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My "wild" blackberries in the back yard have finished fruiting and guess I should cut them back.
can anyone advise me on how to do this? please try to use words of one syllable or less, it would be even better if you could just grunt and point as I don't know anything about cutting back berry canes.
they also seem to have rust as an aside- should I try to treat this now that I am theoretically going to be cutting them back?

All blackberries, cultivated and wild, together with loganberries all fruit on one year old wood.
So if you can see where the berries were on the stems, then those are the ones to cut down to ground level.
The new growth should look strong and healthy with no sign of any dead fruit bunches or branches, and those are the ones to NOT cut off.
It's a lot easier if you do that as soon as they've finished fruiting as it's then obvious which are the just fruited canes and which are new growth.
Err, also thorn proof clothing and gloves, but that's entirely optional.