Are these new plants this year? Assuming yes I would cut the raspberries to the ground after they go dormant in the fall, then remove the plastic. (assuming red raspberry) My experience with raspberries is they will crowd out most weeds once you get them established, and that only take the first year.

The blackberry being a 2yr cane, I would wait until buds start opening next spring, then take a knife and cut up the plastic as needed to remove. My Black berries don't crowd out the weeds as well as my raspberry, but I don;t feel any need to protect it from weeds.

For black berries, pinch off the top of each shoot when they get like 4' tall and they will grow thicker bushes, rather then tall and gangly. the new branches will need pinched as they try to get really tall also.

I'm not sure about raspberries, but I do have blackberries. For most of the new shoots, they shoot out in the near proximity, less than 6 inches. Some do shoot out farther, but about 3 to 6 feet out in few years. Instead of relying on new shoots for propagation, both can be easily propagated by tip layering. This method will allow you to control where your next bushes will be. Also, separating it from the mother tree after a successful tip layering will hinder any continuity of disease in case any one of your berry bushes gets diseased.

This post was edited by Avocado101 on Sat, Jun 15, 13 at 20:43

I grow raspberries and blackberries and I have never used plastic mulch for just the reason you mentioned. I am learning it would have been difficult to gauge the size of the hole to cut in the plastic mulch liner. Instead I've opted for regular black pine bark mulch and its working really well. Mrs. G

I use a woven plastic weed barrier on my raspberries. I place it between the rows (mulch on top), and leave a 9" - 1' gap of the plastic on the row proper. This allows the berries to send up as many shoots as they like in the open space of the row, but keeps them from filling in everywhere. Very little weeding in the row once they are established. However the berries have spread out and started sending shoots up beyond the plastic.

Overall though, this seems like a good compromise for them.

You will be trampling all over the plastic repeatedly during harvest and pruning, unless you intend to pull it away from the row for a month or so. It will probably get torn up and you will step on many fallen berries hard to see against the black plastic. Ground-level plastic in landscaping eventually becomes a mess.

The woven weed barrier is likely more durable, the mulch more versatile.

Weeds only need to be removed to one foot away from any clump of canes; the rest of the patch can be weedy.
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Avocado, what disease(s) does tip-layering hinder?

Does the black plastic weed barrier need anything put on top of it so it doesnt get so hot? Can the blackberries in Missouri tolerate the heat it causes?