Dead-head it & you'll have blooms all summer long. You can dig & divide & replant in the spring.

It should continue to grow all year if you don't have frost. I would just stop fertilizing it for a while in December and January for a resting period and then start it going again when most of your spring flowers start to grow. You can cut it back to keep it manageable towards the end of the resting period and as the Plantlady pointed out if you want more plants dig in the late winter and divide.

Deadhead for continuos blooming, the plant will wither on it´s own, when bone dry just cut the dead foliage and stems about 1" above soil level, you can leave it in the ground to over winter or lift the tuber to divide. I leave mine in the ground and dig them up every 3-4 years.

Thanks guys for the advice. Sounds really easy , I'm so glad no pruning back like roses. How about when it gets really cold :-) like in the 40's Do I have to cover them up or will they be just bulbs in the ground with no plant and leaves??
Thanks

Nothing bad is going to happen to them in the ground.

Dahlias are native to Mexico ( Mexico´s national flower ), more specifically, they are native to the Altiplano ( the platteau that forms the central portion of the country between the Sierra Madre Occidental ( part of the Rocky Mountains ) and the Sierra Madre Oriental ( along the Gulf of Mexico ) ), I live in central Mexico so I´m in the exact geographic area origin of dahlias, I leave mine in the ground year long plant or no plant, I even plant other things on top of the tubers to cover the bed with color, so not even regular watering for the other plants has had a negative effect on the tubers resting dormant in the flowerbed.