How to Grow Boysenberry Plants

Prepare the planting site. Choose a site with full sun exposure and well-drained soil. Till the soil, thoroughly working in compost or other soil amendments to enrich the soil.

Plant boysenberry plants in holes 3 feet apart and at least 1 foot deep. The University of California Cooperative Extension recommends loosening the soil to 2 feet deep if drainage is poor. Spread the roots well in the prepared hole, and apply a thick layer of mulch.

Water the plants thoroughly after planting and throughout the growing season. Boysenberries like moist, well-drained soil. The University of California Extension recommends soaker hoses or drip irrigation rather than overhead watering, which can lead to rot and disease. Stop watering after the first frost to harden the plants for winter.

Fertilize only if the plants do not seem to be growing well. Most berries do not require much fertilizer. If you fertilize, you can use either a 20-20-20 inorganic mix or an organic fertilizer such as blood meal or fish meal. Fertilizer should be applied to the soil and watered in.

Train the canes to a trellis to protect the plants from insects, disease and rot. The most common trellis for trailing berries is three wires stretched between posts, with the wires at 2, 4, and 6 feet above ground. Tie the canes to the trellis as needed.

Watch plants for signs of pests. The two most common are likely to be mites and cane-borers. The best deterrent for cane-borers is to remove the affected canes. Mites are more difficult to treat. The University of Colorado Extension states that pesticides do not seem to be effective against them and often kill beneficial insects. They recommend a light spray of summer-weight horticultural oil if necessary.

Harvest berries in late June to early July. Ripe berries will be dark red or black and will separate easily from the plant. Boysenberries should be picked in the morning, as they become soft in the afternoon heat. They have a short shelf-life and should be used or preserved within two days of harvest.

Prune the plants after harvest, removing the old canes. Boysenberries produce fruit on new growth each year. Allow five to eight new canes to grow and remove all other new growth.

In November, protect the plants for winter. Prune the new growth back to 5 or 6 feet long. Remove the canes from the trellis and lay them flat on the ground, all pointing in one direction. Shovel a shallow furrow alongside each row of plants, and cover the plants with the soil. Do not water the plants or uncover them until after the danger of frost is past. To uncover them, use a pitchfork to lift the canes from the dirt. Rake the excess soil back into the furrow.