Having grown up with an acre of raspberries that we did pick, eat, process, and sell, sometimes I see people do/say things that make me want to tear my hair out.
A previous gardener told my client that 'sometimes raspberries just fade away'. Uh, no. Raspberries are survivors. They will survive neglect, forming thickets over the years, just doing their own thing and slowly spreading. If you want the best fruit yield, you will want to give them a lot of fertility and irrigate properly, but no, they don't 'fade away'.
This gardener also made the choice to cut the canes, floricane and primocane both, down to two feet tall. Just below the bottom wire on the trellis. This is infuriating to someone like me who cares a bit too much about plants for a number of reasons:
1. Reduces fruit yield. Even cutting them back to 6 feet reduces yield, much less to 2 feet!
2. The new growth does reach above the wire, obviously, but it's a lot weaker than the over wintered cane, so if I tie by that, it's liable to rip off the branch entirely.
3. They are now extremely branched and much more crowding is going on knee to waist height. Annoying to pick at that level, and there's less light and airflow. This means less photosynthesis per cane and a greater risk of disease.
4. They left the spent floricanes! Bah.
5. It is my considered opinion that you should prune and tie canes at the same time. Like, even just from an efficiency standpoint. If prev gardener had done this, they might have noticed after the first cane that they were creating a problem.
6. They didn't even remove the canes that were too far away from the trellis.
7. If you did think they were weak and fading away, then why in the good green world would you prune away three quarters of their mass?
Ah. Between 3 & 7 I just realized why prev gardener thinks raspberries just fade away sometimes. Don't prune them like this.
I feel like I shouldn't leave without saying what I do so here:
1. Grow in a row with a trellis consisting of two sturdy posts and a wire strung at 5 feet/150cm ish.
2. In the fall, after they have dropped their leaves, cut out the canes that overwintered last year and fruited in the summer. These would be good to use in a bug hotel or deadhedge.
3. Tie any canes that are within a foot of the trellis at their base to the wire. Use a square knot around the canes so it can't sinch down:
4. Once it's tied at the wire, arch the tip over and tie that down too, so it's not flopping all over the place. You can shorten it a bit if you want. This is a simplified drawing to give you the idea of what I mean:
5. Dig up any canes that are too far to reasonably tie to the trellis, and either give them away or replant them closer if there's space. Or make a new row.
6. Mulch over the winter, and then feed with rich compost in the spring. Cover that with a layer of mulch.
7. If you're in a dry summer area, you will get a much better harvest by irrigating. One good strategy is a soaker hose run under a layer of mulch.
If you want a second opinion, I'd recommend your local extension office, or WSU's "Commercial Red Raspberry Production in the Pacific Northwest" and modify to suit your conditions.