Do I have to trellis blackberry plants?

Blackberries do not need a trellis for support. However, they can be trained to a trellis no higher than 6'-8

They do not need a trellis to grow...although they are a lot easier to manage when they are on a trellis or support of some kind. Otherwise, they can tend to ramble and run and grow everywhere, making it hard to get to the vines to get the berries.

You could you came and string and make a ladder

Trellis blackberries on two or three wires strung horizontally between wood posts firmly planted in the ground similar to the way grapevines are grown on wires. To remain productive, blackberry canes that produced berries the summer before must be pruned to the ground and removed from the bed. The will never produce again. Blackberries produce berries on canes that are two year old, i.e., a cane sprouts from the ground one year and grows to maturity. During the winter, it sets flowering buds and prepares to blossom. During the summer of its second year, the cane blossoms, and produces berries. Throughout that summer, its leaves feed the roots and stores energy for new canes that will sprout in the spring after a winter of dormancy. In the late winter of the third year, before the plants sprout new canes, you should cut the old canes back to ground level. Often, old non productive canes will die spontaneously after they have produced a full crop of berries. The only real reason for growing blackberries, and similar berry vines on support wires is to keep the canes off the ground and allow them to remain up and make it easier to pick the berries without having to stoop or bend over too far. And, you can gather the berries from either side of the bed. A flat trellis would make it difficult to keep the canes pruned out and/or let you pick the berries from both sides.

Sorry Sue I don't understand what you are saying...reword please...thanks.

No they usually are strong enough to support themselves.

You don't have to trellis, but you'll wish you had. Best wishes!

No, but it won't hurt them and will give them (and you) a better way to "discipline" their growth