Jan, I always plant mine the same time I plant cannas and glads. They are in bloom by July. They continue to bloom right up to frost. Dead heading does help to keep more blooms on.

I have started them in the house and then planted them outside around May 15th (plant a little deeper than what it was in the pot). This gives a good month earlier bloom and they will bloom until frost. I love dahlias and would plant more but the aphids and ect. love them and I get tired of washing or spraying them. I can spray them and the following day they are covered again with aphids. Anyone else have this problem. I live on a farm and fields are all around my gardens so I do have more problems than people that live in town.

I also plant mine in pots first , then set out Second week of May. Have never had trouble with aphids. And I'm in the country. Maybe all the bird feeders I have out. The birds eat the bugs also. Or maybe the lady and Asian bugs.

OK, my ignorance is showing. Sanfan suggested deadheading for extended bloom. I assumed that they would be like lilies, with each bud blooming once, but no reblooms. Are you saying I'm wrong? If so, it's really nice to be wrong! Thanks for your advice.

Here's an article from a Seed company:
General Care of Dahlias
Water established Dahlias thoroughly and deeply once a week. Water more frequently if it's very hot.
To promote a compact, bushy growing habit, with more flowers, pinch back the new growth when your dahlia is about a foot high. If your goal is to produce massive sized flowers, remove all of the side buds at the end of each branch throughout the growing season. If you want your Dahlia to provide a continuous, extended flower show, you will need to remove the spent buds promptly.
The tender new growth of a Dahlia is a favorite entree of slugs and snails. Take the necessary precautions to protect your plants from these evil lawn prawns.
If the identity of your Dahlia is important to you, be sure to add a tag to the stake at planting time. This will be an invaluable help when it is time to dig your tubers in the fall.
For the finest cut flowers, cut them early in the day, when they are first opened. Place them in water which has set for 24 hours to allow any chlorine to dissipate, and change the water daily. Cut Dahlias will last from 5-7 days.