Blackberries are perennial growing fruit producing plants that are also referred to as brambles. Blackberries will grow in a wide variety of soils and growing conditions, which makes them an ideal crop to grow in California. Whatever area of California you plan to grow blackberries, they require three elements to perform well: good drainage, lots of sun and keeping them weed-free.
Choose the planting location for growing blackberries where there will be full sun. Turn over the soil to mix in 1 pound of decomposed compost or manure to every square foot of planting area. Do this between 2 and 3 months prior to planting blackberries. This is an important step for areas that have semi-arid soil, or poor soil, like areas around the central valley (Stockton and Lodi).
Create rows for the blackberry plants that are approximately 2 feet wide and spaced about 3 feet apart. In the center of the row, dig 12 inch deep by 10 inch wide holes about 3 or 4 feet apart. Remove the blackberry plant from its growing container (if planting container grown blackberries). Flip the container upside down and pull the container from the root system. Snip off any broken roots, if planting bare-root blackberry plants.
Set a blackberry plant into a previously dug hole. Scoop in soil around the root system and tamp it down firmly. Cut the stems, or canes, back to six inches from the soil. Then water each blackberry plant with at least 1 gallon of water.
Construct a support system for the blackberry plants. Set 8 foot high fencing posts (metal or wood) at the beginning and end of each row so that at least 6 feet of post is above ground. Secure two rows of 10-gauge aluminum wire using wire clips (or staples if you are using wooden posts) between the two posts so the blackberries have something to grow on.
Spread 3 to 4 inches of pine bark, sawdust, or peat moss for mulch down the length of each row. Provide 1 to 2 inches of water a week starting in mid-May through October. The Sacramento, Los Angeles and San Joaquin Valley areas may require more frequent watering depending on summer time temperatures. The soil should be moist at all times, but never water-logged
Fertilize the blackberries using a 20-20-20 fertilizer. The best way to use fertilizer in California is dispensing it three times a year, as suggested by Sunset Wester Garden Book. Fertilize beginning right as new growth appears in spring, mid-March, again in mid-May and the final application in June or mid-July.
Prune back the blackberry plants to 5 or 6 branches per blackberry plant during the winter. Cut them back to approximately 16 to 18 inches from the ground. If any weeds appear carefully pull all weeds by hand to avoid injuring the shallow root system of blackberries.