> In New Zealand, Hebe species can be found growing in a wide range of habitats, from sea level to alpine regions, so it is no surprise that cold hardiness of the species, and the cultivars derived from them, varies widely as well. There is truth to the old saying that hardiness of Hebe is related to leaf size. As one goes up in elevation from sea level to alpine areas in New Zealand, the leaf size of the Hebes tends to decrease, and overall plant size decreases as well (Kristensen, 1989). Other characteristics like coriaceous and/or glacous leaves and white flowers are also typical of alpine Hebe species (Wardle, 1978). So, generally speaking, you could say that the larger the leaf of the Hebe, the less cold hardy it tends to be. As with all living things, the rule is not perfect, but the most tender Hebes are usually the largest-leaved, and the hardiest are those with the smallest leaves.
> The most extensive study of cold hardiness of Hebe was undertaken by Warrington and Southward (1995), who assessed summer and winter hardiness of 35 species and cultivars. This study showed that large differences in hardiness existed among the various selections. Not surprisingly, the hardiest of those tested were two whipcords, H. cupressoides and H. propinqua, both of which are typically found in alpine or subalpine regions and have very tiny leaves. More important than altitude however, this study observed significant differences in hardiness of species from northern or southern parts of New Zealand, with species of southern origin exhibiting greater overall hardiness.
> Some of the research on hardiness of Hebe has utilized excised shoots as sample material for laboratory studies. Bannister (1986) found that detached shoots of Hebe albicans, a South Island species often found in subalpine scrub above 1000m, withstood mid-winter temperatures of -10°C, one of the hardier of the native species studied. Other Hebe species were not tested. In a more extensive study, Bannister (1990) found that mid-winter freezing resistance of foliage of H. buchananii, a diminutive shrub of alpine areas in Canterbury, was as low as -11°C. Freezing resistance of foliage of H. rakaiensis and H. salicifolia, both of which are found at lower elevations on the South Island, was found to be -5.2°C and -6°C respectively. Buds of H. rakaiensis were found to be hardy to -11°C, significantly hardier than leaves. Testing of both H. speciosa, a tender species from seacoast areas of the North Island, and H. odora, a widely distributed shrub of subalpine scrub, found early-winter hardiness of -6°C and -9°C, respectively. Sakai and Wardle (1978) tested excised stems of a wide variety of New Zealand trees and shrubs in mid-winter for hardiness. H. brachysiphon was rated as one of the hardiest species tested, having leaf and bud hardiness of -10°C and -13°C, respectively.
> Probably the most extensive anecdotal evidence for hardiness of a wide range of Hebe species and cultivars is provided by Trees and Shrubs hardy in the British Isles (Bean and Clarke, 1991). This monumental work, in 5 volumes, lists descriptions and basic hardiness information on over 100 Hebe species and cultivars. The most extensive field trial of Hebes which included cold hardiness results was the trial of RHS Garden Wisley between 1980 and 1982. In addition to selecting 33 cultivars or species for awards, the trial also indicated which of the trialed plants failed to survive the winter of 1981. Harris and Decourtye (1995) evaluated cold hardiness of many New Zealand plants in field trials over several years in Angers, France. This trial indicated that H. dieffenbachii was tolerant of prolonged cold weather with temperatures as low as -12°C. H. pauciramosa, H. pinguifolia and H. amplexicaulis showed some foliar damage from these conditions. Later data from this trial (Harris et al. 2000) showed that H. albicans and H. subalpina were not injured or suffered only slight injury from prolonged winter cold snaps with temperatures as low as -15°C.
> In North America, Hebes are grown primarily as a landscape plant. Because of the intolerance of most Hebes for excessively hot or cold weather, cultivation of Hebes in North America is almost entirely limited to west of the Cascade or Sierra Nevada Mountains in California and the Pacific Northwest. Elsewhere in North America, the climate is generally too cold or hot, or both, to allow for outdoor cultivation, although dedicated enthusiasts have been successful with some varieties in many other areas.
> Even in the Pacific Northwest, Hebes are sometimes thought of as too tender for general landscape use, a reputation which is primarily the result of experience with a few popular cultivars which are not particularly cold hardy. Plantings of ‘Amy’, ‘Tricolor’ or ‘Patty’s Purple’ have been severely damaged in cold events on a regular basis, which has unfortunately given the entire genus this reputation. The key to growing these tender cultivars is to provide a protected location near a house or nearby plant.