USDA Plant Hardiness Zones, California Native Plants, and Such

If you look at the map of USDA plant hardiness zones for the South-Western region, you can see that the zones in California range from about 6a to 10a, meandering casually, and not perfectly from the mountains to the coast.

To determine your USDA zone, you can use the USDA website, or use this site, click here to check using your zip code. Alternately, you can also use this site to get your USDA zone by zip code, click here.

For convenience we also provide a table to convert from one system to another.

The USDA zones on the map indicate the average annual minimum temperature range for that region. And smaller zone numbers imply lower minimum temperatures. So how does this map help? At a high level, this map is useful because if you know your zone, then when you are buying plants, you will know if the plant you picked will make it through your winter. Most plants sold by reputable nurseries will have a tag that shows the plant’s hardiness zone classification.

When I first looked at this map it was conceptually very confusing. I wondered how one could use the average minimums to decide which plants will grow in any given region. Surely the summer high temperatures and other stressful conditions had an impact on plants and their survival. And how was I to go about evaluating each plant for best fit for my specific garden conditions? The answer is that, actually you cannot, not very exactly and scientifically. I have bought many plants with the correct hardiness zone for my garden, lavished hours of care on it, and they have died anyway. Perhaps because the plant liked a higher humidity, disliked the chilly California summer nights, westerly summer winds, or other such reasons. And one plant, forgotten in a corner, has surprised me by turning into a 6-foot bush by the end of the second growing season.

There is a lot written about the Mediterranean climate in California, which usually means warm dry summers with wet and mild winters. Plants that grow in this climate, including those from other similar climatic conditions around the world, are adapted to survive this climate and do well in many parts of California. These plants, due to their adaptation to the dry summer conditions, are by and large drought-tolerant and are included on water-wise lists published by many Californian water districts and other resources for drought-tolerant landscaping.

But most of the guesswork is eliminated if you choose natives that grow where you live. These plants are already adapted to the highs and the lows, the soil, and the rainfall where you live. There are literally thousands of beautiful native species of plants to choose from for a given zone. Until a couple of decades ago, it was not possible to find a lot of these plants in nurseries. But hundreds of different species are now available in nurseries that specialize in natives.

Note that you must never go to an open space, pull up plants and bring them back to you garden. Not only is this not legal in California, but it is also very unlikely that these plants will survive this transplant.Most water-wise native plants have put down very long roots to enable them to survive the long dry months, and uprooting them will surely kill them. Always buy plants from a reputable source like native plant societies and nurseries.

Let us go back to the zones for a little bit. A strip on the California coast is zone 9b (minimum temperatures 25-30F), with a bulging inwards of this zone in the middle of the state behind the Bay.  There are many exceptions of course, but this is generally the case.

Just inside zone 9b is zone 9a (minimum temperatures 20-25F), which covers a lot of the central valley farming regions. The exception is the southern part of the state where there is the true desert, the maximum temperatures are higher here, and some of this is zone 10a (30-35F).The dry and hotter regions behind this are dominated by zones 8a and 8b, with a bulging of this region towards the coast in the middle and lower quarter of the state, and into the upper end. Then there are the Sierra foothills and the mountain regions themselves, represented by zones 7b to 6a.

What this really means is that, given that large swaths of California are covered by the same zone, many native plant growers and nurseries have found that plants that don’t grow in your immediate area, but are from similar zones in other parts of the state are also great choices for your garden. With this expansion, there are now literally thousands of plants that are available to an urban or suburban gardener. The benefits from choosing a drought-tolerant California native plant over other drought-tolerant plant from another Mediterranean region of the world are obvious by now. The benefits to the local ecology are immense. Especially in the two areas of California (the Bay Area and LA), where development has eliminated large tracts of old-growth land and support to the local insects and birds, you can provide the much needed come-back in your small garden. Ever since I converted my lawn to native plants, I see lots of bees, butterflies, and birds in my garden. My fresh-water pond provides a source of fresh water to a large variety of birds all year through, and birds are regular visitors to the yarrow plant for seeds and berries from other plants such as the huckleberry and berberis.

But which plants do you choose for your garden? Our plans provide you help as to their general applicability for different counties. And you can also contact us if you are not sure.

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