Blackberry bushes have two different growth habits. They either grow strong canes that can stand erect or grow weaker canes that trail. The thornless blackberry variety is of the trailing kind and is usually grown on a trellis. It is less hardy than their erect counterparts and therefore needs winter protection in climates that have cold and freezing winters.
Cut off the canes (branches) that produced blackberries after harvesting. These canes, which are called floricanes, will no longer produce fruit and will die after the first hard frost. Use hand held clippers to do this job and remove the debris away from your plants.
Prune off all but 6 to 12 of the canes that did not fruit this year. These canes, which are called primocanes, will produce fruit next year. Choose the healthiest and strongest looking primocanes and try to evenly space them out.
Remove the ties if you tied the primocanes to a trellis or other support system and lay the canes on the ground.
Cover the primocanes with straw after the first hard frost (also called a killing frost), which typically occurs when temperatures dip below 30 degrees Fahrenheit for at least two to four hours. Cover all the trailing canes with about 3 to 4 inches of straw. You can use another organic mulch, such as pine needles or leaves, but straw can be easily removed in the spring.