Marin Master Gardeners
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Marin Master Gardeners

Marin IJ Articles

What does 'hardiness' mean, anyway?

July 14, 2012
Martha Proctor

What helpful, readily available tools can a Marin gardener use as a guide for choosing plants that will be successful in the garden?

One basic tool is the USDA's 2012 Plant Hardiness Zone Map. This newly released edition of the hardiness map, online at http://tinyurl.com/752vadg, is the standard by which gardeners and growers across the country determine which plants are likely to withstand frost and the average low temperatures during winters in their vicinity. This new edition boasts more accurate data, because of better collection methods and improved weather analysis.

The map divides the country into 13 zones based on the "annual minimum temperatures" recorded from 1976 through 2005. Each zone is a 10-degree band, further divided into 5-degree zones labeled as "a" and "b."

The hardiness zones in California range from 6a to 10a as the land tapers off from the mountains to the coast. Much of Marin falls into areas with minimum annual winter temperatures of 30 degrees to 35 degrees, defined by the map as hardiness zone 10a. Several communities including Fairfax, San Rafael and Corte Madera contain sections which fall into both 9b (minimum temperatures 25 degrees to 30 degrees) and the slightly warmer zone 10a. Inverness, San Anselmo and Larkspur fall into 9b.

Interestingly, a number of Bay Area communities have been shifted to slightly warmer zones in the 2012 edition. Check out the lowest minimum temperatures in your zip code.

Marin's Mediterranean climate, which typically means long dry summers followed by mild, wet winters, provides gardeners with a vast choice of plants that will thrive here. Plants that are native to California and Marin in particular, or those from Mediterranean regions, are already adapted to the highs and lows, soil and rainfall here.

To be sure a perennial plant (one that lives from year to year) will survive in your garden, check the label describes the plant as hardy to Zone 9b/10a or higher.

The USDA Plant Hardiness Zone Map does not include annuals because these plants only survive during one spring, summer and/or fall. Hardy annuals can withstand a little frost and bloom again the following year. Frost tender perennials from zone 9a or lower may need to be grown as annuals unless they reseed. For example, lantanas are considered to be perennials in Marin but are treated as annuals in regions too cold for them to complete their life cycle.

A tiny sample of the flowering plants Marin's climate allows us to enjoy includes daffodils, hydrangeas, irises, rhododendrons, alstroemerias, geraniums, dahlias and a host of drought-tolerant plants. In addition, those who have raised beds or grow plants in containers know the joy and nutritional benefits of growing the multitude of delicious fruits and vegetables that flourish in Marin.

The USDA's hardiness zone map is best if used in combination with the American Horticultural Society's Plant Heat Zone Map. The AHS divides the country into 12 zones based on the average number of days in the year that the daily high temperature reaches or exceeds 86 degress, the temperature at which plants begin to suffer damage from heat.

Marin communities fall into six heat zones ranging from Inverness in Zone 1 defined as less than one day at 86 degrees or above to Corte Madera in Zone 6 with 45 to 60 days. It is important to note that the AHS Plant Heat-Zone ratings assume that adequate water is supplied to the roots of the plant at all times. The accuracy of the zone coding can be substantially distorted by a lack of water, even for a brief period in the life of the plant.

While these national hardiness zone definitions provide important information for serious gardeners, Marin gardeners are best served by utilizing Sunset's Western Garden Book's comprehensive definition of the microclimates in the Western U.S. In addition to including average hot and cold temperatures, Sunset divides each of 26 prescribed areas into zones based on a range of issues including summer fog, humidity, wind, elevation, influences imposed by continental air and proximity to the ocean, as well as latitude and the length of the growing season. Marin gardens fall into Sunset zones 15 (Novato), 16 (San Anselmo) or 17 (Inverness).

Most gardens contain numerous microclimates — areas that are colder, warmer, shadier or windier than the rest of the garden. Thus, while zones are a good place to start to determine a plant's adaptability in your region, many other factors such as microclimate, wind, soil type, soil moisture, exposure and humidity play an important part in plant survival.

Experiment with the hardiness ratings — plants often grow outside the ranges indicated — you never know what might survive. The bottom line is that if you want a shrub, perennial or tree to sustain itself year after year, the plant must tolerate the lowest and highest temperatures and the amount and distribution of rainfall in your garden's microclimate.

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