And now, being raspberry harvest time I am seeing that they have a very disappointing yield. It isnt genetics, they have yielded pretty well in previous years, black raspberries always seem to do better but I have always gotten at least a few containers of them. The little red mulberry tree in the background of the picture yields 10x the berries of that entire space this year. I have only lived here one year but picked berries from here since I was a little kid and even last year got a decent amount from here.
There are almost no berries anywhere to be found and the ones that are there are like comically small, only 5-6 little ball/cells on them. Some of the other plants scattered around the edges of the field are doing decent and i have black raspberries not even 10 yards away dripping with berries, its just this patch. My thought is that they have become overcrowded due to me encouraging nothing but raspberries to grow in this spot.
Is there any way to bring them back to life, I've thought of mowing them in alternating strips but i'm not sure when is the best time and whether that would even help the problem, plus they will probably grow right back again. What should I do?
I'm not an expert, but I would look around the edges for new offshoots that you can dig up and move to a new spot. Now is not likely to be a good time for that, but you could start work on a couple of new spots, preparing them for the raspberry transplants going in this autumn.
If the problem is not a disease, it may just be that they have depleted some nutrient that they love. This is one of the places where polyculture is a plant's friend: different plants have different needs, and they share with each other. Think about how you might guild your raspberries, both this patch and your new (smaller) patches.
If you have a brush mower, you could try going right down the middle, clearing that out and then piling on some compost in the alley you made. This might reinvigorate the younger canes on either side.
Monoculture tends to lead to problems, I recommend diversifying your plantings.
Thinning out any growth older than two years should help a lot, though it might not go into effect until the next year. I seem to recall that the new growth of one year becomes the fruit bearing growth of the next year, but that yields drop drastically after a cane is past the second year.