Quantitative studies of blackberry-plant yields suggest that annual crop yields can vary significantly depending on cultivar, location, weather and growing conditions. Annual yields should fall within a range of 2,000 lbs. per acre, for a young crop, to as much as12,000 lbs. per acre, for an extremely productive and mature crop. Yields of at least 6,000 lbs. per acre, or 24 gallons per 100 feet of crop rows, are considered good. Individually, blackberry plants can produce up to 50 lbs. of fruit per plant annually.
Blackberry fruit is an aggregate of drupelets that takes about 40 to 70 days to develop on plants following flowering. Blackberry plants begin producing fruit, to a limited degree, the year after planting. The first-year harvest typically yields no more than 2,000 lbs. of fruit per acre. Well-maintained plants will continue to produce crops up to 20 years; but in general, years three through eight are the blackberry plant's most productive years.
Blackberry plants grow in U.S. Department of Agriculture hardiness zones 7 through 9. This warm-climate crop performs best under full sunlight in well-drained soil with a pH between 5.5 and 6.5. Avoid flood-prone sites and poorly drained soil. Sites must have good air and water drainage, as well as protection from strong winds. Temperatures less than 28 degrees F will damage tender blackberry blossoms; consequently, avoid low-lying frost-prone areas.
Adequate watering is the single most critical factor influencing primocane development, optimal fruit development and crop yields. Drought-like conditions before or during harvest can severely reduce the size and amount of crops, as well as the diameter of primocanes. The current-year fruit yield as well as the following-year yield is affected. The majority of moisture taken up by blackberries comes from the rooting zone of the blackberry plant, which is primarily in the top 6 inches of the soil. Provide at least 1 inch of water per day during the growing season, although significantly less water is required if drip irrigation is used.
Growth Habit and Type
Blackberries are classified according to their growth habit, which includes erect, semi-erect and trailing types. Spacing, which impacts plant density and ultimately fruit yield, is cultivar dependent. Typically, erect and trailing blackberries are spaced 2 to 4 feet apart, within rows and 3 to 5 feet between rows. Recommended cultivars vary by area and growing conditions. The number of plants per acre can vary from as few as 400 to as much as 2,800.