For a long time, I have suspected that earthworms kill potted plants by feeding on their roots. Whenever I cleaned out such pots, I found earthworms in them.
Ever since Charles Darwin wrote of the earthworm in 1881, “It may be doubted whether there are many other animals which have played so important a part in the history of the world, as have these lowly organised creatures”, few have dared to say anything nasty about earthworms, so I kept my suspicions to myself while trying to figure how I could prove my case.
As a scientist I am aware that to prove cause and effect, I would have to set up a comparison between two potted plants in which everything would have to be identical except that one pot would contain worms while the other would have no worms. However, with just two pots for comparison, the results could be muddled up by unknown variations despite efforts to ensure absolute uniformity. It would be better to enlarge the comparison to say 20 pots per treatment. Comparing 20 pots against 20 would be statistically more robust than 1 against 1. I still had to figure out how to measure plant performance and for how long to run the experiment. Things got so complicated in my mind that the experiment never got done.
Last year, I raised half a dozen seedlings of the rare endemic Malayan witch hazel, Maingaya malayana, in individual pots. The seedlings grew at different rates, which was to be expected since they were raised from seeds and could be expected to be genetically different from each other. I had also not taken the trouble to ensure that the soil was exactly the same in all the pots. Also the pots were in different parts of the garden, under different environments. However, one plant was particularly stunted. I thought this was a genetic dwarf because it did not respond to any of my efforts to get it to grow beyond its first few leaves. Finally when it was clear that the plant was about to die, I tipped the plant out of the pot and found earthworms wriggling in the soil. I threw out the earthworms and repotted the plant. It recovered immediately and produced new leaves. The recovery was so striking that I have no doubt the worms were the cause of the stunting and slow decline of the plant.
For those who still doubt, I can now suggest an easy test. Add earthworms to a pot containing a healthy plant. Watch the plant decline over the next few months, then repot the plant after removing the worms, and see if the plant recovers.