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ZONE 7: California’s Gray Pine Belt, Oregon’s Rogue River Valley, and Southern California mountains
Zone 7 encompasses several thousand square miles west of the Sierra Nevada and Cascade ranges, and in the mountains that separate the Southern California coast from interior deserts. Because of the influence of latitude, this climate lies mostly at low elevations in Oregon’s Rogue Valley, middle elevations around California’s Central Valley, and at middle to higher elevations farther south. Gray pines define the heart of Zone 7 around the Central Valley, but more adaptable incense cedars replace them farther north and south.
Hot summers and mild but pronounced winters give Zone 7 sharply defined seasons without severe winter cold or enervating humidity. The climate pleases plants that require a marked seasonal pattern to do well—flower bulbs, peonies, lilacs, and flowering cherries, for example. Deciduous fruit trees do well also; the region is noted for its pears, apples, peaches, and cherries.
Gardeners in a few spots around the San Francisco Bay will be surprised to find their gardens mapped in Zone 7. These areas are too high and cold in winter to be included in milder Zones 15 and 16. In the mildest parts of Zone 7—in the extreme southern Salinas Valley, for example—you can get away with growing borderline plants such as citrus, oleanders, and almonds if you choose a spot with good air drainage to take the edge off winter chill. At weather-recording stations in Zone 7, typical winter lows range from 35 to 26°F (2 to –3°C),with record lows averaging from 18 to -0° F (–8 to –18°C).
ZONE 8: Cold-air basins of California’s Central Valley
Zone 8 makes up most of the valley floor in California’s Sacramento and San Joaquin Valleys. Only a shade of difference exists between Zone 8 and Zone 9, but it’s an important difference—crucial in some cases. Zone 9 is a thermal belt,meaning that cold air can flow from it to lower ground—and that lower ground is found here in Zone 8. Citrus furnish the most meaningful illustration. Lemons, oranges, and grapefruit, which flourish in Zone 9, cannot be grown commercially in Zone 8 because the winter nights are frequently cold enough to injure the fruit or the trees; the trees would need regular heating to deliver decent crops. The same winter cold can damage many garden plants. That cold often shows itself in winter, when cold air rolls off the Sierra Nevada and pools on the valley floor, condensing into thick tule fog. Zone 8 differs from Zone 14, which it joins near the latitudes of north Sacramento and Modesto, in that Zone 14 occasionally gets some marine influence. Low temperatures in Zone 8 over a 20-year period ranged from 29 to 13°F (–2 to –11°C). Certain features that Zones 8 and 9 share are described under Zone 9.
ZONE 9: Thermal belts of California’s Central Valley
As cited in the description of Zone 8, the biggest readily apparent difference between Zones 8 and 9 is that Zone 9, a thermal belt, is a safer climate for citrus than Zone 8, which contains cold-air basins. The same distinction, thermal belt versus cold-air basin, determines which species and varieties—hibiscus,melaleuca, pittosporum, and other plants—are recommended for Zone 9 but not for Zone 8. Zones 8 and 9 have the following features in common: summer daytime temperatures are high, sunshine is almost constant during the growing season, and growing seasons are long.Deciduous fruits and vegetables of nearly every kind thrive in these long, hot summers; winter cold is just adequate to satisfy the dormancy requirements of the fruit trees. Fiercely cold, piercing north winds blow for several days at a time in winter, but they are more distressing to gardeners than to garden plants.You can minimize them with windbreaks. In both Zones 8 and 9 tule fogs (dense fogs that rise from the ground on cold, clear nights) appear and stay for hours or days during winter. The fogs usually hug the ground at night and rise to 800 to 1,000 feet by afternoon. Heat-loving plants such as oleander and crape myrtle perform at their peak in Zones 8 and 9 (and 14). Plants that like summer coolness and humidity demand some fussing; careful gardeners accommodate them by providing filtered shade from tall trees and plenty of moisture. In Zone 9,winter lows over a 20-year period ranged from 28 to 18°F (–2 to –8°C).
ZONE 14: Northern California's inland areas with some ocean influence
Marine air moderates parts of Zone 14 that otherwise would be colder in winter and hotter in summer. The opening in Northern California's Coast Ranges created by San Francisco and San Pablo bays allows marine air to spill much farther inland. The same thing happens, but the penetration is not as deep, in the Salinas Valley. Zone 14 includes the cold-winter valley floors, canyons, and land troughs in the Coast Ranges from Santa Barbara County to Humboldt County.
The milder-winter, marine-influenced areas in Zone 14 and the cold-winter inland valley within Zone 14 differ in humidity. For example, lowland parts of Contra Coasta County are more humid than Sacramento.
Fruits that need winter chill do well here, as do shrubs needing summer heat (oleander, gardenia). Over a 20-year period, this area had lows ranging from 26 to 16º F (–3 to –9ºC). Weather records show all-time lows from 20 down to 11ºF (–7 to –12º C).
ZONE 15: Chilly winters along the Coast Range
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.Also worthy of note is that although Zone 16 is within the Northern California coastal climate area, its winters are milder because the areas in this zone are in thermal belts (explained on page 28). 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. Most of this zone gets a nagging afternoon wind in summer. Trees and dense shrubs planted on the windward side of a garden can disperse it, and a neighborhood full of trees can successfully keep it above the rooftops. 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).
ZONE 16: Central and Northern California Coast thermal belts
This benign climate exists in patches and strips along the Coast Ranges from western Santa Barbara County north to northern Marin County. It’s one of Northern California’s finest horticultural climates. It consists of thermal belts (slopes from which cold air drains) in the coastal climate area, which is dominated by ocean weather about 85 percent of the time and by inland weather about 15 percent. Typical lows in Zone 16 over a 20-year period ranged from 32 to 19°F (0 to –7°C). The lowest recorded temperatures range from 25 to 18°F (–4 to –8°C). This zone gets more heat in summer than Zone 17, which is dominated by maritime air, and has warmer winters than Zone 15. That’s a happy combination for gardening. A summer afternoon wind is an integral part of this climate. Plant trees and shrubs on the windward side of your garden to help disperse it.
ZONE 17: Marine effects in Southern Oregon, Northern and Central California
The climate in this zone features mild,wet, almost frostless winters and cool summers with frequent fog or wind. On most days and in most places, the fog tends to come in high and fast, creating a cooling and humidifying blanket between the sun and the earth, reducing the intensity of the light and sunshine. Some heat-loving plants (citrus, hibiscus, gardenia) don’t get enough heat to fruit or flower reliably. In a 20-year period, the lowest winter temperatures in Zone 17 ranged from 36 to 23°F (2 to –5°C). The lowest temperatures on record range from 30 to 20°F (–1 to –7°C).Of further interest in this heat-starved climate are the highs of summer, normally in the 60 to 75°F (16 to 24°C) range. The average highest temperature in Zone 17 is only 97°F (36°C). In all the other adjacent climate zones, average highest temperatures are in the 104 to 116°F (40 to 47°C) range.