Re: How late can I split?
The answer to your question is highly dependent on your local conditions. I would suggest locating a local experienced beekeeper and ask him/her for their opinion.
In general, you can surely split a hive (that is large enough to be split) as long as swarm season is still in progress in your area. Here, in Denver, our swarm season is just getting into full swing.
The earlier in the flow that you do a split allows a smaller a hive that can be split and still survive the winter.
Also, if you feed the hives, you can still do splits after swarm season has ended.
Providing the daughter split with a mated queen gives them a 3 - 4 week advantage over a hive that must raise and mate its own queen after splitting.
(7th year, 10 hives, Zone 5b, 5500')