Container Gardening: How to Improve Drainage in Potted Plants
Drainage in potted plants is uber important.
In fact, it’s the most important aspect of container gardening. Ignore it at your own risk.
Studies show that plants need air, water, warmth and light to have a chance of making it in their boring container life.
In addition, two of these elements are very critical for plant root development.
On one hand, plant roots need water since all the chemical processes and the transport of nutrients through the plant, take place in solutions made up mostly of water.
While on the other hand, roots need air too. If roots have no oxygen for a long time they will suffocate.
Take one element out and their chances of survival drops by 35% on average. Having a reasonable proportion almost always guarantees successful growth rate.
Even worse too much or too little of one element will affect the availability of the other.
For example, too much water will inhibit air availability while too much air means inadequate water or moisture.
Things get much complicated when plants are planted in containers.
This is why drainage in potted plants is uber important.
But what is drainage in the first place?
According to encyclopedia of Soils in the Environment, drainage is a natural process by which water moves across, through, and out of the soil as a result of the force of gravity.
Sidenote: Feel free to replace the word “soil” with whatever media you’re using to grow your plants.
Other authorities define drainage as how well the soil holds water. I like this second definition better 🙂
If you think about it, it’s not just how much water drains away from the growing media but also how much is retained.
This brings us to two classes of drainage problems that we should avoid:
- Poor drainage or water not flowing through.
- Excessive drainage or too much water moving through and being lost.
My goal is to show you how to avert these two problems and find a sweet spot – optimum drainage!
In today’s post, I’m going to explore various ways you can use to improve the drainage of your pot plants.
Consequently, optimum drainage will improve your soil or pot mix structure; increases efficiency of phosphorus fertilizer; conserve nitrogen; and control water-logging, leaching, and salinization caused by constant irrigation.
There’s a good chance that you’ll not use all of them, but picking just a handful will guarantee instant results!
Let’s dive in:
How to Improve Drainage in Potted Plants
1. If you’re using the soil add plenty of compost
If you’re experiencing excessive drainage, it means that you need frequent watering. Otherwise, the water is lost too quickly and your plants might wilt and die.
The root cause for this problem could be that your soil has a high proportion of sandy soil. Universally, sandy soils are large textured and have poor water holding capacity.
On the other hand if your soil has too much clay, drainage will be hindered causing stagnation.
When this happens, the roots of your container plants will lack oxygen and suffocate.
Here’s the solution:
Add compost to your soil and mix properly before planting.
In a way, this is like killing two birds with one stone since compost will solve the two extreme problems.
The organic matter in the compost acts as a sponge, holding water in the soil where the plants can reach it.
For the clay soils it opens up the porous structure allowing water to drain through.
2. Improve the texture of the soil
One of the easiest and fastest ways to improve the texture of your soil is by adding a course material such as sand or gravel.
Let me explain:
Fine textured soil contains very small particles packed close together. This means that the pore space is reduced.
Sidenote: the pore space is an area between one soil particles to the next and is often occupied by air.
Now these pore spaces are reduced dramatically once they get into contact with water, replacing any existing air.
In turn, this causes massive water-logging.
To solve, this problem it is important to mix this type of soil with an opposite textured soil preferably; sand, coco coir, and even compost.
You can either mix these materials together or arrange them in layers within your pot.
Some gardeners go to an extent of using rocks in their pots.
Rocks for drainage in pots
Adding a layer of rocks can be done to a non-draining pot as well as one with holes.
The idea behind this technique is that the rocks will act as a reservoir, storing excess water until the plant draws it up from the roots.
While for a pot with holes, they permit the excess water to flow away at the same time preventing the dirt from escaping.
You can use several types of rocks for this purpose.
Most gardeners use gravel made of granite or pebbles. In fact, this is the most popular tactic used by gardeners to improve the drainage of their potted plants.
However, being popular doesn’t mean it’s effective. This explains why very few of them succeed in container gardening.
Gravel or rocks at the bottom of the container doesn’t necessarily help.
To be precise, it’s ineffective.
There’s a phenomenon in container gardening called the Perched Water Table.
And I’ll discuss about it next.
This is a water saturation zone that could potentially deny the roots enough oxygen. Adding gravel elevates this zone causing problems to your plants.
The Concept of perched water table in pots
As I’ve mentioned earlier, in gardening, especially container gardening, you will often run into a phenomenon called a 'Perched Water Table' (PWT).
So what is PWT and how does it form?
This is a common scientific phenomenon known to scientist. But it is a foreign subject to most gardeners despite the fact that it affects them every day.
I’m going to explain this concept in the context of container gardening.
Let’s find out:
When watering your container plants, you might notice some sogginess at the bottom of the pot.
This water that doesn’t drain as intended is what we call PWT.
How does PWT then form?
The image below illustrates the formation process of a perched water table in pots.
Don’t mind my mediocre drawing because I’ll explain exactly what’s happening.
I’ll get a bit scientific but will try to explain as simply as possible.
First, let’s start with the basics:
Your favorite potting mix or growing media holds water by means of two natural forces.
The first force is through adhesion and the second one, cohesion.
These two forces are generally referred to as Matric Potential (MP). Simply put;
Adhesion + Cohesion = Matric Potential (MP)
MP is responsible for retaining water within the pot so that the moisture is available to the roots.
You should also note that MP is always uniform within the entire container.
However, when you water your plants, there’s another opposing force that ensures, water penetrates into the media, moves through and across, and finally drains from the bottom of the pot.
We call this force – gravity.
It’s also the reason why the planet holds everything together – otherwise we would all be floating in the galaxy – I don’t know if that’s even possible.
Scientifically, it has been proven that gravitational potential (PG) is higher at the top of the container and lower at the bottom of the container.
Now, here’s the catch:
From the illustration above, part A, which is also the upper zone, gravitational potential (GP) is higher than Matric Potential.
This means that irrigation water will flow seamlessly down the container without much resistant. Hence, the upper zone (A) is always the most porous and well drained.
Point B is the area of equilibrium, where GP is equivalent to MP. At this point the water slows the movement down the pot considerably.
The last part (C) is the saturation zone. There’s also another name for this zone. And yes, you guessed right.
It’s called the perched water table (PWT).
Formed as a result of higher matric potential than the gravitational potential.
I hope I’ve not lost you.
Your key takeaway should be, PWT is a matter of physics and occurs in all containers and cannot be eliminated. What you can do is minimize its negative effects.
If you think about it, PWT isn’t necessarily a bad thing:
As long as the upper zone has the required air spaces to allow for healthy root growth, you could simply consider PWT or the saturation zone as a "reserve" of moisture for the plant roots.
The container size and height also play a role in the "location" of the Saturation Zone.
Taller, narrow containers will increase the upper zone while shorter, wide container will decrease the upper zone, the relative height / depth changes but the saturation zone actually remains the same.
Since the PWT remains constant regardless of the size or height of the container, it’s important to choose your container wisely.
3. Choose the right container or planter
When choosing a planter or container to host your indoor plants, consider the following aspects;
- Shape – a shallow container will affect the porosity of your soil or mix. While a deep container with identical amount of mix will increase porosity thereby giving a boost to the drainage.
- Material - There are three main container materials. These are; self-watering, porous and non-porous. Self-watering serves as a wick that transfers water to the potting media making it wet as required by the plant. On the other hand the porous one allows water to escape through evaporation while the non-porous does the opposite. If you choose a porous container material, be prepared to water your plants more frequently.
- Size: The rule of the thumb is to choose a planter that is proportional to the size of the plant. But it’s better to err on the higher side. Smaller containers require frequent watering since they drain much faster.
To start off this subject, I’ll let you in on a secret that you’re unlikely to hear it from anywhere else.
And that is; at some point in your container gardening endeavor, after several months or years of sweat and tears, you’ll have to dig up those beautiful plants!
How do I know?
I have been there and done that.
But even more than my experience, is the fact that regardless of the plant you have or choose, the planter will clog reducing the drainage efficiency.
The only question is ‘when’ will that be and not ‘if.’
For some gardeners, it might be immediately thereafter, three months, six months, one year or ten years.
This time depends on the quality of your planter, and the thought process that went into the planter’s drainage system.
So if you have plans for specimen plants, or are planting trees that will grow too large for workers to lift by hand, make sure you have crane access to lift the trees, or some way to get fork lifts or other types of equipment like tripods in to lift the plants out of the planter for repairs, or to bring in new ones should major plants die.
I’ve exaggerated a bit may be and some planters can be repaired without moving the plants.
But let’s just say it is extremely hectic.
It’s also worth noting that; planter drainage is often the weakest link in the delicate balance of nature required to maintain planters and container gardens.
If you’re like most people, you don’t want to go this route.
Instead, you’d rather choose a planter that can be durable enough at the same time, accommodate a sustainable drainage system that won’t clog the following day.
Consider these simple tips;
- If possible, keep the planters small – this facilitates easy re-potted plantings that you can lift out, in pots which slope evenly outward as the pot wall moves towards the top. This simple classic form facilitates the removal of the plant to clean out blocked drainage holes without damaging either the pot or the plant.
- For large planters, consider a system at the bottom that can help you pump out excess water, especially after a heavy down pour for outdoor pot plants.
- Use corrugated piping also known as tiling - where proper drainage cannot be achieved through natural means, corrugated piping called tiling may be installed below the growing surface to move moisture away from the plant's roots.
4. Instead of the soil, use potting mix
You can save yourself the trouble associated with soil-based media by using recommended potting mix.
Most mixes have good porosity.
And as we already know, porosity is one of the most important properties of a potting mix.
It is the space available within a mix for water, air or root growth.
Small pores contribute to water retention whereas large pores promote aeration.
You can buy a ready-made organic potting mix or if you’re a do-it-yourself freak like me, you can make your own.
Regardless of your option, it’s extremely important to understand the general characteristics of a good pot mix.
To kick you off, I’ll list some of those characteristics so that you can make an informed decision should you choose to buy. It’s even more important if you want to make your own.
Characteristics of a good potting mix
- Drains well, which means an air-filled porosity of at least 15%
- Easy to re-wet – some peat and bark media are difficult to re-wet if they dry out
- Stable – meaning that it does not shrink away from the side of the pot as it dries
- Reasonable weight – not too heavy to lift, not so light as to blow over easily
- Optimum pH, between 5.0 and 6.5 is satisfactory for most plants (all pH values quoted are measured in water)
- Pest and disease-free, for example weed seeds, fungal pathogens, or can be sterilized without producing harmful by-products
- Easy to store - can be stored for short periods without significant changes in physical or chemical properties
- Cheap and readily available.
Source: Agriculture and food
Finally for my fellow DIY freaks, understanding these properties is good but they won’t results to anything substantial until you get your hands dirty.
Luckily I’m here to tell you exactly what to do.
However, I won’t go in to much detail because there are better resources out there on this subject.
But first, let’s take a look at the popular mix components.
This are; peat, coconut coir, vermiculite, pine bark, perlite, sand, zeolite, soil, animal manure, wood chips, and sawdust.
The perfect soilless potting mix recipe
Soilless mixes do not contain any soil – as the name suggests, but generally consist of peat moss combined with horticultural grades of vermiculite and/ or perlite and thereafter adding a fertilizer.
Peat-based media are useful for seed germination because they are relatively sterile, light in texture and weight, and uniform.
The light texture enables seeds to readily germinate and emerge, allowing tender roots to grow, and makes transplanting seedlings easier later on.
In general, standard media recipes are created based on the types of plants being grown (ex. bedding plants, potted plants, or for seed germination).
For general plants, you can use the following mix ratios:
Pinebark: peat: sand
Pinebark: peat: sand
Pinebark: sawdust: peat
Pinebark: sand: soil
1:1 to 3:1
Sawdust: pinebark: sand
Sawdust: pinebark: peat: sand
Sawdust: sand: peat
A standard recipe for a homemade soilless mix consists of half sphagnum peat moss and half perlite or vermiculite.
To mix ½ bushel basket or four gallons of media:
1. Start by pouring two gallons of peat moss into the bushel basket.
2. Add two gallons of either perlite or vermiculite and mix thoroughly.
3. Moisten the mix before using in pots or flats.
4. Add limestone to condition the mix
5. Add a fertilizer as required
Source: PennState Extension
Case Study: Fred’s soilless mix success story.
A few years ago, Fred was having a heated debate with his uncle over some aspects of container gardening.
His uncle always liked to win arguments and this occasion was not exceptional.
At the time, the uncle was a to-go-to person about all things gardening.
But he had one problem; he believed in his own righteousness regardless the facts presented.
For instance, he held a view that soil alone was enough to sustain pot plants.
When Fred pointed out the issues of soil compaction and subsequent reduction in porosity, his uncle wanted to hear none of it.
Fred was rudely interjected by a now visibly furious uncle, “you don’t know what you're talking about. I have been growing plants a lot longer than you and therefore I have more experience.”
And that was the end of their conversion for that day.
Fast-forward and Fred was having a family gathering at his home the other day.
He decided to brag about his new green friends growing in containers.
When he showed his family members all the plants he had, from citrus, to gardenias, to strange and exotic plants from Africa, to the deep Orient, gardenias, and many others, growing as happy as they could be, they all got jealous!
They all wanted answers:
And for that they asked lots of questions.
They wanted to know what in the world he was doing different! Fred picked each plant up and showed them the soilless mix they were all planted in.
Some were neatly planted in the 5.1.1 mix and some in the 1.1.1 gritty mix.
Fred’s family members couldn’t believe he even knew of the stuff. He went ahead showing them even the fertilizers he used.
Funny enough, his uncle was there too.
Yes, the very same man that couldn't keep much of anything alive in containers except seasonal stuff and kept bragging of years of experience, now was convinced that his nephew had something there.
Every year most of the family members had to buy a new gardenia, because by the time spring approached, the peat mix they use seemed to suffocate the roots by the following summer, not to mention the mites and the knats they had to fight all winter in their bed room
But now thanks to Fred with his discovery, they had finally found a solution.
Today they commend him and say in all their years, they never thought Fred could grow such beauties through their cold long lack of sunlight winters.
By simply doing minor tweaks, Fred was able to succeed where others were failing. He was successful because he paid attention to the well-being of his plant roots and so can you.
If you want to succeed in container gardening, take care of the soil rather than the plant.
If you want to succeed in container gardening, take care of the soil rather than the plant. #containergardening
5. Drill Drainage Holes in your Container
New gardeners keep asking the following questions; whether you need drainage holes in pots.
If so, how many drainage holes should there be? Finally, whether it’s possible to drill these holes in different container materials such as plastic, ceramic, and resin planters.
I’ll attempt to answer these questions as comprehensive as possible.
Do you need drainage holes in pots?
To answer this question, I’ll go back to the basics definition of drainage as discussed earlier.
Here’s the definition;
Drainage is a natural process by which water moves across, through, and out of the soil/potting mix as a result of the force of gravity.
Pay attention to the words in typed in bold, “out of the soil or whatever media you’re using.”
Therefore, it is a definitive yes that you must have drainage holes in your pots to allow the excess water to exit.
However, there’s one exception:
Not all container plants are the same. Neither do they have the same water requirements. This means that for water loving container plants such as Houttuynia cordata ‘Chameleon’, Pickerel rush, and Dwarf sweet flag, Acorus gramineus ‘Ogon,’ the rules might be different.
Meaning that if you’d rather not have drainage holes in your beautiful pots, you can be forgiven.
As long as you take the risk with aquatic plants only and not those that requires proper ventilation.
How many drainage holes do you need in a pot?
The beauty of gardening is that the rules are very few. Even better, you can break the few that exists and still get away with it.
The beauty of gardening is that there are very few rules. Even better, you can break the few that exists and still get away with it.
When it comes to the number of drainage holes required in a pot, there are no exceptions.
You’re allowed to go with a number that pleases you as long as the excess water can flow freely.
This is the reason why most ‘green thumbs’ will just tell you “drill drainage holes at the bottom of the container.”
They won’t tell you how many holes you need leave alone how to drill them.
However, through personal experience I find three or four small (1/4 inch) holes in the bottom of the container most ideal.
Holes larger than 1/4 inch in diameter will allow too much soil or dirt to escape.
Placing a layer of gravel or broken pottery pieces on the bottom of the container, below the soil, can help stop the flow of dirt through larger holes.
I will briefly discuss how to properly cover these holes later.
A vital step in ensuring the holes are covered just enough to prevent the dirt from spilling.
Drilling drainage holes in different pot materials
There are so many ways to make holes in pot materials such as plastic, ceramic, metal, and resin.
Again there are few rules here:
The bottom line is that you make your holes large enough to allow water to pass through.
At the same time, they shouldn’t be too large that the soil or potting mix can escape through them.
Drilling drainage holes in plastic can be very easy and time-saving if you have a drill.
Furthermore, the drill will allow you to be more precise with the size of your holes.
For instance, you can set your drill bits at between 3/8 and 5/8 inch. Once you’ve done this, go ahead and make three to four ¼ inch holes as we’ve already discussed before.
But please remember that it’s not mandatory to follow my recommendation in terms of the number of holes.
You can make as many or as little as you wish.
Sadly, not everyone can afford a drill, or those that can, may be reluctant since hard drilling if not done properly can cause cracks on plastic.
In that case, you can use a hot nail or a soldering iron with a sharp tip.
I know of a gardener who has been very successful in doing this.
She doesn’t use a drill to make the holes, since she does not have one either way. She declined the offer from a shop assistant in the store, as she thought hard drill on cold plastic may cause cracks.
Instead she uses a soldering iron with a sharp tip, plugs it in a power outlet and inserts it into wherever she wants to make the holes.
To enlarge them a bit, she moves the iron in the hole circularly so that it'd melt down more of the area. Finally, she detaches the extra bits of melt plastic while it hadn't completely cooled down.
How cool is that?
Try it yourself and see how it works.
There you have it.
As I wrap up, it’s my desire that you become very successful with your container gardening.
Whether you are short of space and want to grow some food sustainably in your home, or you want to enhance the aesthetic value of your property by adding a splash of green in those containers, mastering the basics is very important.
But most especially, learning how to improve drainage in potted plants is a basic skill that I can’t emphasize its importance enough.
As we’ve learned in this article, plant roots cannot survive without two important elements. These are; air and water.
In nature, these two elements can rarely exist together.
When one is in abundance, the other one becomes limited and vice versa.
In addition, we have seen that there are two scenarios affecting the drainage in container plants. The first is excessive drainage which in turn causes the plants to wilt, and the second is poor drainage that limits aeration.
I’ve discussed some of the tactics you can use to solve those two problems.
It’s your turn to choose the ones that best suits you.
Back to you:
Let me know if I’ve left out any other actionable tactics in the comments.