How to kill a plant: death by drowning

If Trevor is “the guy that talks about cows,” then I’m the girl that writes about plants.  Being said person, I have a lot of plants at home.  Since I don’t have any actual pets (or children) my houseplants are given plenty of attention.  Despite years of practice caring for plants, last summer I slowly killed two via drowning. 

If this were a murder mystery, we would now meet the cast of characters, namely the victims Ms. S. Bridgesii and Mr. P. Americana.  Both were residents of The Windowsill and were subjected to a slow death by overwatering. 

The two victims. Photo by Claire Collie

The two victims. Photo by Claire Collie

To understand what went wrong with my overzealous water use, we have to go underground to the dark place where plant roots live: the soil.

The mess of minerals, organic matter, water, gases, and microorganisms that makes up soil is, perhaps surprisingly, pretty detrimental to plants.  Soil scientists will espouse on this fact, and explain how important soil structure is for plants to thrive.  Minerals (the percentage of sand, silt and clay) and organic matter in soils come together to form soil crumbs called aggregates, and give soil it’s structure.  Most plants need well-structured soil where aggregates aren’t too big or too small.  Water and gases fill the spaces between the aggregates, and a balance between liquids and gases is needed throughout the roots zone.  Generally soil is made up of 45% minerals, 5% organic matter, 25% water and 25% air.  This is an approximation; soils are different in composition depending where they are and their geological history. 

In well-structured soil, oxygen can defuse several meters deep. But, if the spaces between aggregates are filled with water for an extended period of time, roots can be severely damaged, plant growth is delayed and future yields are reduced.  The soil saturation time depends on the species and size of the plant, but for many crop plants the time can be as short as 24 hours. For this reason, it’s important that farmers know just as much, if not more, about their soil as their crops.

Geranium roots.  Notice all the root tips where growth, cell division and nutrient uptake occurs.  Photo by Claire Collie

Geranium roots.  Notice all the root tips where growth, cell division and nutrient uptake occurs.  Photo by Claire Collie

Plant roots meander through soil aggregates absorbing water and nutrients.  Both are needed to help the plant grow below and above ground.  All the action below ground happens at the very tip of each root: cell division, cell growth, absorption of stuff from the soil.  As such, water, nutrients and oxygen need to be readily available in the surrounding soil.  

What happens when things go wrong and there isn't enough oxygen in the soil?

Turns out, this is what was happened to my poor houseplants, Ms. S. Bridgesii and Mr. P. Americana.  Continually watering them meant the soil pore spaces were filled with water and there wasn't enough oxygen to support important metabolic processes.  The nutrient and water transport assembly line had stopped.  The pH inside each cell was becoming more acidic, eventually leading to cell death and rotten roots.  Damaged roots can’t support plant shoots.  Leaves wilted since water and nutrients weren't being transported up. Older leaves began to fall off prematurely.  I mistook these signs for those of a happy, growing, transpiring plant and continued to pour more water on.  By the time I figured it out and pulled the plants out of their pots, each had one delicate root left.  I tried to save them, but it was too late. 

Mangrove roots in the Caribbean. Photo by Claire Collie 

Mangrove roots in the Caribbean. Photo by Claire Collie 

In my last post, I wrote about growing lettuce plants in artificial environments with no soil at all, just a liquid nutrient solution that roots dangle in.  If oxygen is so important for roots, how do these plants survive?  Air stones or pumps, like those used in fish tanks, pump air into the continually circulating water.  If this stopped, due to a power outage or system breakdown, the plants could die in just an hour or two.

What about wetlands, swamps or ponds?  

There are plenty of plants that grow and thrive in these water-saturated environments.  Usually these species have some sort of adaptation for bringing oxygen to the roots.  Mangrove trees are a well-known example.  These trees grow on the waters edge where the soil is completely saturated. They send up pneumatophores, or aerial roots, which are covered with spongy lenticels that take up oxygen.  Other examples are plants like water lilies and cattails that grow in ponds or swamps.  The mud at the bottom of these swamps contains little to no oxygen, so the plants pipe it down from the leaves to the oxygen-deprived roots through long interconnected tubes.  A method of removing invading cattails is to cut the stems off underwater.  Without a supply of oxygen, the Cattail roots cannot survive and the plant drowns.    

By now, I may have scared you off from plant parenthood.  There’s no fail-safe way to keep plants happy. Too little water will kill a plant, but so will too much water.  And what about light, nutrients, toxins or pests? There are lots of ways to kills plants, but their beauty more than makes up for the effort, right?

On my windowsill, goldfish plant from the genus Nematanthus. Photo by Claire Collie

On my windowsill, goldfish plant from the genus Nematanthus. Photo by Claire Collie