Frost hardiness and winter hardiness in collector practice

Frost hardiness and winter hardiness

A plant may survive a brief temperature drop yet die after a long winter. It may also lose individual segments and rebuild shoots from surviving buds. “Withstands” is therefore incomplete without knowing what was examined. Cell survival, retention of above-ground parts, root health and flowering in the next season are separate endpoints. Plant comparison is meaningful only after a shared criterion is established. [Goldstein and Nobel 1994] [Loik and Nobel 1993b] [1-WWW]

Frost hardiness concerns tolerance of subzero temperatures under defined conditions. Winter hardiness is broader, expressed at a particular site and season. It also includes excess water, duration of soil freezing, thaws, mechanical damage and later regeneration. Ranking species by one minimum temperature obscures these differences. In a bed, the sequence of events matters, not just the coldest night. [Goldstein and Nobel 1994] [Loik and Nobel 1993b] [1-WWW]

Freezing and cell dehydration

Water in living tissue behaves differently from pure water in an open container. It contains dissolved substances, occupies different cellular compartments and adjoins cell walls. Ice formation outside cells alters water-equilibrium conditions. Water may leave protoplasts and their volume decreases. Damage therefore depends on ice formation itself and the ability of membranes and other structures to survive dehydration and subsequent rehydration. [Goldstein and Nobel 1994]

Goldstein and Nobel's 1994 paper compared three opuntias with different cold tolerances. Acclimatization accompanied changes in water relations and dissolved compounds. In material then identified as Opuntia humifusa, increased mannitol content was among the changes described. The study also linked resistance to the course of extracellular freezing. It does not show that drying alone can harden a plant or that watering with sugar solution reproduces natural acclimatization. [Goldstein and Nobel 1994]

Seasonal acclimation

The same specimen may respond differently to frost in autumn, midwinter and after spring growth starts. Gradual cooling and changing plant activity create different conditions from abruptly moving a plant from a warm home to a freezing greenhouse. Acclimatization changes living-tissue physiology. It does not replace the species' genetically determined capacities or make a tropical plant winter-hardy. [Goldstein and Nobel 1994] [Koski 2019]

Autumn wrinkling and drooping of segments in some opuntias may accompany reduced hydration. This observation is also known from Fleischer and Schütz's book. A wrinkled surface alone is insufficient to identify proper dormancy, however. Check whether the change is evenly distributed, tissues retain integrity and no wet, expanding patch develops. Abrupt watering solely to restore summer appearance may worsen root conditions. [Goldstein and Nobel 1994] [Koski 2019]

A thaw does not end winter. A few warm days may alter plant activity, while another frost arrives before lasting substrate warming. Temperature ranges may be especially large under cover. Ventilation and measurements near plants describe this situation more accurately than readings from a distant weather station. [Goldstein and Nobel 1994] [Koski 2019]

Meaning of laboratory LT₅₀

Nobel's 1996 experiment compared tissues of plants kept warm and after two weeks of acclimatization at day and night temperatures of 10°C and 0°C. Excised samples were cooled at 4°C per hour, held at the target temperature for one hour and then warmed. Neutral-red uptake by living photosynthetic parenchyma cells was assessed. [Nobel 1996]

For material identified as O. humifusa from Point Pelee, Canada, LT₅₀ after acclimatization was about −23.8°C. This temperature halved cell staining relative to the control. It did not mean absence of damage or safe overwintering of the entire plant. [Nobel 1996]

Results of a study of 73 opuntia varieties

Valdez-Cepeda and co-authors described a natural frost episode on 11–14 December 1997 in the El Orito collection near Zacatecas. They examined 73 collection entries, with five plants older than five years each. Nearby measurements recorded a minimum of about −11°C, and cold persisted much longer than a single night. The proportion of dead segments was assessed after one week. This field experiment concerns damage to mature plants after a particular event. [Valdez-Cepeda et al. 2001]

Responses differed greatly. The authors recorded no dead segments in entries named Duraznillo and Tapón Aguanoso, while some Copena-group material was very severely damaged. The result does not establish a resistance threshold for entire species O. leucotricha or O. robusta, to which the first two entries were assigned. It includes no long-term assessment of overwintering in Poland, separate measurement of root damage or response of young cuttings. Its value is documenting differences between materials exposed to the same weather episode. [Valdez-Cepeda et al. 2001]

Historical names and plant provenance

The archival page groups cacti by several temperature limits. It does not provide an experimental protocol for every entry, so the lists help locate candidates and literature. They are not a modern table of guaranteed survival. Former Opuntia imbricata now appears in POWO as Cylindropuntia imbricata. The names Coryphantha vivipara and Escobaria vivipara correspond to the atlas's accepted Pelecyphora vivipara, while retaining information on the different genus treatment by de Vos and co-authors. [Sánchez et al. 2022] [2-WWW] [5-WWW]

A name change does not explain a particular plant's provenance. A widespread species may include populations living under very different conditions. A label should therefore retain field or collection number, seed source and propagation history if known. Do not add a location based solely on appearance. The former broad concept of O. humifusa requires particular care. A result for historically identified material does not automatically apply to every taxon now separated from this complex. [Sánchez et al. 2022] [2-WWW] [5-WWW]

Soil profile and water drainage

In 1978, Fleischer and Schütz already distinguished frost tolerance in the habitat from survival of a wet Central European winter. This is a more useful starting point than copying their temperature list. Drainage must function beneath the whole root zone. Surface gravel does not remove water retained lower down, and a thick layer of fine substrate in a depression may remain wet despite a dry-looking surface. [4-WWW] [1-WWW] [Koski 2019]

Raising the site and creating a slope can facilitate drainage. Their effectiveness depends on the ground below and incoming water from the surroundings. Rain protection should permit air exchange. Tightly wrapping a plant in plastic changes humidity and heating rather than simply making it “drier”. A pot also creates different conditions from the ground. Its volume is small and it loses heat on many sides, so bed experiments do not directly establish root resistance in containers. [4-WWW] [1-WWW] [Koski 2019]

Documenting a garden trial

Start with a small number of well-documented plants, retaining reserve material under appropriate conditions. For each, record identification, source, age or approximate size, planting date, rooting method and prewinter condition. Add soil, covering and surroundings descriptions. Record temperature at the site, noting sensor position. Air temperature does not measure temperature inside the stem or root. [Loik and Nobel 1993b] [Koski 2019]

After winter, assess segment damage, collar condition, root retention and renewed growth separately. Supplement a March photograph with observations in May or June. A plant that looked healthy in cold weather may reveal damage later. Record lost specimens and undamaged seasons too. Such a series is more valuable than memory of one record. Differences may arise from microsite, so one success does not establish superiority of an entire provenance. [Loik and Nobel 1993b] [Koski 2019]

Planning the overwintering place itself is discussed in Overwintering conditions in Poland. Cards in the Cactus and succulent atlas retain the distinction between species data and conclusions for cultivation. Read these two levels together, especially when purchasing material of undocumented provenance. [Loik and Nobel 1993b] [Koski 2019]

New field data and comparison limits

The supplied publications allow comparison of different outcome criteria. Canadian opuntia work tracked absence of necrosis and subsequent root and shoot formation. In Mendoza, segment damage after natural frost was counted, excluding mother segments. In Romania, an ordinal damage scale compared portions of one bed. Results from these methods should not be combined into a single minimum-temperature ranking. [Ishikawa and Gusta 1996] [Guevara et al. 2000] [Szekely-Varga et al. 2022]

The Bozeni trial, published in 2022, included ten plants per taxon and covering treatment. The reported minimum was −21.9°C, but internal tissue temperature and covering construction details were not specified. All uncovered Echinocereus coccineus died. This shows failure of particular material at that site. It neither disproves nor confirms winter hardiness of all populations of the species. [Szekely-Varga et al. 2022]

Cactus frost-hardiness physiology develops freezing mechanisms, the roles of ABA, sugars and mucilage, and species and population tables. The papers by Goldstein and Nobel 1994 and Loik and Nobel 1993 are now discussed using their full texts. [Goldstein and Nobel 1994] [Loik and Nobel 1993b]