• Tya Shannon

Overwatering: the root of the matter

Updated: Jun 12, 2020

Most of us know the basics of what plants need to survive: water, light, nutrients, and carbon dioxide (CO2). But of course it’s more complicated than that. For instance, did you know that roots need oxygen to survive, and like us they can drown? I’m not a botanist, and chemical equations are like Klingon to me. So, I’m going to explain as simply as possible why plants can drown.

When I was five years old, someone told me trees breathe the opposite way to people. How else do you explain to a child that plants use CO2 and produce oxygen, while humans use oxygen and produce CO2? I remember thinking trees must move their lungs the opposite way to humans when they breathe. So I used to sit very still, pretending to be a tree by pushing my tummy out when I breathed out, and pulling it in when I breathed in. Try it, it feels really weird.

But it’s not actually true that plants breathe the ‘opposite’ way we do, at least not all the time. Plants need to respire, i.e. use oxygen to convert sugars to energy, the same as us. When the sun is shining, plants produce more oxygen through photosynthesis than they use for respiration. Most plants don’t use their roots to photosynthesise. But, roots do need to respire. Plants don’t have an efficient circulatory system that can take oxygen from the leaves to the roots. Instead, roots need a supply of oxygen right on their doorstep.

Roots take in oxygen from air spaces in the soil, and from the dissolved oxygen in the water in the soil. When the soil is very wet, there are no air pockets and the water can’t absorb more oxygen from the air. After a while, the dissolved oxygen will be used up by by the roots and other organisms like bacteria and fungi. Then the roots start rotting, and are no longer able to transport water or nutrients to the rest of the plant.

So what’s the lesson here? Depending on the species of plant, let the compost dry out at least a little on the top of the compost before watering. For some species, like Monstera deliciosa, you should let the compost dry out a lot more. Aerating the soil regularly is also important. I do this at least monthly to all my plants – see my blog post ‘Be the Worm’ for instructions on aerating.

If your plant is looking yellow and/or wilted but the soil is wet, or has dark patches on the leaves, it might be suffering from over-watering. In which case, it’s worth easing it out of its pot and checking the roots. If they’re white and plump, they’re healthy, in which simply case aerate the soil and let it dry out before watering again. If the roots are black, slimy and smelly, they’re rotting. You’ll need to remove the rotting roots and repot the plant in new compost.

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Tradescantia pallida propagated in water by cutting. The roots are using the dissolved oxygen in the water. Compared to soil, there are fewer bacteria and other organisms using the oxygen, leaving more for the plant.