It really depends on the type of plant in terms of when and how much to prune. Some plants you can shear off or prune up to 1/3 or more of the plants overall size (perennials all the way to the ground, which it looks like you have quite a few), other plants not so much. There are plant identification apps out there you could use to snap pictures of the leaves for each plant species you would like to prune, from there you can look up pruning advice for each one (assuming its spring or summer and has leaves). Also key to pay attention to is when to prune each plant, most like spring or fall or a combination of both. Best of luck with your yard, feel free to contact us if you are looking to replace these areas with more low maintenance plantings.