Answer: There are two types of blackberries, erect and trailing. The primary difference is the growth habit of their canes. Erect blackberry types have stiff, arching canes that are somewhat self-supporting.
Trailing blackberries, also called dewberries in the East, have canes that are not self-supporting; they include the Marionberry, Boysenberry, Loganberry, Youngberry, and Thornless Evergreen.
Erect blackberries are more cold-hardy than trailing types. However, you can grow trailing types in colder areas if you leave the canes on the ground and mulch them in winter.
All blackberry plants are perennial, with roots living for many years. The canes are biennial; they grow one year (primocanes) and produce fruit the following year (floricanes). The floricanes die after they have fruited. New canes are produced each year from roots or the base of old canes. The floricanes need to be removed each year after harvest, but the new primocanes, which will fruit the following year, need to be thinned and trained.
It's a good idea to trellis all blackberries. You can grow erect blackberries without support, but trellises keep the planting neater and make both cultivation and harvesting easier.
A simple trellis system of wire supports strung between posts is preferred. You may use either wood or metal posts for the trellis. Treat wood posts with a copper-based preservative. Use heavy end posts with lighter posts spaced 20 feet apart in the row.
About 4 1/2 to 6 feet of post should show above ground. A two-wire (12-gauge or heavier) system with the top wire at 5 feet and the other 18 inches lower is common for trailing types.
If you're growing only a few plants, train canes to stakes placed beside each plant. Wrap and tie the canes spirally around the stake. Training the canes to a multiple-wire trellis or an existing fence are other options.
In the summer, remove the top 1 to 2 inches of new primocanes when they're 3 feet tall. This causes canes to branch, and these branches will produce fruit the next year. After harvest, remove the floricanes; these 2-year-old canes will die by the end of the growing season.
In late winter, when plants are dormant, thin primocanes to three or four of the strongest canes per plant. If you have plants growing in a hedgerow, thin to one large cane every 5 inches in the row. Prune lateral branches on these canes to 12 to 18 inches long.
In most trailing types, new primocanes are produced in the spring at the crown of the plant. After growing upright for a time, these canes turn down and grow along the ground. To avoid injury to new primocanes, keep them trained in a narrow row beneath the bearing canes.
When harvest is over, remove the floricanes. Thin primocanes, leaving 6 to 12 of the sturdiest canes on each plant to bear next season.
As for your hydrangea, you can cut it back to 12-18". New growth should emerge from the old wood and produce flowers this summer.
Best wishes with your garden!
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