This time of year, there are tons of free plants just waiting for you to claim them, right in your own garden. There’s one small benefit to tidying up your garden in the fall: all those perennials you’ve cared for over the years have continued to grow faithfully, and they sometimes benefit from being divided. Here’s what you can and can’t share, what to look for, and how to do it.

Here’s a rule of thumb: Never divide a plant that’s still flowering, but a plant that’s just finished flowering is ready to be picked. Right now, we’re dividing plants that will flower in the spring and summer. There are three main reasons to divide: Either a plant has outgrown its space, it’s had a bad year and could use a makeover, or you just want to double it and divide it so you can move a division elsewhere.

This includes irises, phloxes, lily of the valley, lilies, hostas, veronica, salvia, coreopsis, and the whole daisy-like flower club: black-eyed susans, shasta daisies, coneflowers, and gaillardia. You can also use this time to divide perennials like artichokes and horseradish. If you are unsure whether a plant should be divided, take the time to look it up.

Some shrubs and flowers do not do well with transplanting and they do not do well with division. These include poppies, woody shrubs such as lavender, and flowers such as lupine, butterfly weed, fireweed, and shoots. The good news is that in a few cases, such as butterfly weed and lupine, these plants will do the work of self-seeding for you. In the case of woody shrubs such as lavender, you will find that some of the lower branches will root themselves and those divisions can be separated and transplanted.