Zinnias are usually healthier and more vigorous when grown from seed directly sown where they will grow. If you do purchase zinnias in nursery pots, look for plants that are not obviously root bound since they may have reached the point of being permanently stunted. When growing from seed, it is best to transplant them at a very young stage where the resentment of the transplant will be less severe. Annuals grow very quickly and can reach the point of no return in their little nursery pots before they are purchased.

Molly

I find the zinnia pretty easy to collect seeds from . If you like that variety in the growing pot, maybe you can put it outside to get pollinated, then collect the seeds.

Part of transplanting will let you learn which plants do better than others in transplanting, but in general, if I want the least disturbance to a root ball, I tease the root ball a bit, then plop the entire old root ball + soil into a new pot, usually bigger and with new soil.

If I am more confident that the root ball can handle a bit of pruning, then I might take a knife and make cuts to get it smaller. Then it goes right back into the same pot or a similar sized pot, or a bigger pot, but it now has the chance to grow into the pot.

For zinnias, I think I would treat it as the first case, and disturb the root ball as minimally as possible. Even if planting it in the ground, I would plop that entire pot's contents into the ground.

I have rather a large area of zinnias, and I just scatter the seeds in the fall and they come up in the spring. I also allow the seed to fall during the blooming time and this gives me new plants that bloom all season long. This would also work in a small area where you want to keep them confined.