How to Grow Boysenberries

Boysenberries (Rubus ursinus x ideaus) stand out among other blackberry varieties with their exceptionally large, juicy fruit. Healthy, established boysenberries can live for eight to 10 years, bearing a sizable crop every two years on their biennial canes. Boysenberries need minimal hands-on care and will yield four to eight quarts of fruit per plant each year. They must be planted under the right conditions, however, to produce the best yield.

Boysenberries grow best within U.S. Department of Agriculture plant hardiness zones 6 to 9 (Ref 2), although the thornless boysenberry (Rubus ursinus x ideas 'Thornless') also grows in USDA hardiness zone 5. Established plants will withstand temperatures as low as -10 degrees Fahrenheit without protection, although they may need a little extra help if temperatures continue to fall. Lay their trailing canes on the ground and cover them with a thick layer of straw until spring. Remove the straw when temperatures warm, and reposition their canes back on the support trellis.

Boysenberries need six to eight hours of sun exposure daily and moderately fertile, fast-draining soil to perform well. A soil pH between 6.0 and 6.5 is best, although most brambles will tolerate acidity down to 5.8 with no trouble. Amend heavy clay soil or sandy, porous soil with a 2- to 4-inch layer of compost to improve its texture and drainage. Boysenberries need 6 feet of space and a hefty support structure to keep their canes healthy, so choose a planting site with plenty of room away from walkways where people might accidentally brush against their sharp thorns.

Established boysenberries rarely need heavy irrigation, although newly installed plants benefit from regular, deep watering during their first season to encourage root establishment. Provide new plants with 1 inch of water each week, wetting the top 10 inches of soil. Water established boysenberries only if less than 1 inch of rain falls during any 10-day period. Mulch reduces the need for supplemental water, especially in warmer climates. Spread a 3-inch layer of pine needles or straw around each plant to help conserve soil moisture during the summer.

Fertilizer needs of boysenberries vary according to the plant's age and the time of year. Newly planted boysenberries benefit from a starter solution two to three weeks after planting. One tablespoon of 12-12-12 analysis fertilizer sprinkled in a 12-inch circle around the base of each plant provides the necessary nutrients. Starting in their second year, boysenberries need feeding only after fruit begins to form. Feed each plant with 1/4 cup of granular 12-12-12 fertilizer. Boysenberry roots can burn when exposed to fertilizer, so always apply fertilizer to wet soil and water deeply afterwards to dilute it.