Suburban Myths: Water Temperature

All of us have heard our share of “urban myths,” those widely believed but untrue factoids that periodically make the rounds of popular culture. Gardening has its own popular lore that is commonly passed on – often by noted gardening experts – but is not upheld by rigorous scientific investigation.

One such suburban myth, as I call them, is that the temperature of the water used to hydrate plants is crucial: it should always be at room temperature.

As someone who has grown orchids for many years, I’ve long bought into that idea. Orchids are notoriously finicky about their conditions (though not nearly as finicky as many people assume.) It is universal in orchid care that only lukewarm water is recommended.

The same advice is usually applied to growing foliage, flower and vegetable plants outdoors. No less a gardening icon that Mel Bartholomew states in his wildly popular book, Square Foot Gardening:

“Plants are a lot like us – they like warm water, not cold. Cold water causes plants to go into slight shock, and when you water from the hose or the cold water tap, your plants will stop growing momentarily.”

Sounds perfectly reasonable, doesn’t it?

And authoritative, coming from a justifiably revered expert like Mel B.

Which means that I’ve been lugging buckets of sun-warmed water around my yard and traipsing in- and outdoors with heavy, sloshing watering cans filled with tepid water from my kitchen tap.

That’s NOT painless gardening, so I undertook to see if there was another, easier way. Turns out there is: just use most any temperature water that comes out of your hose.

What Science Says

The only science I could find on the subject are reports of studies done at the Wisconsin and Utah Agricultural Experiment Stations, where agri-scientists did field and greenhouse trials to determine if cold water was detrimental to plant growth and water transport through plants.

In Wisconsin, coleus planted in lots of equal size and vigor were hydrated with water at 35°, 50°, 65°, and 86° F. At the end of 60 days, no difference in growth was noted.

So, they tried again with beans watered at 32°, 40°, 70°, and 100° F. The results were pretty similar except that the beans watered with the coldest water –32° and 40° – showed slightly better results.

To re-test those findings, the scientists tried lettuce; the lettuce watered at 32° F. – on the verge of freezing – yielded slightly more than the other lots. (Lettuce is a cool-loving crop after all.)

From these experiments it was concluded that water may be used freely at any time of the year without warming.

In the Utah study, it was shown that very cold water – below 35 degrees – could potentially slow water transport through plants. My deep scientific reaction is “duh!” They proved that semi-solid frozen water didn’t move through plants as quickly as more liquid liquids.

None like it hot

Actually, hot water is much more destructive to plants than cold water. Water over 70° can literally cook delicate roots. In fact, a proven natural way to kill weeds is to just pour boiling water on them.

I did my own experiment today and took the temperature of water that first came from my garden hose at high noon. It was 118 degrees. Ouch!

So, the best advice I can give to make watering your plants as painless as possible is to apply water at any temperature that’s convenient for you but in summer, always let your hose run a minute or two until any sun-warmed hot water in it is gone.

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