THE COASTAL GARDENER:Planting in the zone

Having returned from garden visits this summer to places such as 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 Vancouver. Bananas are regular components of gardens in London. 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 U.S. 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 its zones again. The USDA uses a one-to-11 numbering system, which is further divided into A’s and Bs.

In 1967, after 30 years of publication, “Sunset Western Garden Book” introduced its own 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 one to 24, but recently five more zones were added to cover Alaska and Hawaii.

Are you confused yet?

The point of all this zone stuff is supposed 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 canyon 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, S.C. Surprisingly, we share the same plant zone as Orlando, Fla., whose latitude is 10 degrees less and is almost 500 miles farther 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 the biggest factors affecting 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.

Hostas, for example, will usually decline in USDA zones 9 and 10 and “Sunset” zones 21 through 24. Our problem is that temperatures do not drop far enough in the winter for a hosta to go completely dormant. Hostas, 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.

ASK RON

My Western garden book does not address how to handle a divided clivia. Cut leaves back? How much? Prune roots lightly? Heavily? What type of soil and any soil additives?

There is very little soil remaining in the pots, mostly roots. (It was largely adobe originally.)

Thank you for your help.

BARBARA

Newport Beach

I have found clivia very easy to divide and replant. Once the clump is dug up, the individual divisions can be broken apart from one another by hand, even when rather small. Knock off most of the soil; rinsing the roots with water works well for me. Leave the attached roots intact, only removing dead, broken or decaying parts. Remove any dead, brown leaves; otherwise do not cut back the leaves. Allow the divisions to air-dry in a shady spot for a few days.

Clivia like an organic, slightly acidic soil, so mix in liberal quantities of a quality planting mix, as well as some acidic azalea-camellia mix. After replanting, keep the area dry for another week, then add water sparingly.

In Orange County, clivia can be divided any month of the year, but fall or spring is probably best.