This year I am hoping to obtain a cutting off of a neighbor's hydrangea and I am trying to get some forsythias to take root (as much as I love the bright yellow, they may end up being bonsai-ed, not sure where I would put a full size forsythia- but never say never!) In years to come I am sure that I will obtain other bulbs, cuttings and divisions, and will pass along more than a few to other gardeners.
I'm telling you all of this for two reasons: one, I am a sentimental fool, and love to look around my garden and see little bits of friendship from all of the people who have given me plants and two, dividing plants is a necessary part of gardening.Plants getting root bound in their pots; carrying so many baby suckers that they are trailing on the floor; bulbs, rhizomes and root stocks getting so big and crowded that the plant can't get enough nutrients to bloom correctly, all of these are reasons to divide. Each type of plant has its own set of rules about how and when to divide it, any good garden book should be able to guide you. But what to do with all of the "baby" plants when you do divide?! (yes, there were 6!!! plants in a 6" pot!) Passing them along to friends, especially those that you know are starting a new garden, looking to expand their garden, or want to add more perennials instead of annuals, are good places to start. If they are newbies, you may even get lucky and they will accidentally kill it and in a few years you can pass more along when it is time to divide again. There are groups out there (like the seed saving group that Amy is now part of) who not only trade seeds, but also divisions of plants. It sure keeps the cost down if you can find somebody with the kinds of plants that you want, and somebody else probably wants the kinds of plants that you have. Guerrilla gardening is another way... only a few of the irises that I dropped along the train wall seemed to take this year, but that means I can try again 2 years from now when they need divided again! Just be careful not to introduce invasive non-natives such as bamboo or honeysuckle- in the right- controlled- location they are great, but they can cause destruction of the habitat rather than enhancement.
If you can't find somebody who wants them, another option is always the compost pile. Now when it comes to things like the peony and irises that came from someone else's lovingly cared for garden, and especially because they were given to me when I was starting my own garden, I try extra hard to find someone to pass the starts on to. Sometimes though, it isn't practical. Many plants, such as thyme, mint and oregano crowd themselves out of their pots so fast that I am chopping a chunk of the plant out every year and adding fresh soil. Often times someone will need more herbs and will take them (speaking of... does anybody need oregano, mint or thyme for this year? I thought I had divided them in the fall, but the roots still seem kind of tight, so I will gladly pull a start off for you if you need them) but just as often, everybody else I know will have a bumper crop of herbs, that's where the compost pile comes in. Even though it is "throwing a plant away" I like to remind myself that once decomposed, it will add nutrients back to my garden so that other plants will be big and healthy. If a flower start is extra tenacious and lives through being thrown in the compost bin, I take it as a sign, replant the start somewhere else in my garden or a pot and enjoy the beauty of the flowers. One of these is going on my desk at the office, the other 5 are getting added to my shade garden in the alley as soon as the temperatures go up just a bit higher.