Good drainage is extremely important if you want to keep potted plants healthy. Without drainage, plants can develop a whole range of problems. If you want to avoid them, let’s find out how to get proper flower pot drainage.
Why do flower pots need drainage?
Flowerpot drainage is absolutely critical for plant health. Without drainage, watering should be micromanaged to keep plants healthy. If you accidentally overwater a potted plant without drainage, you’ll need to take immediate steps to fix the problem.
Overwatering leads to problems such as root rot due to the lack of oxygen in the soil. This will cause the plant to become stunted, the leaves will turn yellow and start to fall and the roots become mushy and turn black. The plant becomes unable to absorb any more water and will eventually wither and die.
How pot plants drain
To stop problems such as root rot and waterlogged soil, it’s important to understand how plant pots drain.
Water in planted pots doesn’t simply drain all the way through. Some water is retained in the potting medium. The amount of retention depends on the wicking ability of the potting mix.
Wicking ability is very similar to capillary action. This ability is driven by the cohesion of water to itself as well as sticking to other media. To increase drainage, we need to influence the adhesion of water to other materials since we cannot influence the water molecules sticking to each other.
To influence wicking, you need to understand what increases and reduces water retention in potting media. To increase wicking, you need to maximise the surface area that water can cling to. This means to reduce wicking, you need to do the opposite to increase drainage.
Another thing that influences wicking is particle size. The larger the particles, the more airspace is created between them, the smaller the particles are, the smaller amount of airspace will be between the particles. This influences how quickly water can move through the media.
How to improve drainage in potted plants
The only way to increase drainage in potting soil is to change the composition of the soil. You also need to make sure the pot you’re using has drainage holes. Here are some examples to give you an idea of how changing the properties of the soil will influence the water table in your pot.
The size of the airspaces between the soil particles affects the speed of drainage. If you use soil with very small particles such as clay soil and silt, there is almost no air in the soil, and water will take much longer to drain through the medium. This type of soil also retains much more water than soil with larger particles. If you have soil with large particles and large airspaces like sand, for instance, water will drain through very quickly. The adhesion of water to sand is also very little which means the soil will retain little water.
The best type of medium retains enough water for the plant to use, while still allowing the water to drain quickly enough without suffocating the delicate roots. To get this type of medium, you can mix soil types with both small and large particles. You can also use soil amendments to help.
What to use for drainage in pots
Soil amendments are materials used along with potting soil to improve water drainage by increasing the airspace between the particles. If your goal is to improve soil drainage, consider the following amendments for your potting mix:
Vermiculite is a soil amendment material that’s used to retain water and nutrients in the potting soil. This material is made by heating mineral mica to extremely hot temperatures, causing it to expand and create the flaky, porous material we refer to as vermiculite.
This amendment increases soil aeration by creating air pockets in the otherwise densely packed soil but also improves water retention by absorbing some of the water passing through. By doing so, it keeps the soil moist without suffocating the roots. This substance also releases vital nutrients that plants can use over time.
Perlite is another very popular material that is often used to increased drainage. Perlite is volcanic silicate rock that’s been rapidly heated, causing it to expand. As a result, perlite is extremely light and floats in water.
Perlite is used to increase drainage and soil aeration by expanding the air pockets in the soil. This substance is also non-absorbent which forces water to move through.
3. Coarse sand
Coarse sand is another excellent amendment for waterlogged soil. The large particle size creates air spaces that allow water to flow through more easily. Keep in mind that this amendment is very heavy.
If you have soil with very small particles, you’ll need a lot of sand to make any kind of difference. The small particles will usually only fill up the space between the sand particles instead of staying out of the newly created air pockets.
To make any kind of difference, you’ll need to have more sand than soil in the mix to improve drainage. This is where the weight of sand might count against you.
Pumice is a very porous type of volcanic rock. This soil amendment helps with improving water retention in very sandy soil but it also absorbs excess moisture to prevent root rot.
This type of rock is often used to improve soil absorption, aeration and health. It helps to create air pockets in the soil and won’t compact over time. Once you’ve added pumice, you won’t need to worry about it decomposing. This substance is there to stay.
How to test the drainage of a potting mix
The easiest way to test the drainage of your potting soil is to add water to the pot and time how long it takes to start draining out of the bottom. In very well-drained soil, the water will pass through very quickly and should start pouring out of the bottom of the pot almost instantly.
In soil that retains water, it will take much longer for the water to pass through. If you’re wondering how exactly to measure the drainage capability, try the method below:
Use about one litre worth of unamended potting soil and place it in a flowerpot. Do the same with amended potting mix. Once ready, place something underneath the pots to capture any water that drains out.
Now you can pour 500 ml of water into each pot. Wait a few minutes to allow the water to drain. After about 5 minutes you can take the trays away and measure the amount of water that drained out of the pot.
In a well-draining mix that works for most plants, around 250 ml should have drained through. Pots with less water than this retain a lot of moisture while pots that drained more water than this drain far too quickly to support most kinds of potted plants.
Always aim for a mix that retains at least half of the water given.
How pots affect drainage
You might be surprised, but the soil isn’t the only thing that affects how well your flower pots drain. The shape and size, as well as the material your pot is made of, will affect how well it drains.
Pots that are shallow and wide won’t have a very impressive water table. The water will always pass through quite quickly. Pots that are narrow and deep tend to have a more substantial water table. This means more water will be retained for your plants to use.
Pots made from a porous material such as terracotta tend to absorb some of the moisture in the soil. This means more water will be drawn out of the soil. It also means that water can evaporate through the pot so more frequent watering will be required.
In pots made from non-porous materials such as plastic or glazed ceramic pots, the only way out is for the water to drain through. These pots tend to retain moisture much longer than clay pots but won’t allow any air into the soil.
It’s best to put a layer of gravel at the bottom of a flower pot. This will prevent the drainage holes from becoming blocked by the potting soil and will lower the water table in your pot. If the water table is too high, your plants may drown and develop problems such as root rot.
Why do some pots not have holes?
Some pots are purely decorative and usually have another pot with drainage holes placed inside them. If you decide to plant directly into a pot without holes, you’ll need to carefully manage watering to prevent drowning your plant.
Is one drainage hole enough?
It depends on the material your pot is made of. If you have terracotta or ceramic pots, one drainage hole will be sufficient since the pot also absorbs some of the moisture in the soil. If you have a plastic pot, more than one drainage hole will be required to prevent too much water retention inside the pot.