I think of April, May and June as the English season in New York gardens. In the cool sunny days early flowers like violas, pansies, ranunculus, primrose and alyssum bloom in dizzy profusion. These, combined with flowering shrubs and roses, are our opportunity to have an "English garden" that will soften the heart of a minimalist -- if he admits it or not.
So go for it. Plant window boxes with violas and alyssum. (I took this picture last month in Pacific Grove, California, where they have English weather year round.) Time is short, only a couple of months before hot weather comes. Then the cool season flowers flag and it's on to tropicals that the English envy us for.
Besides, you can't plant impatiens yet. It's pansies or nothing.