The Purdue Plant and Pest Diagnostic Laboratory

P&PDL Picture of the Week for
April 13, 2009

Rhubarb

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!

Click image to enlarge

Rhubarb plant

Rhubarb plant

Rhubarb stalks

Rhubarb stalks

Rhubarb leaves

Rhubarb leaves

 

Purdue Plant & Pest Diagnostic Lab Purdue Cooperative Extension Service