Although in many parts of Colorado, blackberries and boysenberries are considered marginal plants that won’t grow well, this is not the case for Denver. Denver is considered a “heat island,” according to Carl Wilson of the Colorado State University Cooperative Extension. This means that while surrounding areas of the state are zone 5, Denver itself is zone 6. Blackberries and Boysenberries that are hearty to zone 6 will grow well in this region.
Select a location for boysenberries that receives at least six hours of sunlight. Although boysenberries will do well in partial shade, they will not produce berries in full shade. Avoid locations where you have planted root crops, tomatoes, strawberries or eggplant in the past three years to avoid spreading common pests and diseases.
Break up your soil to a depth of 8 inches with a rototiller up to one month before planting. Spread organic amendments such as compost, peat moss, manure to improve the nutrient structure of your soil. Since Denver soil is high in clay, you should also add gypsum to improve drainage to your soil. Boysenberries dislike wet feet, so improving drainage is important for the plant’s health.
Select a late-blooming cultivator. Early blooming cultivators may produce blossoms that are then killed by late-season frosts.
Dig a planting hole that is as deep as the boysenberry roots and slightly wider. Place the root ball of your boysenberries in the planting hole and cover with soil.
Sink fence posts 1/3 of the length into the ground on either side of your trellis. Stretch fencing wire between the two fence posts. The bottom wire should be 3 feet from the ground. The top wire should be 6 feet from the ground.
Separate new growth canes (primocanes) from one year old canes (floracanes). Tie both types of canes to your trellis. The floracanes produce blackberries, and should be pruned away after the fruit harvest.