[ralphyboy]

My mother has some raspberry canes which are at least 15 years old and are not very productive now. She wants to replace them. I am not sure if this is the right time to dig out the old and replace with new vanes or whether we should let this season pass amd replace in the autumn. Any advice would be greatly appreciated

[Trillium]

If they're not producing, there's little point in waiting for fall. The new plants will be bare root or potted, either of which can go in now. Potted can still go in later this season. Depends on the variety you buy but it's not likely you'll get any crop this year, but as you aren't already, you won't be missing anything 

[Kristen]

When I were a lad ... it was said that you should replace the soil when replanting Raspberries (I think it was because yield diminishes over time as the plants become increasingly virus ridden).  Is that still true I wonder?

[Nikkithefoot]

Raspberries are quite hungry surface feeders. Have you mulched them with good compost / manure at the start of each growing season? If not they might just be starving!! My raspberries have been in situ for about 10 years, but still crop abundantly. The one year I didn't manure them was the only year I've had less of a crop.

Choice is yours really. Even if you get new canes you will have to thoroughly feed the ground with well rotted compost before replanting.

Good luck

[Trillium]

When I were a lad ... it was said that you should replace the soil when replanting Raspberries (I think it was because yield diminishes over time as the plants become increasingly virus ridden).  Is that still true I wonder?

Years back, I used to go to a pick-your-own raspberry place and the owner complained about grubs constantly in the fruit beds ruining his crops so it was his last year growing them. But we found out that he strictly fertilized them (never manured) and he sprayed with legal chemicals at the time. In actuality, he was trying to feed the plants rather than the soil which only aggravated his problem. In nature, raspberries will produce forever but you normally find healthy stands among trees where they'd get yearly dumps of leaf compost to feed them. In the gardener's quest for neat rows in optimal sunny conditions, the plants no longer get what they normally would nor in the environment they prefer.
So, if you manure and add leaf mold yearly, the raspberries should bear almost forever.
Ralphyboy could start adding more manure to his raspberry bed but I suspect there are now grub problems to deal with so I suspect new plants in a new location which is well manured and watered would be the better solution.