My seeded dahlia grow fine on 6,000 lumens per square foot, so tubers should do the same. They do require the lights be on 17 hours a day, however, and won't think about blooming in that no matter how early I start them. Most flowering full sun annuals do OK under those lights, but only the zinnia and marigolds will blossom in that lighting.

In my case, I'm using T4 lights, one warm white and one cool white to balance the spectrum a little better.

Wow. That means I need 36,000 lumens for a 6 square foot area ? That is SEVEN 4 foot T5 tubes !

I just want to get a 4 week head start compared to direct planting outside...

It would help it if I'd check the lumens and not wing the math. 2,800. Sorry!

I use four T4 bulbs per six foot shelf (24 bulbs total for the entire rack). Like I said, it's tolerable enough for them to grow, but not bloom.

Not quite sure what you mean when you say, "#1 tuber," as tubers come in all shapes and sizes, and as long as there is a viable eye on the head, you can get a perfectly fine plant regardless of the size of tuber. Lots of advanced growers prefer the smaller tubers, in fact.

Dan, you seem to have done some research into lighting, so I say this simply for the benefit of others that may have no clue what lumens and T4/5 bulbs are...

The majority of dahlia growers I know use plain old four foot shop lights, two strips per shelf, (which would be four bulbs of any lumen and any spectrum... No obvious difference in my experimentations) that sufficiently cover four standard trays. It is prudent to figure out how to suspend the lights so you can easily adjust them as the plants grow. Last year, I started a bit too early and ended up with 18 inch plants that were smack up against the lights, leaving burn marks on the leaves. That was with a 28 inch shelf space.

I use reflectors to bounce back light onto the plants, and keep the warmth/humidity up, but watch out for too little circulation that can cause a host of other problems. Fourteen hours of light is fine.

I bought six shelf 4 foot bakers rack at home depot for 90. I'm using 4 of the racks with just 2 regular shoplight over each rack with 4 40w cool white tunes and everything is growing just fine. Short and stocky. I have also started shishito peppers under this setup and they are blooming and starting to set fruit. I also wrapped it with panda film since its in an unheated attic and this winter has been really col and keeps the tuners warmer. I've moved the dahlias under a 400w hps on a light mover to cover a 9x3 table after moving them into square gallon containers. I don't think I will be starting them as early next year. I just realized that power in the Boston metro area goes up to about 20 cents per kilowatt hour during winter as opposed to about 8 cents during the spring to fall months. So it's costing me about 120 more per month or 240 so far, just to get them blooming a few weeks earlier.

Solomon,

How many weeks do you have your dahlias indoors before you plant them outside ? When do you plant them outside ?

Thanks to all that replied !

I was conversing about this very question to a group of grower friends a couple months back, wanting to nail down what the 'ideal' timing would be. Enthusiasts tend to jump the gun a bit (me, me!), needing to see life growing before everything really needs to be started. The result is often higher bills and an extra month of tending to foot-tall plants that really just need warm ground to be planted into. This schedule is what we came up with...

Here is a general guideline for starting dahlias in the spring, from seed, tuber or cutting:

Seeds- 5-6 weeks prior to planting
Tubers for cuttings- 10 weeks prior to planting
Take cuttings- 8 weeks prior to planting
Tubers for plants- 6 weeks for most varieties, 7-8 for giant varieties

Planting differs depending on your location, but a good rule of thumb is when you would plant tomatoes... When the ground is warm, and the last frost is behind you. Too early will set back the plants even without the frost, as they won't continue root growth in cold soil.

Thanks Cici !

How tall will the plants (from tubers) be at week 6 ? Week 8 ?

Honestly, I've always jumped the gun on timing and have 12-18 inch plants to set out, so I couldn't tell you exactly even if there were exact heights to share with you... Depends on if tuber was sprouted when you preplant it, and on variety. Some, like Pooh, will spring into action and be over a foot tall before you know it. Others seem to take forever to wake up and get height.

If you don't have adequate light, they will be more leggy between the leaf sets, hence taller.

Most experienced growers I've talked to prefer the plants to be around 8-10 inches high. This avoids the stalks from being 'flimsy' from the pampered inside, but enough height to avoid the worst of the slug nibbling. I typically plant my tall ones a bit deeper, taking any leaves off that would be underground. Tubers will form at those leaf junctions by the fall.