Know your planting zone when purchasing new plants

If you recently moved from a northern state to Yuma, you may be confused as to what to plant this fall.

Plants that tolerate northern cold don't tolerate our Southwestern heat; but, mild winter temperatures allow gardeners to grow vegetables, herbs and annuals from October through May. Our long winter season has made this area the winter lettuce capital of the world.

The USDA Agricultural Research Service states that the newly-released 2012 USDA Plant Hardiness Zone Map is the standard by which gardeners and growers can determine which plants are most likely to thrive in a certain location. The map is based on the average annual minimum winter temperature divided into 10-degree Fahrenheit zones and was originally devised during the mid-1900s by the U.S. Department of Agriculture (USDA). These zones help the agricultural community rate its plants for sale to the public. The zones also help gardeners choose correct plants to grow successfully in their location. Yuma, USDA Zone 10b, has average minimum temperatures between 35 and 40 degrees F. (the earlier USDA range was 30-35 degrees F.). The new map can be found at www.usda.gov by searching for “zone map.”

In 1990, the zones were further divided into an “a” zone which included the lower five degrees of a zone and a “b” zone which included the higher five degrees. However, plant breeders have yet to start using these smaller distinctions on their nursery tags.

Like everything in nature, there are always exceptions to the USDA Zones. Some plants listed for zone 9 will grow in Yuma (zone 10b) if placed in a shady location. The extra shade creates a micro-climate favorable to plants that need cooler temperatures. For instance, I am growing wisteria, USDA zones 3-9b, along my back fence. Because it is shaded by a large cottonwood, it is doing well, although it did take awhile for it to acclimate to Yuma's hot weather.

Another example of a plant growing outside its proper zone is a sweet gum tree my mother planted. This tree is zoned USDA 6-9 but is doing well in full sun. The secret to her success was keeping it shaded each summer until it was established and about 7 feet tall. Once established, it has grown to over 25 feet in height, in full sun, and shows off beautiful red, purple, and yellow leaves each fall.

Just to make gardening interesting, there is a second climate zone map developed by Sunset Magazine, a longtime magazine which offers great articles about gardening in the southwest. In the Sunset Zone System, Yuma is zone 12-13; a low or subtropical desert area. Sunset Western Garden Book describes the area as having possible frosts from Dec. 1 to Feb. 15.

Sept. 22 is the official start of fall; but, in Yuma, mid-October usually begins our cooler weather. Now is a good time to set out transplants or seeds for carrots, artichokes, broccoli, Brussels sprouts, cabbage, cauliflower, collards, kale, beans, beets, bok choy, lettuces, mustard greens, spinach, Swiss chard, turnips, bunching onions, summer squash, leeks, and radishes. Plant herbs and strawberries, cold-hardy trees, such as Chinese pistache and desert willow, sow wild-flower seed and annual seed, and set out transplants of annuals and perennials.

No matter which zone map you use as a reference when choosing plants, your garden's success is determined by the right location, proper soil preparation and correct watering.

Happy Gardening!

Karen Bowen is a master gardener and member of Yuma Garden Club. This column is sponsored by the Federated Garden Clubs of Yuma.