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River mallow or rose-mallow has the potential to grow quite large so moving one of your plants is probably a good idea. Since they've been in the ground for 3 years, they will each have large root systems but if you time the move right, they won't suffer too much from root loss. I'd wait until early next spring, before the plants produce new foliage, because they will be mostly dormant at that time. You can cut back the stems on the one you plan to move, just to make it easier to handle. Use a sharp spade or shovel and cut straight down all around the plant to sever the roots that are extending out more than 12-18" from the base or main stems or trunk of the plant. Expect the roots to be in the top 12" of soil, extending out 12-18". Dig the plant and then replant as quickly as possible so the roots don't dry out. Fill in around the remaining plant to cover the roots and once you've filled in all the holes, water both plants well. Water will help settle the soil and give the roots a chance to re-establish themselves. By the time the plants begin to grow, they should both have recovered from their ordeal.
Best wishes with the move!
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