We have periennial sweet peas that seem to be taking over the whole garden bed. Can they be cut back and when should this be done - spring or fall?
Yes, perennial sweet peas can (and should) be cut back to prevent them from spreading too far. You can cut back in either spring or fall.
In the areas where you don't want them growing, dig up or mow the plants, then put mulch over the area to prevent re-growth. Also, collect any seed pods that have fallen to prevent the plants from reseeding themselves and spreading.
For the main area where you want the plant to grow, you can cut back 1/3 of the vines each year. Cut off any part of the vine that is spreading into an area where you don't want them.