Free plant stock is rare, unless you know what to look for. It may already be in your ground, waiting to be recognized.
You also have to not be spraying herbicides and weed killers, as many free native varieties are often seen as weeds!
Catnip is everywhere in Michigan, it's in the Mint family and has a square stem; along with all the other mint varieties. If you have Cats, it's priceless! Michigan grows fantastic Catnip!
Wild Violets are abundant in Michigan, and are just popping out in early spring, usually in patches, possibly by rivers; and woodland areas. They are tubers buried in the ground, and when conditions are right, they take hold and flourish. Flint has Wild Violets.
Spiderwort is everywhere as well. Once a popular garden plant, it grows well in Michigan weather, and shows up when totally unexpected; when conditions are right for it to flourish. Often, all that is required is some naturalization techniques, and the elimination of foot, or dog, traffic.
Milkweed, often scourged as a nuisance, is vital to Monarch Butterflies, and quite striking when left to grow and go to seed.
Some other Mints can turn up, Spearmint, Peppermint, and some Lemon mints...will often grow wild among flower beds; especially if you have an heirloom situation with old beds and old plant stock.
Birds and animals can bring any number of additions of seeds to your garden, particularly if you happen to provide habitat for them. Volunteer Berry bushes and Sunflowers are possibilities, and anything else in a birds diet of seeds, is possible. In Flint, the Mulberry is abundant, thanks to the birds.
Depending on where you live, many native plants are the easiest to cultivate, and flourish in our 'harsh' (the 4 seasons of Michigan) weather changes. Wild Onions, Asparagus, Rhubarb, and good old Dandelion are all Michigan hardy food sources; and volunteer tomatoes are not rare!
If there's nothing but bare ground where you are, consider a compost pile and bird feeder to improve your chances. The secret is to not rip out the little green shoots that we all call weeds. They are something! There isn't a plant called "weed", with the exception of the marijuana plant we are all so familiar with.
Nettles, Purslane, and Clover...a perfectionist's nightmare, are often the plants sprayed out of the cracks. The dark green plant with the watermelon shaped leaf, that grows in a rosette formation, can be used on a bee sting to remove the swelling; a remedy my Grandmother used.
It's not easy to identify 'sprouts' from one another, one green shoot looks like all the others, generally. It's in letting them grow that the identification process can happen. Often some part of the plant will give clues to follow; such as scent, shape (like the square stem of the mint family), flower, or lack of flower? Is it growing in full sun, full shade, or a combination of both? What is the leaf pattern in relationship to the stem, and what is the texture or pattern on the leaf? Moss is a good example, it is most often on the North-side of an object or exposure; because it requires a great deal of shade and moisture.
Volunteer Ferns are also out there waiting for conditions to be right, often a mere digging in the soil, turning the topsoil around will expose hidden seeds and spores waiting for air and warmth, to bring them to life. Newly dug flower beds are often full of wonderful little survivors laying dormant, that are now exposed to what is needed for their growing to begin.
Have you ever looked in your garden and wondered where did that come from? There you go! Volunteer plants!