While much of California has a Mediterranean climate with hot, dry summers and cool, moist winters, there are many distinct microclimatic variations within our state.  A microclimate is defined as a local atmospheric zone where the climate differs from the surrounding area.  It can be as small as a few square feet or cover hundreds of square miles. 

Selecting plants well adapted to a particular microclimate is probably the single most important way to improve the odds they will survive and flourish.  The Sunset Western Garden Book provides maps of large-scale microclimates, or planting zones, for the western U.S.  The majority of Butte County residents live in Sunset Zones 7, 8, or 9. 

Paradise, Magalia, Forest Ranch and Berry Creek lie within Zone 7.  This area has hot summers and relatively mild, but pronounced winters.  Late-blooming deciduous fruit trees (apples, pears, cherries), do well in this zone.  Winter lows range from 23° to 9°F and the frost-free growing season normally lasts from the beginning of May to mid-October.

Zone 8 is a cold-air basin located on the valley floor which includes Chico, Gridley, Biggs and Richvale.  Hot, sunny summers and a long growing season that extends from mid-February to late November allow deciduous fruits and vegetables of nearly every kind to thrive in Zone 8.  In winter, cold air flows from higher elevations and settles on the valley floor, bringing winter lows that range from 29° to 13°F.  These cold winter temperatures are frequently low enough to injure citrus trees and other cold-sensitive plants.

Sandwiched between Zones 7 and 8 is the thermal belt of Zone 9.  Cold air flows downhill from Zone 9 to the valley floor.  Oroville and other lower foothill areas are found in this zone.  Zone 9 has a similar growing season to Zone 8 (mid-February to late November), but the lows here are not quite as extreme, ranging from 28° to 18°F.  These slightly higher winter temperatures mean that not only can the plants and crops that do well in Zone 8 thrive, but many cold-sensitive plants too can succeed in Zone 9. 

Small-scale features can affect your garden's microclimate as well, so it is important to consider the following:

1. Low spots collect rainwater and/or cold air resulting in localized frost pockets or boggy areas. Hilltops are more exposed to drying winds.  Barriers across slopes, such as walls, fences or rocks can provide shelter from winds, but can also cause cold air to pool behind them.  Making gaps in the barrier can allow the air to drain and prevent possible frost pockets.  Water flows off sloped areas before completely soaking in, so slopes tend to be drier than flat surfaces. 
2. South-facing areas get more light and heat than areas facing in other directions; a south-facing wall accentuates this as it absorbs and reflects heat.  Conversely, a north-facing slope or the north side of a fence or a wall will be more shady and cool.  Temperatures in west- and east-facing areas fall in-between north and south aspects, with western aspects being slightly warmer since they are exposed to the hot afternoon sun.
3. Clay soils tend to hold water more than sandy soils, so poor drainage can be associated with clay soil, while sandy soils can drain too fast and dry out too quickly.  Loamy soil strikes a balance between these two extremes.  Heat is stored in soil water during sunny days and that heat is re-radiated at night.  Moist soil can store more heat than dry soil, thus reducing frost risk.  Dry sandy soils or recently cultivated soils with many air pockets reduce the storage of heat in soil water during the day and are therefore colder at night, increasing the risk of frost damage. 
4. Trees and shrubs create shade and can provide windbreaks as well.  A dense tree canopy may trap heat at night, moderating nighttime temperatures.  Because trees compete for sunlight and for water and nutrients in the soil, it can be difficult for other plants to survive beneath them.

Man-made Structures.  A house absorbs heat during the day and re-radiates heat at night.  A house affects wind patterns, creating turbulence and causing higher wind speeds to occur along its walls.  The lee side may be in a “rain shadow” when rain is driven by strong winds.  Rainwater flowing off roofs or out of downspouts can create wet spots if it doesn't drain properly.  Balconies high above ground level may avoid frost, but they are often exposed to more wind.  Raised beds usually warm up and dry out earlier in the spring.  Paved surfaces act as “heat islands,” absorbing and reradiating the sun's energy.

Take these characteristics and features of your home landscape into consideration when selecting plants and choosing locations for them.  Doing so can make a big difference in the success of your plantings.

UC Master Gardeners of Butte County are part of the University of California Cooperative Extension (UCCE) system. To learn more about us and our upcoming events, and for help with gardening in our area, visit our website. If you have a gardening question or problem, email the Hotline at mgbutte@ucanr.edu or leave a phone message on our Hotline at (530) 538-7201. To speak to a Master Gardener about a gardening issue, or to drop by the MG office during Hotline hours, see the most current information on our Ask Us Hotline webpage.