Dahlias, tuberous perennials native to Mexico, are excellent flowers to grow in your California garden. They thrive in California's widely varied climates, which are spread through USDA Hardiness Zones 4 to 11. Although you can grow dahlias from seed and cuttings, the easiest way to grow them is from tubers. No matter which California climate you live in, plant your dahlia tubers in a location that receives at least six hours of sunlight each day.
Work 2 to 4 inches of compost or peat moss into the top 12 inches of soil. Also incorporate 2 to 4 lbs. of a balanced fertilizer (8-8-8 or 10-10-10) for every 100 square feet of planting space. Do this about six weeks prior to planting.
Select dahlias that grow well in your California climate, particularly in zones 9 to 11 where summers are extremely hot. There, choose varieties that can withstand the summer heat, such as the Prince Noir and Winsome. In cooler zones, most dahlias will grow well.
Plant your tubers directly outdoors after the last spring frost, which varies among the different California climates. If you live in an area where frost is not an issue, plant them anytime in the spring.
Loosen the soil to a depth of 12 inches and then plant your tubers on their sides about 6 inches beneath the soil with the eyes facing up. Fill the holes with 2 to 3 inches of soil and only fill in the rest as they grow. Exact spacing between tubers varies among the different varieties, but in general, about 2 to 4 feet apart will suffice.
Apply 3 to 4 inches of mulch such as bark or pine needles. During the hot California summer, be sure to add more mulch as it decays and settles.
Water your dahlias to keep the soil moist, usually about 1 inch of water a week. However, during the hot summer months in the coastal and southern portions of the state, you will most likely need to water them two or three times a week to keep the soil moist.
Thin the shoots as they grow, if desired. Each tuber can grow more than one stalk. To get the best blooms, clip off the extra shoots so all the plant's energy will go into just one flowering stem.
Insert a stake for tall dahlias once they reach about 12 inches tall or add the stake at planting. Place the flower stake next to the planting hole and loosely tie the stem to the plant with twine as the dahlias grow.
Fertilize dahlias in July with the 2 to 3 lbs. of 5-10-10 or 10-10-10 fertilizer. Follow the dosing instructions on the label for correct application methods. Water your dahlias well after fertilizing.
Cut the foliage in the fall after the first killing frost in USDA Hardiness Zones 4 to 7. Dig the tubers up and store them in damp perlite, vermiculite or peat moss in an area between 35 and 50 degrees Fahrenheit. In zones 8 and warmer, cut the foliage to about 2 to 4 inches and cover them with mulch during the winter months.