Go ahead and water from that cold tap!

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What you see here may be an aloe's dream, to receive lots of special attention. However, in our house, it's a pipe dream. We explain to each new plant that professional growers don't heat the water that comes in from their wells and mains, and neither do we. All our plants have grown well on these terms. 

Oft-repeated advice questioned and de-bunked.

Just a warning about what you're about to read: Some of the most often repeated advice about indoor gardening is "Don't use cold water on houseplants." Yet we find plenty of data developed for greenhouse growers that provides more specific directions. It's stuff to allow a home gardener to rest easier.

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A pro's livelihood depends on reliable, healthy potted plants, and that pro knows the scientific definition of cold is "less than 50°F." Simple tests place even the chilliest tap water above that mark.

In short: It's okay to use water direct from a cold tap on all but African violets and others with furry leaves.


The question:

One thing I like about winter is that the water from my tap is so cold. I do love a cold drink of water. But I hear that my growing collection of indoor plants doesn't feel that way. Do I have to warm the water for houseplants? - S.V. -



Dear S.V.,

Unless it's ice cold it's probably fine as is. Since it's really rare to receive water from a tap that's below 50°F -- approaching the chill of refrigerated water -- you're almost certainly safe. Our home's tap water is refreshingly cold in winter, like yours, but still short of tooth-numbing. We've measured it at 57°F in the depths of winter. That's no surprise, since it runs through pipes in soil below the frost line where temperatures also dip to the mid 50's in winter.

57°F from the tap? OK!

As cold as 57°F may feel to us, it's okay for general use. It's water below 50° that's trouble.* Then, it's trouble for those few tropical species we grow indoors which may develop leaf spots or stunted root tips from cold watering. African violets (Saintpaulia varieties), Gloxinia, other furry-leaf species and Croton are in this lot. They'll develop those leaf and root problems (and probably more trouble, over time) when they're splashed with or allowed to sit in water  below 50°F.

If in winter we add a new plant that we've heard has cold sensitivity, for its first watering when it may yet be warmer than our house in general we add a bit of warm water to the mix in filling the watering can.

Let stand to warm, still only low 60's

Alternatively, we let the can sit and absorb heat from the room -- what flows from the tap at 57°F reaches 62°F after a couple of hours and holds steady there. Chances are pretty good once plants have been in the house about that long, they too hold at room temperature. Thus they're unlikely to be shocked by water in the tap- to air-temperature range.

In summer, our tap delivers water at 65°F. That still feels cool to drink, and it would absorb some ambient heat if it sits. Yet, why bother? At 65°F it nearly matches what comes year-round from the well that serves my friend Kurt in Florida. His tropicals -- indoors and out! -- give it a "thumbs up." So does the University of Florida in bulletins for greenhouse growers, citing studies that water right from the well, in the 60's and low 70's, provides good growth in all except certain seed germination processes.

Warm water may be more trouble

On the other hand, warmer is not necessarily better. If you blend your taps, use "tepid" as an upper limit. Water in the high 70's can still feel cool on a testing wrist, yet treating plants regularly with water at temperatures above 80°F** can cause trouble via an increase in root rot fungi. In some cases, problems can occur with anything warmer than 60°F.***

Straight from the tap into controversy

We water most interior plants with water just as it flows from the cold tap, even in winter. This flies in the face of advice found in many garden books, bulletins and on websites, including some linked to universities. Those say, "Don't use cold water on tropical plants, it can shock the roots."

Our contrary, cold approach began from a pinchpenny tendency. "If it can't live here without running up the hot water bill," said our wallet, "better it should die." After a few decades, wondering why there had been no noticeable ill effects, we revisited that old advice.

Looking for specifics seemed a reasonable start: "How cold is cold?" "What temperature is best?" and "Lacking a specific water temperature, can we define cold water shock symptoms so we know when we've committed a water temperature offense?"

After some concerted digging, we found answers:

  • From plant production studies: 50°F is cold water.  +70°F is warm.
  • Many greenhouse growers make effective use of cold water sprays to control mites and scale on interior plants.
  • Research results explaining why we should avoid water over 70°F for general irrigation: It cannot hold as much dissolved oxygen as colder water.


Re *, ** and ***:
We try to revisit and update external links but it's not possible to keep them all current without your help. Let us know which to check and we'll look into them as soon as we can.

In the meanwhile, here are the URLs behind the links you were given earlier on this page. All were valid early in 2014; if they go to a dead end for you today, you may find if you delete suffixes from the URL you may access the relevant home page, then search from there. That may net you a quicker answer than we can return.
* http://www.ag.auburn.edu/hort/landscape/Gloxinia.htm
** http://edis.ifas.ufl.edu/CV264
*** http://apsjournals.apsnet.org/doi/abs/10.1094/PDIS-94-1-0054











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