In U.S. Department of Agriculture plant hardiness zone 9, the average annual winter temperatures range from 20 to 30 degrees Fahrenheit. When planting your garden, you need to consider other factors regarding the influence of climate on plant growth in USDA zone 9, such as local climatic conditions, in addition to the USDA classification alone. The right timing for successful summertime plantings in USDA zone 9 varies considerably among the diverse geographical regions that fall in this nearly subtropical zone.
Regional Climate Characteristics
Zone 9 has a tremendous variation in summer temperatures and precipitation. Much of the Pacific coastline of California and Oregon falls in USDA zone 9, but has extremely cool summer temperatures because of the moderating effect of the ocean and the foggy conditions it creates. Many of California's interior valleys are also in USDA zone 9, but there summer weather is scorching hot. USDA zone 9 also includes areas of low elevation desert in the southwestern corner of the country, where the summer is even hotter and drier. Much of Florida and some of the immediate coastal areas of other southeastern states lie in USDA zone 9. In this region, summer temperatures are hot, but they are also extremely humid, unlike their counterparts in the West.
Mild Maritime Areas
The foggy summers of the Pacific Coast are conducive to planting almost anything -- annuals, perennials, shrubs or trees -- because the plants are unlikely to suffer from transplant shock in cool weather. The one caveat is that they must be irrigated. The air temperature may be cool, but there is little or no rainfall in these areas, despite the fog. Also consider that there's not always enough heat to ripen all the summer vegetables and fruits you might plant.
Hot Areas, Wet and Dry
Summer planting in the desert areas of USDA zone 9 is unadvisable. The extreme heat and aridity make establishment difficult even if you give plants ample irrigation. Using shade cloth over newly planted areas may be the only way to sufficiently mitigate the intensity of the summer sun in this region. In areas of USDA zone 9 in the Southeast, summer planting is feasible because of frequent rainfall, and the effect of the heat is diminished somewhat by high humidity. However, summer is not the ideal time to plant in these areas -- spring and fall are preferable -- and consistent irrigation will be necessary to ensure the plants' survival.
Special Types of Plantings
Warm-season annual plants grow well in the summer in USDA zone 9, but you must plant them in spring. You can plant cool-weather annuals for fall in late summer in the areas of USDA zone 9 in the Southeast, California's interior valleys and on the Pacific Coast, but they're best left until early fall for the hottest parts of the zone in the desert Southwest. The subtropical plants that grow in USDA zone 9 can be planted in summer in all areas except the desert, where spring planting is preferable. Summer planting of these tender species is advantageous in the other regions because it gives ample time for their root systems to develop before there's any threat of freezing weather in winter.
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