I am planning to plant some flowers in two half barrels that sit in front of my brownstone apartment building (I rent the ground-level apartment). The half barrels get about 4 hours of full sun a day, and each already contains a boxwood (see photo). My objective is to add a little color to a rather drab-looking area, so ideally I would like flowers that would bloom all summer. I'm not sure what would look right with the boxwoods and would love to hear any ideas to help get me started. Thanks!!
We have found that Impatience plants do well in shade especially when you get the amount of sunlight that you have mentioned. You can get these in several different colors any of which would look great when grown next to your boxwood. When we water we usually mix fertilizer at 1/4 strength and apply the fert. at least once a week.
I also agree with flowAjen that Coleus would look great in your planters. It also comes in several colors.
Coleus you plant for the color of the foliage and Impatience you plant for the bloom. Either would work well in your situation.