ZONE 15: Inland Slopes of the Coast Range and Siskiyous, from Willow Creek to Orleans, with Marine Influence.
Head west from route 96 on a hot summer day on any of the roads that climb steeply from the Klamath River Canyon and you're liable to encounter coastal fog that spills over the mountains and gives rise to a dense forest that includes coastal species such as rhododendron, azalea, salal and Port Orford Cedar. The slope and coastal influence tempers the extremes of summer heat and winter chill found at the valley floors for the Klamath and Trinity. Zones 15 and 16 are areas of Central and Northern California that are influenced by marine air approximately 85 percent of the time and by inland air 15 percent of the time. The cold-winter areas that make up Zone 15 lie in cold-air basins, on hilltops above the thermal belts, or far enough north that plant performance dictates a Zone 15 designation. Many plants that are recommended for Zone 15 are not suggested for Zone 14 mainly because they must have a moister atmosphere, cooler summers, milder winters, or all three conditions present at the same time. On the other hand, Zone 15 still receives enough winter chilling to favor some of the coldwinter specialties, such as English bluebells, which are not recommended for Zones 16 and 17. Lows over a 20-year period ranged from 28 to 21°F (–2 to –6°C), and record lows from 26 to 16°F (–3 to –9°C).