Self seeders can get a bad name if they cross the line and become "weeds" (which is really just a loosely applied term for plants you don't want — even in the plant world, one man's trash can be another man's treasure). They can behave wildly differently in various regions. Some are invasive in some areas, but mild mannered cherished garden plants in others — so take my list with a grain of salt and use logic based on where you live. (I live in Zone 5/6 (New England).) Here are some of my favorite self-seeding plants:
Oriental Poppies (Papaver orientale) - Once these showy flowers are finished, the seed heads remain. These funny upside down cone shaped seed heads are fascinating to open up. The insides are chock full of poppy seeds (just like the ones on your bagel). Let it dry out and shake it around wherever you want to plant. I did this freely for years as a kid, much to my mother's appreciation, and we always had the most beautiful mix of shasta daisies and orange poppies in June.
Foxgloves - (Digitalis var.) - My own are the inspiration for this post. Right now, they are in full bloom and will continue to be for at least another week or two (it takes time to flower from the bottom to the top of those lanky stalks). They are also easy to manage, as you can see where they are going to grow a whole season ahead of time. After these finish flowering, the seeds will spread and those seeds will grow plants that I can move around later this fall. Actually, those new babies that will pop up through the summer are probably from seeds from last year, but the plants that grow this year, will bloom next year. (These plants are bi-ennials) I love picking off these stems, when they are ready, and by shaking the stalks around I can migrate my floxglove collection through the garden.
Nigella hispanica - "Spanish Love-in-a Mist" - I have struggled to get these going in my garden, but I love those blue flowers so much that I persist in my efforts. Maybe you have had better luck and can share a tip?
Knautia Macedonia - These dainty red heads on wire-y stems give an airy feeling to any planting group. I love to have them mixed with anything, as I think they do a great job of loosening up a planting that might be just a little too uptight.
Asclepias speciosa "Showy Milkweed" - In general I'm a fan of all Milkweeds, but the speciosa is particularly special — they are great for the butterflies and their healthy silvery leaves and big showy flower heads (that spread seeds liberally) are so fun to watch open. And when the seeds set in, you are in for a real treat if you dismantle one of the pods. These seed heads showcase the best of nature's ability to pack a big punch in a small package. The feathery stems on the seeds (that help them fly away) are packed into the pod in such an amazing way it is hard to imagine such perfection comes completely naturally. In case that isn't enough to make you love Milkweed, the shells of the seed pods are fantastic material for all sorts of fall and holiday crafts (I have a huge collection of milkweed Christmas ornaments).
Do you have a favorite self-seeder to share? Let us know where you live so we can more better sort out where your suggestion does well.