I bought a box of snapdragons 3 years ago and wonder how to overwinter again this year. They have continued to flourish and even grow in some of the cracks of my stonework.
It sounds like your snapdragons (Antirrhinum spp.) or snaps have self-seeded themselves for the past couple of years.
In Toronto’s hardiness zone (Canadian Zone 6/USDA Zone 5), especially if we have good snow cover over winter, sometimes snapdragons may act like a tender perennial, returning from the same plants in spring. However, if they are not deadheaded, they will often self-seed in the garden. They act like what is often called a “hardy annual” which is an annual plant that spontaneously returns from its own seed that is not killed by winter cold. (Another popular hardy annual is Cosmos.)
Snaps will grow best in cool weather and show their best colours in spring and fall. Deadheading will increase the number of flower buds that will be set, to encourage the longest-possible show of blooms over the growing season. If your snapdragons continue to self-seed, after several years the flower colours may not be as vibrant.
Good luck and have fun with these happy, old-fashioned garden flowers!