Rhubarb (Rheum rhabarbarum) is botanically a vegetable, though often used as a fruit due to its tart flavor. Safety concerns surround the plant because only the bright red or green stalks (petioles) are edible. The large, leafy blades are toxic to humans and pets. This difference in edibility leads to a persistent question: does a hard freeze cause the otherwise safe stalks to become dangerous?
The Role of Oxalic Acid in Rhubarb Toxicity
The compound responsible for rhubarb’s toxicity is oxalic acid, found in varying concentrations in many plant species. Consuming excessive amounts of this acid can lead to the formation of insoluble calcium oxalate crystals, which contribute to kidney stones.
The concentration of oxalic acid differs significantly across the plant. The edible stalks contain low levels, generally between 0.2 and 0.5 grams per 100 grams of fresh weight. The leafy blades, however, contain a significantly higher concentration, ranging from 0.5 to 1.0% of the leaf’s weight. Ingesting the leaves can cause severe poisoning, including vomiting, abdominal pain, and ultimately kidney failure. The leaves are permanently inedible regardless of weather conditions.
Why Freezing Poses a Risk to the Stalks
The concern that freezing makes rhubarb stalks poisonous stems from potential cell damage. Rhubarb is a hardy plant, and a light frost (temperatures in the upper 20s Fahrenheit) typically does not harm the stalks. However, a hard freeze (mid-20s or lower) can severely damage the plant’s cellular structure.
When water inside the cells freezes and expands, the cell walls rupture, causing tissue breakdown. This cellular damage compromises the plant’s structural integrity, allowing chemicals to move freely. The hypothesis suggests this damage enables the highly concentrated oxalic acid from the leaves to translocate down into the petioles. This process would significantly increase the oxalate concentration in the stalk, potentially rendering it toxic. While definitive scientific studies proving this translocation are limited, the physical damage from a hard freeze certainly compromises the stalk’s quality.
Guidelines for Post-Frost Rhubarb Harvesting
Gardeners should use visual cues to determine the safety of post-frost rhubarb. Stalks subjected to a hard freeze show clear signs of damage, appearing limp, waterlogged, or mushy, with blackened or shriveled foliage. Any stalk exhibiting these visual signs of freeze damage should be immediately pulled and discarded.
If the plant sustained only a mild frost and the stalks remain firm, upright, and visually healthy, they are safe to harvest. After a severe freeze, wait two to three days before inspecting the plants. Remove and compost any damaged material, and cease harvesting until new, healthy stalks and leaves emerge. Never consume rhubarb material that appears visibly compromised by freezing.