I had never done that and my colleague explained we were cutting back 1/3rd of the older branches to the ground, then arranging the remaining ones on the arch and shortening laterals to 2/3rds.
|All branches untied and spread out
|A previously pruned rose
It looked to me the plant was rather top heavy, with plenty of laterals and sublaterals on a single stem, so the most challenging part was to reconcile the principle of keeping suitably shortened branches and allowing some distance between them to avoide rubbing (that might cause dieback and provide access to other pests and diseases). By the end, I found what I thought was a good compromise, but I had cut more than the expected one third of the plant.
|The final pruned rose, tied back on the arch
Roses usually respond well to hard pruning, and there were some vigorous new shoots in this one that I left, but I am a bit worried that the plant might put too much energy in vegetative growth next year, as opposed to flowering, which would be a pity as this is a very attractive spot for visitors.
Well, one can only wait and see...