To answer your question, we must first discuss what we Grevilleas need to thrive. We are a diverse genus, primarily native to Australia, and our cold tolerance varies significantly by species. Our fundamental requirement is a climate that avoids prolonged freezing. Our tissues are not adapted to survive sustained sub-zero (°C) temperatures. When the ground freezes, it damages our root systems, and ice crystals form within our cells, causing fatal damage. Therefore, our survival outdoors is entirely dependent on whether the minimum average winter temperatures in your zone align with our specific hardiness.
The majority of popular Grevillea species and cultivars, such as myself Grevillea rosmarinifolia or the stunning Grevillea victoriae, are best suited for USDA zones 9 and 10. In these zones, winter lows typically dip no lower than 20°F to 30°F (-6°C to -1°C). Here, we can grow outdoors year-round as permanent landscape plants. We may experience some tip damage in an unusually cold snap, but our root systems and main branches will survive. Zone 9b is often considered the safest bet for a wide range of cultivars.
Zone 8, where winter lows can reach 10°F to 20°F (-12°C to -7°C), is a marginal and risky environment for most of us. Only the very hardiest species have a chance of survival here, and even then, it is not guaranteed. The robust Grevillea juniperina and some tough forms of Grevillea lanigera are known to occasionally persist in protected Zone 8a/b microclimates. Success depends heavily on perfect drainage (we abhor wet, cold roots), a full sun position against a heat-reflecting south or west-facing wall, and excellent air circulation. A thick layer of mulch can help insulate our roots, but the risk of a fatal freeze remains high every winter.
For zones 7 and colder, where temperatures routinely fall below 10°F (-12°C), it is simply not feasible for us to survive outdoors in the ground. Our cellular structure cannot endure such deep and prolonged freezes. Attempting to plant us there would result in our certain death during the first hard winter. In these regions, we can only be grown as container specimens that must be moved to a protected, frost-free location like a greenhouse or a very bright sunroom for the duration of the winter.
Even within a suitable zone, our survival depends on more than just temperature. We demand two non-negotiable conditions: full sun and excellent drainage. We are sun-worshippers and will become leggy and fail to flower without it. More critically, our roots are highly susceptible to fungal root rot in heavy, waterlogged soils, especially in cooler weather. Planting us on a slope or in a raised bed with amended, gritty soil is often essential for our long-term health, even in zone 9.
