No light is required for tubers to eye up or develop shoots. The stimulus is warmth , not light. Remember, one common method of growing dahlias is to begin by totally burying the tuber, eye and all. Once the shoot appears, then light is required.

What nemo2009 says is true. However, when starting up my dahlias, I usually set then into a damp growing medium with heat below and light above. This makes the growths upright and easier to see. It is then easier to split, if you haven't already done so. The lamps I use are 4 foot florescent bulbs which I rescued when my friend was remodelling.

Hi pdshop, you can bring dahlias into flower by altering the photo period, a bit like poinsettia.
If you get your tubers to shoot roots and foliage early by heat, Then make sure your light period is the same or longer that the natural day in summer and the plants will try to flower as though it were summer.

In my climate the winter is warm enough to grow dahlias only the day length is too short, ie the nights are too long.
So what I do is put a light on their foliage for approximately 1 hour in the middle of the night, as dahlias like other plants affected by photo period tell what time of year it is by the length of night not the day so if you reduce the night into 2 short nights then the plant thinks it is summer and will continue growing and flowering not dormancy.
allan (only dropped in to this forum by chance and may not be back to answer questions ? my appology)

Hi pdshop, I bought a 400 MW Metal Hallide lamp last year, and had dozens of large dahlias ready to go by planting time. Since they were so far along, I was able to plant them much earlier than recommended. They thrived and produced hundreds of flowers from early July onwards.

I haven't seen a 400 MW Metal Hallide lamp in my reading? Maybe I wa in the wrong placce. Can you tell me the brand or anything that might lead me to where I could get it? Thanks

The article below has developed over the past 15yrs as I have worked with propagating dahlia seedlings and cuttings.

Here is a link that might be useful: Plant Lighting for Dahlia Seedlings and Cuttings

Sorry that I have not been checking this forum. There is a good article on using the metal halide and high pressure sodium lights here:

http://www.jasons-indoor-guide-to-organic-and-hydroponics-gardening.com/indoor-grow-lights.html

If you search "metal halide for growing," you should get the information you need. My dahlias just exploded last year with the metal halide light.

Here is a link that might be useful: garden lights