A boysenberry vine is neither a tree nor a true vine, even though most people call it one. If you have asked yourself is boysenberry a tree while shopping for plants, the honest answer is no. The plant grows as a trailing bramble cane in the Rubus genus, with long flexible stems that you must support to keep them off the ground.
I planted my first boysenberry crown ten years ago and watched the canes flop right onto the soil. The boysenberry growth habit fooled me at first because the stems looked like they would stand up on their own. They never did, and that lesson cost me a whole season of fruit.
Trees have woody trunks that hold themselves up year after year, but your boysenberry has soft green canes that bend under their own weight. True vines like grapes use tendrils to grip and climb, while boysenberry canes have no climbing parts at all. They sprawl and drape, which is why the trailing bramble label fits this plant best.
The plant works on a two year cycle from a perennial crown that lives 15 to 20 years. First year canes called primocanes grow long and leafy without fruit. Second year canes called floricanes flower, set berries, and then die back to make room for new growth.
When I tried to grow mine without a trellis, the canes piled onto the wet soil and half of them rotted by spring. Mature canes stretch out to 8 to 10 ft (2.4 to 3 m) by the end of their first year. UC Cooperative Extension recommends a three wire trellis at 1.5, 2.5, and 4.5 ft so each cane has a spot to rest.
Proper boysenberry trellis support keeps the fruit clean and lets air move through the leaves to stop fungal problems. You tie your floricanes to the lower two wires and let new primocanes grow up the top wire. This split keeps next year's canes away from the fruit you pick today.
Install your trellis before you plant the crown, never after. Setting posts later means you risk cutting roots when you dig the holes. Tie the first season canes to your wires once they hit 3 ft (90 cm) tall. Your boysenberry will pay you back with buckets of fruit for many years. The plant rewards good support more than any other care step. Mulch heavily around the base and keep weeds away from the crown for best results.
Read the full article: Boysenberry Plant Growing Guide
