hold, the better its drainage. And vice versa.


A short video showing how to choose a pot that has exactly the right size for your autoflower.


The size of the pot is also important. Maybe not so much in terms of proper drainage, but very much so for establishing and keeping a perfect irrigation plan

Cannabis Soil Conditioners/Amendments

If you don’t want to see your cannabis soil staying wet or not draining, make it as airy as possible. There are basically two types of amendments for cannabis soil aeration — perlite and vermiculite. They are not interchangeable, so we’ll talk in more detail about both.

Perlite

This is the most commonly used soil conditioner. It is a part of many store-bought soilless mixes, but cannabis usually requires a greater percentage of it than your regular house plants. So you’ll probably have to add more perlite to improve the drainage of the medium you’ve bought.

15 to 20 percent should be about right, but it’s not an exact science. The ratio can be as low as 10/90 and as high as 50/50. Just remember that with more perlite, you’ll have to water more often (which is not always convenient). With less perlite, water retention of the medium increases, and you can fertigate less frequently, but your marijuana plant may suffer from overwatering due to poor drainage.


Drainage for Cannabis Plants: Why is it Important and How To Improve it - Perlite and its features as a soil conditioner

Characteristics of perlite as a perfect soil amendment for drainage.


So what is perlite exactly? It’s a coarse-grained, white, lightweight material with closed-cell pores. As such, it has the following features:

  • can’t hold water inside its particles,
  • doesn’t contain nutrients and can’t buffer them,
  • is chemically inert and pH-neutral,
  • prevents cannabis soil compaction.

As you can see from the list above, perlite basically just sits there and makes your medium airier, thus creating an ideal environment for the roots of your plants.

Vermiculite

Vermiculite is a completely different story. It’s also an expanded, very lightweight material, but its brownish flakes don’t have a closed-cell structure. So—unlike perlite—they do absorb water and nutrients.


Drainage for Cannabis Plants: Why is it Important and How To Improve it - Vermiculite and its benefits

Vermiculite and its features as a soil conditioner.


The following features of vermiculite are the most important:

  • it’s excellent at pH buffering,
  • it contains K, Mg, and Ca, absorbs other nutrients from the solution, and then can slowly release them,
  • has a high water-retention rate.

Despite its high water holding capacity, vermiculite can still be used as a drainage amendment because its particles are large and create a lot of air pockets on their surfaces.

Other Cannabis Soil Conditioners

There are several other amendments that can improve drainage for marijuana plants, but they aren’t as common as perlite and vermiculite in cannabis gardens. Let’s overview them briefly:

  1. Coarse sand (0.5–1 mm). A very fast-draining but heavy medium. Sand is best used in greenhouses and outdoors.
  2. Expanded clay pebbles. Most often, growers use them to cover the surface of the pot (for mulching). But you can also dig them into the soil mix to create large air pockets.
  3. Pumice. This naturally expanded volcanic rock can hold a lot of air in its foam-like structure.

Drainage for Cannabis Plants is a Fairly Easy Concept

There is nothing difficult about the idea of soil drainage for cannabis. Let us repeat the key points here.

Cannabis plants need lots of water, but their roots also need lots of oxygen. So you should not only make drainage holes so that the excess water can run off, but also make sure your medium is constantly oxygenated. Better use special containers with breathable walls and prepare well-drained soil for pots by amending it with perlite or vermiculite or similar drainage material for potted plants. Do all this, and you’ll never run into any issues. Happy growing!

External References

  1. Effect of soil addition on physical properties of perlite based media and strawberry cv. Camarosa plant growth, Selda Ors, Omer Anapali, Scientific Research and Essays, November 2010
  2. Influence of Perlite in Peat- and Coir-based Media on Vegetative Growth and Mineral Nutrition of Highbush Blueberry, Patrick H. Kingston et al., HortScience, March 2020

 

06 September 2021
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