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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