Don't know about alstromeria but I cut penstemons back to within a few inches when there's lots of new growth coming up from the base. You should be able to get some cuttings from the old stems if they are still flourishing.

The alstromerias if the foliage looks ok I would leave if not cut down to tidy them up and you will soon have new shoots coming up to replace them , I do have several in the greenhouse since last year all look great with being protected

a fresh top dressing of compost they would appreciate.

Gg

I'd cut the penstemnons down and use the cut stuff for cuttings . as for the alstros never had that problem.