Usually by this time of year, my windowbox impatiens are obstructing the view from the lower third of the windows. This year, the plants are still small and the flowers sparse. (The tuberous begonias and lobelia in the same windowboxes are doing okay) Other planters of impatiens around my yard are thriving and bushy, so it isn't this year's coolish summer that is to blame.

Does anybody have any idea what I can do to rev them up a bit?