Ann Wright: Zone in on plant zones |

Ann Wright: Zone in on plant zones

Ann Wright
The Berginia crassifolia, shown here, is a plant for USDA zone 2-9 and Sunset zone 1-9, and is currently in bloom.
Photo by Ann Wright |

As seed catalogs start to arrive in mail boxes and gardeners are planning spring gardens, it is a good time to explore plant hardiness or climate zones. Every region in the world has climate zones — each with a unique combination of heat, cold, precipitation, wind and other climactic factors.

Gardening books, seed catalogs, seed packets and some plant labels refer to hardiness zones for groups or individual plants. Planting the “right plant in the right place” is one of the guiding principles Master Gardeners describe to help plants have the best chance for survival.

Understanding climate zones will help in choosing plants for particular environments. Typically based on expected temperature highs and lows, knowing the hardiness zones will serve only as a guide in plant selection.

The two primary sources of information relating to hardiness zones are from the United States Department of Agriculture and Sunset growing zones. Additionally, the American Horticultural Society has a U.S. zone map based on heat zones using the average number of days above 86 degrees Fahrenheit (which is the point at which plant cells are damaged in many plants).

Know your zones

The USDA Plant Hardiness Zone map which was updated in 2012, is one of the standards used by gardeners and growers to determine which plants are more likely to survive in a particular place. The interactive map, available on the website is based on the average minimum winter temperatures, and is a useful guide to determine which plants will withstand frost and low temperatures.

In Nevada County, some of the zones are 9b for areas around Smartsville, 9a in Penn Valley, 8b in Grass Valley and 8a in higher elevations. Truckee is zone 6a on the USDA map.

The low temperature ranges for these zones is from -10 degrees Fahrenheit in zones 6a to 30 degrees Fahrenheit zone 9b. Although the USDA mapping is useful as a tool for plant hardiness, there are some drawbacks as the map puts the Olympic rain forest in Washington (zone 8b) in a zone shared by parts of the Sonoran desert near Indio in southern California.

The Sunset growing zones are based on minimum winter temperatures and summer highs, but also on a broader range of factors such as humidity, precipitation, wind, proximity to the Pacific Ocean, snow cover, and length of growing season. Considered one of the standard gardening references in the west, the Sunset zone maps are more precise than the USDA map since the broader range of factors that influence growing environments is considered when the zones are mapped.

Locally, Grass Valley, Nevada City, and Colfax all lie in zone 7; the lowest elevations of Nevada County may lie in the zone 9 range with Lincoln and Oroville. (Yuba City and Marysville are in zone 8.)

Check the label

Nurseries and garden centers stock plants from commercial growers and plants may be labeled with USDA hardiness zones. But the nurseries may also have plants labeled with Sunset zones.

Sunset zone 7 (winter lows from 9 to 23 degrees Fahrenheit ) is warmer than USDA zone 7 (range of 0 to 10 degrees Fahrenheit), so the zone numbers are not interchangeable. Further descriptions for local zone 7 includes California’s higher foothills suited for plants that need dry, hot summers and moist, only moderately cold winters.

If uncertain about whether the plant will grow in your area, it is best to consult with the nursery staff. However, it is good to know the growing zones as a guide to search seed catalogs, or when going to make purchases for plants. Make sure the right plant is going in the right zone.

The Nevada County Master Gardeners are on hand at public workshops which are back in full swing. Today, find out about “Making More Plants: Propagating Hardwood Cuttings” from 10 a.m. to noon at the Grass Valley Elk’s Lodge, 109 S. School St.

On Feb. 17, “Wasps of Nevada County: Friend, Foe or Both?” will be offered. “Native Plants” will be the focus of the workshop on Feb. 24.

Check the website at for more workshop information, check the Facebook page, or call 530-273-0919.

Ann Wright is a Nevada County Master Gardener.

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