Best to wait till October - you can do it now if you like, but you'll need to frequently water it in the ground with a good gallon or two twice a week up till late autumn.

Hi, welcome to GoY, I think if you have had it in the same pot for some time, I would get it planted in the ground asap, you will probably find that the roots are going round and round the side of the pot, and are just a mass of roots, with hardly any sign of any compost, try to untangle the roots, to break the cycle of going around in circles, it won't matter if you break 1 or 2, just make sure that as mny roots as possible are facing outwards, and get it into a well prepared planting hole, you will find your plant will be much happier, just be prepared to do as Bamboo says, and you will have more flowers next year, Derek.