I've seen paulownias before, but only really as single specimens, now and then, and of course when they aren't in flower one isn't particularly likely to notice them. The experience of rounding a corner and finding a whole square of trees which appeared to have been painted purple was a new one, which seemed to add a fairy whimsicality to the city.
It was pointed out to us that they are really only in this state for a week or so of the year, so we were lucky to be there at just that moment. Which might be a(nother) good reason to go to Paris in the first week of May every year.
To look at foxglove trees in bloom
Twenty springs is little room
As Housman didn't say, but Haussmann might have done.