Zonal geraniums are a perennial. In warmer climates where they only get a few degrees of frost they can get very big, unless they sucumb to some disease.

That probably was just an ordinary geranium that had been wintered over for quite a few years. Victorians grew zonal Geraniums as a house plant, because as long as they have sufficient light they made a beautiful houseplant that required very little attention and they tollerated indoor growing conditions

Geraniums grow from seed or cuttings but, I have never heard of a zonal Geranium growing from a bulb. I don't profess to know everything and I've been wrong before, but I have never heard of it.

I agree with oilpainter - what you saw was most likely a mature form of the common zonal geranium or Pelargonium.

And no bulbous geraniums :-) There is one that is tuberous - Geranium tuberosum - but that's as close to a geranium "bulb" as you're going to get. Quite a few species of pelargonium that have a tuberous or geophytic growth habit as well but they are rather uncommon out of their native habitat unless cultivated by collectors. What may be confusing the issue is that there is a spring flowering narcissus (daffodil) with the cultivar name 'Geranium'.....and that's the closest to a geranium 'bulb' I can think of :-)

Thank you for the replies, oilpainter and gardengal. :)
Last week I brought in some of my potted geraniums before the frost could get to them. They had been dying down but now they look so happy and perky and have fresh new blooms. I'm going to see how long they remain happy and healthy.
What should I feed them during the winter? Will they go dormant?
I did a search for geraniums and found pics of narcissus. Tricky plant names.

Grow them like a house plant. Don't over water. Use the finger test--Put your finger in the soil, and if it feels wet an inch down don't water. Fertilize with a 1/4 strength water soluable fertilizer about 3 times during the winter, and you can put them outside next summer and then treat them like you did this year. Be sure to harden them off when you do.

I save my geraniums every year and grow them inside. I Don't let them flower and cut them back if they get leggy. The reason I don't let them flower is that I want the good to go into making new shoots not flowers. I want to take cuttings in the spring. If you want to do this then check out my clippings