Foxglove is very easy to grow from seed, as you have done. After the plants flower, they will set seeds, a lot of seeds. You can easily harvest the seed and resow in the ground around the mother plant. I started out with 2 foxglove and have dozens now. Some years I don't get a lot of flowers, but if you keep doing this, you will have flowers most of the time. Here in Georgia, they bloom early (May) and are mostly done by June. Sometimes they get a second stalk, but not all the time.