Every year I try to keep an eye out to see which flowers are the last to go before winter hits for good, and even though it is now the second week of November and we are getting our first real snow today, it's not even close to the end of flower season. The bergamot in the yard still has a few frail wisps of color, and on my walk to/from the bus stops this morning (a total of about 4 blocks), I saw white sweet clover, red clover, tansy, tansy ragwort, European bellflower, dandelion, sow thistle, cress, and evening primrose, and I'm quite sure that there's lots more out there.
Depending on the weather, I've seen some flowers (sweet clovers, tansy ragwort, and once liatris) hang on until early December, and I've seen dandelions as early as mid-March. And of course, pussy willow catkins are technically flowers, too, and they'll start coming out in late February sometimes. Even up here in the frozen north, we only have about a 10-14 week period with no flowers.