Hi there! I'm in NY, about an hour North of NYC. So we get a real winter.
Do I need to wait until spring to get pullets, or will they do ok anytime?
I would wait.
Chicks take weeks and weeks to get fully feathered (even under the wings) to thrive even in cool spring temperatures without a fluffy mama hen around, and you would need to keep your pullets heated practically through the winter. They would have little time to adjust in your climate (actually no time), and I don't think I'd try even in mine.
The longer you can wait in spring, the better, depending on your set-up for the chicks. Ours was pretty sketchy and not designed to hold chicks for the amount of time we had to have the supplemental heat. We purchased ours in March so they would be old enough for fair in August. Last time I do that if I can help it, but it was an unusually cool and slow spring.
I know it's hard to wait, because as soon as the hens get old enough to lay the light starts fading and production dips. If, on the other hand, you have a fully enclosed set-up, with supplemental heat and a generator to back it up (imagine if you lost power!) then you *might* be able to pull it off. But all that effort! When you can wait until April or May and have little trouble whatsoever, and the pullets would have plenty of time to grow their feathers, and then time to acclimate to the progressively colder temps.....
As long as you can maintain their environment, even through power outages, this should work in theory. And replacement bulbs for the lamps, because they will be on 24/7 for months!
The type of brooder I really like is one that hovers and maintains that proper temperature locally, but allows the chicks to move outside that if they wish. It simulates a mother hen. Naturally brooded chicks can be outside in cool temperatures, if they have their mother nearby to warm up under. In a frigid winter, they can roam the shed or garage and duck under the brooder. My chicks this year really loved the bricks I put in under the heat lamps!
The brooder temp starts at 95 degrees for newly hatched chicks, and drops about 5 degrees for each week until the temperature inside matches the temperature outside. If the weather is warming up significantly, you can fudge on this, but if it is getting *colder*, you can't.
You need enough enclosed space for all the pullets-- depending on the breed, they are nearing their adult size at 5 months (the approximate laying age, though that varies wildly between breeds). I was taken off guard with my set-up this spring. It was waaaaay too small because I was expecting warmer temperatures we just weren't getting. What a mess!
You need a generator, but I'm guessing a lot of folks up there (technically down from where I am, but, anyway....) have one on hand anyway.