Hi Padma,
I plant my tubers about 20" apart. I also stake them while planting so I won't disturb the tubers later. For stakes I use metal stakes used for electric fencing. They are about 3' tall and have the triangle at the bottom for stepping on to push into the ground. They are silver colored and round. I got them at a lumber yard for $1 each about 5 years ago. They are a bit rusty but will probably last forever. Once the Dahlias are up you can hardly see the stakes. Good luck to you with your new bed.
Tina

Padma,
As Tina said you definitely want to give them "room to grow", even as much as 24 inches apart.

My soil is VERY rich from combost/bonemeal and TLC.
I say this, because, the dahlias I grow, end up being closer to 6 ft tall. So I have, from the beginning (as per Bernie :o), use 8 foot lengths of reinforcement bars (re-bar) when I plant.

The tubers could have been a gift from last year of "bedding" dahilas, that grew first year 18 inches tall. Second year was almost 4 ft.

My point, other than I LOVE to garden, is that it is better to go taller, then to disturb the tubers later and risk losing them.

Good luck,
God Bless,
Di

Hi Padma,
In a 4 x 6 foot bed, I'd barely have courage to plant more than 6-8 dahlia tubers if they are not the short 'bedding' dahlias. I've had luck planting some shorter ones 12-18 inches apart, but more often 24 inches is the minimum distance. Some of those monsters get big, as Di said.

Ten inches deep sounds ok- a nice bed for tubers to rest in as long as it is well-drained.

Precautions to take: don't break your tubers. Stake when you plant so you don't stab them later. Some people use bone meal dug into the planting hole to help the plant roots along, but I find that it is a huge dog magnet and have lost tubers to inquiring paws. grrrrr....

Don't over-water early, and don't plant when it's too cold, soil below 60º or so. Wet and cold are a bad combo.

Good luck with your planting!
Poochella