A lot of project management tasks are simply housekeeping: Maintenance tasks that keep things running smoothly. For a project, that could be doing things like closing out old work items that were successfully completed, but which the developer failed to mark as complete in the work item tracker. Or it could be verifying that all the milestones for open work items are set correctly. Or it could be looking for work items that are assigned to people who have since left the team. Or it could be closing out stale pull requests.
You get the idea. There are always some housekeeping tasks that need to be done to ensure that the project runs smoothly. The Microspeak for performing these housekeeping tasks is gardening, as in, “I did some gardening over lunch and moved all the unfinished work items from the previous milestone into the current milestone.” Or “Bob, can you do some gardening on the wiki and delete the outdated information?”
As with most housekeeping tasks, gardening tasks are largely mechanical and do not require the person performing them to make any decisions that alter the project. Gardening is primarly about getting the bookkeeping up to date with the current reality.
I suspect that this Microspeak term is not in use in the UK. In the UK, if you ask a work colleague to do some gardening, that is a euphemism for asking them to resign.