Understanding Plant Zones in Orange County, California

Having returned from garden visits this summer to places like Portland, Vancouver and England, I feel compelled to write about a common frustration of the weekend gardener – “Zones”.

It’s hard to pick up a seed packet, read a plant tag or browse a garden article without being a victim of “zone” talk. “Hardy to 7B”, “Grows in 8,9,12-24”, “Heat Zones 7-9”, “Perennial in Zones 21-24, annual elsewhere”, etc.

Palm trees DO grow in both Vancouver. Bananas are regular components of gardens in London. Conversely, Lilacs and Redwoods DO grow in Orange County. Nonetheless, gardeners in Orange County can’t grow a decent peony, lily-of-the-valley or aspen tree. Don’t zones have something to do with this?

In 1960, the US Department of Agriculture published its first zone map. A few years later it was significantly revised. In 1990, in cooperation with the National Arboretum, the USDA updated their zones again. The USDA uses a 1 to 11 zone numbering system which is further divided into a’s and b’s.

In 1967, after thirty years of publication, The Sunset Western Garden Book introduced its on plant zone system. Likewise, Sunset’s zones have been revised several times, essentially every time a new edition is published. Sunset’s zones are numbered from 1 to 24, but recently five more zones where added to cover Alaska and Hawaii. Are you confused yet?

The point of all this zone stuff is supposed to be to let you know what plants you can grow in your yard and which ones you can’t. It’s a simplistic shortcut to a complex topic. Gardeners, like most people like simple, straightforward, black and white answers. Here’s another overly simplistic shortcut . . . a 15-30-15 fertilizer is a better value than a 5-10-5 fertilizer, right? Not necessarily.

At best, plant zones are a guide.

Probably everyone reading this column is, according to the USDA, gardening in zone 10a or 10b. If you’re a Sunset reader, you’re almost all gardening in zone 24. This is certainly true of the communities of Huntington Beach, Costa Mesa, Newport Beach and Corona del Mar. A handful of residents in the canyons bottoms of Newport Coast might drift into zone 23 or even 22.

Now that you know your USDA and Sunset zone, plant selection will be simple, right. Not exactly. Consider this; Orange County shares the same latitude as Lubbock, Texas or Charleston, South Carolina. Surprisingly, we share the same plant zone as Orlando, Florida, whose latitude is ten degrees less and is almost 500 miles further south. In addition, Orange County receives no summer rain while summer is the wettest time of the year in Orlando. Are Orlando gardens and Orange County gardens really that much the same? Consider that Orange County gets only 13 inches of rain per year, while Orlando receives a torrential 60 inches each year.

You are beginning to see a few of the problems with plant zones.

It gets worse. One of biggest factors effecting plants is summer heat. Many plants from the north are not able to handle southern California’s hot summers. Also, nighttime heat and humidity, not just during the day, has a significant effect on the suitability of many plants for local gardens.

Compounding the situation even further, the fluctuation between night and day temperatures, in many cases, also affects plant health. During the day, plants store up energy. If nights are cool, this energy goes into the growth of the plant. But, if the nights are too warm, this stored energy is lost.

Now let’s introduce another factor, heat dormancy. Hosta’s, for example, will usually decline in USDA zones 9-10 and Sunset zones 21-24. Our problem is that temperatures do not drop low enough in the winter for a hosta to go completely dormant. Hosta’s, like many other plants, need a dormant period at a specific temperature in order to start growth again in the spring.

In summary, whichever zone system you follow, use it only as a guide, not a rule. Many factors are at play when deciding whether a plant will be a good choice for a garden. Yes, our gardens are in USDA zone 10 and Sunset zone 24, but don’t stop there. There’s much more to know before deciding whether a plant will grow well in your garden. After all, remember that gardeners are like doctors . . . they bury their mistakes.

by Ron Vanderhoff, Nursery Manager at Roger’s Gardens, Corona del Mar