Acclimation and differences between plants
During hardening, hydration, dissolved substances, mucilage and the hormonal response change. Tolerance of extracellular freezing allows some opuntias to survive severe cooling. The magnitude of the response depends on population, organ and season. New growth can be highly sensitive even in a plant whose winter segments tolerate severe frost. [Loik and Nobel 1993a] [Goldstein and Nobel 1994] [Ishikawa and Gusta 1996]
The full account of frost tolerance physiology covers 17 publications, tables for 20 populations of Opuntia fragilis, seasonal measurements from Saskatchewan, and comparisons of roots, stems, fruits and meristems. [Loik and Nobel 1993b] [Nobel and De la Barrera 2003] [Zutta et al. 2011]
How to read hardiness data
| Material | Acclimation and test | Result | What was assessed |
|---|---|---|---|
| Opuntia fragilis, 20 populations | 14 days at 5/−5°C after 25/15°C. Five segments per population. One hour at the test temperature. | LT₅₀ from −15,6 to −48,9°C | Staining of living cells relative to the control. [Loik and Nobel 1993b, Table 2] |
| O. fragilis, Saskatchewan | Winter segments. Initial freezing at −3,5°C, controlled cooling. At least five segments per temperature. | LT₀ −50°C | No visible necrosis after four weeks. [Ishikawa and Gusta 1996, Table 2] |
| O. fragilis, young growth from the same population | June. Freezing initiated. | Death at −3,5°C | Young segments without winter acclimation. [Ishikawa and Gusta 1996, Table 2] |
| O. polyacantha, Saskatchewan | Winter segments. The same protocol as for winter O. fragilis. | LT₀ −40°C | No visible damage. [Ishikawa and Gusta 1996, Table 2] |
| O. ficus-indica, clone 1279 | Seven weeks at 20/10°C. One-hour exposure. n = 5–7 plants. | One-year-old stem −7,6°C, young root −6,5°C | LT₅₀ based on tissue staining, assessed separately for each organ. [Nobel and De la Barrera 2003, Table 1] |
LT₀ and LT₅₀ have different meanings. None of these values is a universal temperature recommended for cultivation. The material description, exposure duration, method of ice initiation and subsequent recovery are part of the result. [Ishikawa and Gusta 1996] [Loik and Nobel 1993b] [Nobel and De la Barrera 2003]
Winter hardiness outdoors and under cover
In field trials, the duration of frost, plant age and cultivated material mattered alongside the minimum temperature. A study from Mendoza showed large differences between opuntia clones after the same event. A one-season trial from Romania also resulted in the loss of plants sold as frost tolerant. A result for a provenance at one site cannot replace observations over successive winters in Poland. [Guevara et al. 2000] [Szekely-Varga et al. 2022]
Keep a record of the source of the material, planting date and shelter. After thawing, check stems, the base, roots and resumed growth separately. Collector practice and overwintering conditions develop these topics. [Ishikawa and Gusta 1996] [Guevara et al. 2000] [Fleischer and Schütz 1978]
Archived temperature list
954 entries from the earlier site’s list, an archival collection list and an account of several nights at −4°C. The copy dates from November 2010. [Anonim b.d.q]
