When plants in pots become too large to be repotted, they are….

Topdressed in spring. Allow the surface soil to dry slightly, then use a small trowel to scrape away the top 1-1 1/2”. Replace the surface scrapings with fresh potting compost. Leave a space between the top of the compost and the rim, so that the plant can be watered when the potting compost becomes dry.