In my travels I am always looking for what is blooming. Driving down hwy 36 and 694, east of St Paul, I am seeing widespread yellow sweet clover and birdsfoot trefoil in bloom. My yard is full of blooming white dutch clover. Red Clover is blooming in the ditches of a new roundabout north of 36.
The conclusion is, that most of us are experiencing or will soon be experiencing a nectar flow. Supers should be on colonies now.
If the deep hive boxes are finished or close to being finished drawing comb, supers should be on new colonies. Supers should always be put on a colony when the colonies are ready for them. Failure to put on supers will possibly make the bees fill the whole hive with honey. This honey bound hive will leave no place for the queen to lay. With no place to lay eggs, the population will suffer and the hive may not survive the upcoming winter because of not enough bees.
The nectar flow can start with a huge intensity. A medium super that is just foundation can be drawn out, filled with nectar and capped in one week. That is why we always put supers on two at a time.