I start my dahlias, caladiums, and tuberous begonias indoors to get a
jump on the season here in Zone 5, Chicagoland. (FWIW, I buy new ones each year, have had no luck storing tubers. My basement is not cold enough. Some of the pots I store indoors dry, complete with tubers and / or begonias in them: sometimes those survive. I have 2 growing right now, I will have to cut them back hard).

My rule of thumb has been to
start at Easter, but since Easter floats around a lot, maybe I should come up
with a new plan. I have had leggy dahlias before, so maybe I HAVE been starting
them too early. I do pinch them if there is too much stem space between leaves.

Last frost date here is traditionally Memorial Day, end of
May. However, the Chicago Botanic
Garden lists May 15 as the last frost date now.

I don’t have lights for these, the pots are set in front of
a sunny window. Caladiums are started in flats, the rest in pots. Caladiums and
begonias will get transplanted into annual pots, when I have the annuals. Most
of the dahlias will go in the ground. The caladiums take the longest to get going,
all I really expect of them is to make roots and wake up by the time I am ready
to transplant them.

They spend a week or 2 hardening off outdoors before
planting; I pull them in if the temperature dips.

Assuming I transplant these outside at the end of May, how
much head start should I give them indoors? A month (start end of April)? Or longer?

Please specify if you start ALL of these at the same time, or Dahlias first, begonias later, etc. Thanks!