Rosie Lerner, Extension Consumer Horticulturist,
Dept of Botany & Plant Pathology, Purdue University
Rhubarb, sometimes called pieplant,
is an herbaceous perennial grown for the unique, tart flavor of
the thick leaf stalks (petioles). The leafy blades contain toxic
levels of oxalic acid and, therefore are not edible, either raw
or cooked. Gardeners often inquire whether a frost or a freeze
causes the oxalic acid to move into the stalks, rendering them
poisonous. There is little evidence that supports the idea of the
stalks becoming toxic following a frost or freeze. Nor is there
clear evidence to say that stalks do not become toxic. The quality
of the rhubarb is certainly suspect if subjected to a hard freeze;
stalks will turn mushy. But what about stalks that have been exposed
to low temperatures without obvious indications of freeze injury?
What we do know is that all parts of the rhubarb plant contain
oxalic acid, which is toxic if consumed in high enough quantities.
The leafy blades of the rhubarb plant contain far more than the
stalks, such that the stalks are safe to eat while the blades are
not. And oxalic acid content varies with the time of year and other
factors. Several other vegetables, including spinach, lettuce,
collards, beet greens and parsley, also contain varying amounts
of oxalic acid.
What we don't know is whether oxalic acid moves from the rhubarb
blade into the stalk during a freeze, and, if so, is it in high
enough concentration to pose a safety threat? Common sense would
indicate that if this were the case, we would see far more cases
of rhubarb poisoning at the poison control centers, and, to my
knowledge, that just isn't the case. Maybe we just don't eat enough
of the affected rhubarb stalks to become ill?
As always, if there is any doubt, it is better to play it safe!
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