By Prof. Dr. Willem VAN COTTHEM

University of Ghent, Belgium


People are asking me why I am in favour of drilling drainage holes in the sidewall of a container.  Why are those in the bottom not good enough ?

Of course, the classic drainage holes in the bottom are functional. Rain or irrigation water is running through the potting soil towards the bottom.  A possible surplus of water is thus easily evacuated through that single hole or the different holes in the bottom.

However, potting soil contains a high number of bigger and smaller cavities, that are filled with air.  When watering a container (pot, bottle, bucket, barrel, etc.), irrigation water is running rather swiftly towards the bottom of the container.  A number of those cavities are thereby filled with water and air is pushed out.  That is visible e.g. when we immerse a pot in a bucket of water: we see the bubbles leaving the potting soil during a short period, until influent water has pushed out most of the air of the bigger cavities. Even if we do not see bubbles leaving the soil anymore, a certain volume of air is still remaining in the potting soil, namely that in the smallest cavities.

This means that, when watering a container with the classic drainage hole(s) in the bottom, water will run quickly through the potting soil, moistening that soil for its major part, but not completely.  A lot of precious water will run out of the container through the drainage hole(s) in the bottom and be lost for moistening of our plants. This means that a number of cavities will still be filled with air (leaving those small parts of the potting soil dry).

Now, imagine what will happen if we put the container (with holes in the bottom) in a recipient, e.g. a bowl.  Water running out of the drainage hole(s) is then collected in the bowl.  From there it will gradually be re-absorbed by the potting soil (and the rootball).  It re-enters our container and after a certain time it moistens the potting soil almost completely.  However, if we exaggerated when watering, too much water will be collected in the bowl and that water will stand there for a longer time, having a negative effect on the roots (e.g. by asphyxiation).

On the contrary, if we did not exaggerate when watering, only a smaller quantity of water will be collected in the bowl, and that quantity will rather quickly re-enter the container, completing the moistening of the potting soil and the rootball.

Considering this phenomenon, it came to my mind that drilling drainage holes in the sidewall should have the same effect as collecting a quantity of irrigation water in that bowl.  My experiments proved the positive effect.

2009-12-30-bottle-preparation-p1030170  2009-12-30-bottle-preparation-p1030171 

Photos WVC: 2009-12-30 BOTTLE PREPARATION P1030170.jpg and 2009-12-30 BOTTLE PREPARATION P1030171.jpg

In a first series of experiments I drilled 2 opposite holes (diameter 0,5 cm) in the sidewall of plastic bottles at 2,5 cm above the bottom.  I preferred to get 2 opposite holes, expecting that one of the holes could be clogged.  Somewhat exaggerating the watering, I noticed that a lot of water was running out of the 2 drainage holes.  Nevertheless, the quantity of water kept in the bottom of the bottle was readily moistening the potting soil above, having a positive effect on the growth of the plant in it.  None of the holes was clogged in a first period, but I expected that it could happen when the growing roots would reach the bottom of the bottle.

p1030643  2010-03-29-avocado-p1030768


Photos WVC: 2010-03-03 BOTTLE PREPARATION (P1030641-P103064 / 2010-03-29 AVOCADO P1030768.jpg and 2010-03-29 AVOCADO P1030765.jpg

Therefore, I have set up a second series of experiments with 2 opposite drainage holes, having a diameter of 1 cm (reducing the risk of clogging).

A third and fourth series of experiments were set up with 2 drainage holes (diameter 1 cm) in the sidewall but respectively at a height of 5,0 cm and 7,5 cm above the bottom.


Finally, I used bigger containers with bigger holes (3 cm) at different height.

My general conclusion of these experiments is that drainage holes in the sidewall of a container are better than the ones in the bottom, because:

(1) one is saving a lot of water (less loss);

(2) one is saving also a quantity of fertilizer (otherwise lost by leaching);

(3) one registers better plant growth.

It is obvious that there is a close relationship between the dimension of the container (and thus the volume of the potting soil) and the height of the drainage hole(s) above the bottom.  The higher the holes in the sidewall in small containers, the bigger the risk of asphyxiation and root rot.  One should also determine the optimal diameter of the drainage holes.

Today, I hope that researchers or students will set up scientific studies to determine the optimal method to improve plant growth in containers by taking into account the position and dimension of drainage holes in the sidewall.

Anyway, container gardeners using planters without any drainage holes are hereby recommended to drill those holes not in the bottom, but in the sidewall.

Published by

Willem Van Cotthem

Honorary Professor of Botany, University of Ghent (Belgium). Scientific Consultant for Desertification and Sustainable Development.