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The Therapeutic Range of Water

What could possibly go wrong? Today at the U of M Conservatory Greenhouse where I volunteer, I was assigned the task of watering the plants in two of their four large rooms, the rooms for tropical and subtropical plants. These two rooms house hundreds of threatened and critically endangered plant species.

The finger test

The time honored finger test: insert your finger up to the first knuckle or two into the soil of any plant pot, if it feels dry water the soil thoroughly, if it feels damp leave it alone - don't drown the plant roots. It sounds like a mom-ism, but there truly is a sweet spot for dampness of the soil in a plant pot. Too dry and the plant withers; too wet and the roots rot.

Basic energy economy of all plants

  • Above the soil surface, plants makes sugar:  Sun's Energy + CO2 + Water  =>  Sugars + O2
  • Below the soil surface, roots use the sugar: Sugar + O2  =>  CO2 + Water + Energy to absorb nutrients and transport of water for the leaves and stems (so they can make more sugar)
Did you notice that the process going on in the part of the plant above the ground is the reverse of the process going on in the roots below the ground? Above ground the plants make oxygen and sugar; below ground the roots use oxygen and sugar. Clearly, if the roots don't have access to oxygen in the soil, they cannot absorb nutrients or water, and the whole cycle stops. The key message is roots need as much air as they do water.

Tillandsia cacticola in bloom, an air plant, receives a brief daily shower only

Beyond the finger test

The finger test doesn't hold true for some plants. Obviously, water plants need their roots in water. Whereas most desert plants need their roots completely dry between waterings. Cacti can absorb water so fast their trunks split open from the pressure of water pushing up from the roots. Then, there are the air plants like Tillandsia which absorb water through their leaves as they hang on rocks or trees (think Spanish moss).

As an example of the variety of options plants use, the rice plant has evolved a tube which delivers air from the surface down to the roots. This allows the roots to survive in the water of a rice paddy, while the weed plants around it are drowned.

The art and science of knowing how to water more than 1300 different species of plants in a large conservatory collection is way beyond the finger test (and often beyond my personal knowledge). 

Everyday, each plant gets attention

Since I am not a botanist with training in the care or each plant, I could easily damage or drown a plant that is considered critically endangered in its native habitat. Wisely, the administrators of the greenhouse have provided training and many visual cues to guide the watering work. There is a defined procedure for unspooling long hoses and connecting them to the appropriate water taps. Different types of water supply either tap water (for washing the floors), and  deionized water with or without fertilizer depending on the plant. Most plants get the deionized and fertilized water which provides a balanced mix of nutrients. Each pot has to be assessed visually for moisture content and watered appropriately. Tags are inserted into many of the pots to flag plants needing more or less water. Some plants are in dormancy and not to be watered at all.

My task today was made easier by the types of plants in these two rooms, where most of the plants are happy to be somewhat damp. I'm fairly confident that I was able to follow the instructions, to provide enough water, of the right kind of water, but not too much...within the therapeutic range.

Deionized water: with or without fertilizer

200-foot hose in each room, enough to reach every plant



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