From a botanical perspective, the cold hardiness of any plant, including Grevillea, is a complex physiological adaptation. It involves the plant's ability to acclimate to decreasing temperatures, often by producing antifreeze-like proteins within its cells and altering the fluidity of cell membranes to prevent ice crystal formation. For Grevilleas, which are predominantly native to Australia's warm and often arid climates, this is a significant challenge. Their survival in USDA Zones 7b-8 and warmer depends on selecting species and cultivars that originate from higher, cooler elevations within their native range. These specific genetic lines have evolved with a inherent tolerance for colder conditions.
Several Grevillea species and hybrids have proven their resilience in cooler climates. Among the most reliable is Grevillea rosmarinifolia (Rosemary Grevillea). This shrub, typically hardy to around 10°F (-12°C) (Zone 8), features needle-like foliage reminiscent of rosemary and produces clusters of spider-like red flowers throughout the year, providing a vital nectar source for pollinators during mild winter spells. Another excellent choice is Grevillea juniperina, particularly its low-growing and spreading cultivars like 'Molonglo'. Its prickly, juniper-like foliage offers textural interest, and it can tolerate temperatures down to approximately 5°F (-15°C) (Zone 7b).
Plant breeding has been instrumental in expanding the cold tolerance of this genus. Hybrids often exhibit "hybrid vigor" (heterosis), where the cross between two species results in offspring that are more robust and adaptable than either parent. A premier example is the Grevillea ‘Canberra Gem’. This robust hybrid is celebrated for its exceptional cold tolerance, reportedly surviving brief periods down to 0°F (-18°C) (Zone 7a) when established. It forms a dense, large shrub with deep green, divided leaves and prolific red flowers. Similarly, Grevillea ‘Poorinda Royal Mantle’ is a highly regarded groundcover hybrid, valued for its ability to form a vast mat that suppresses weeds and its tolerance for cold, damp conditions better than many other species.
A plant's survival is not solely determined by its genetic potential; its environment is paramount. For cold-hardy Grevilleas, the single most important cultural requirement is perfect drainage. Their roots are highly susceptible to fungal rots in cold, wet soil. Therefore, planting on a slope or in a raised bed is highly recommended. Furthermore, these plants are sun-loving and require a full sun position to thrive and harden off properly before winter. A south-facing wall can provide additional radiant heat and protection from harsh winds. Providing a thick layer of gravel mulch around the base, rather than organic matter, helps keep the crown dry and further mimics their preferred well-drained natural habitat.
