The home we moved into 2.5 years ago has a shady patio with two ready-to-plant window boxes at the base of each kitchen window. Since settling in, I have been happily experimenting in the world of window box plants, and I’m pleasantly surprised by how many can thrive in the shade. Among a handful of shade-happy annuals I chose, my favorites by far are my begonia (I don't know the exact variety), Strobilanthes Persian Shield and Euphoriba Diamond Frost. I love how the begonia blooms look with the dark perisan shield foliage and the ethereal nature of euphorbia’s baby white blooms. If you have shady window boxes you would like to plant, consider the following:
Composition. When evaluating what plants to buy, make sure to look at each plant’s tag to understand its height when mature and overall growth habit. Does it trail? Grow straight up? Or grow in a mound? You don’t want all the plants in the box to be 12” high or all of them to be trailing. Try to buy a mix of plants so that some grow high, others form mounds, and some trail. This variation will create visual interest. My plants are still growing so they'll need a little more time before they fill out the boxes as I envision.
Palette. If you are as interested in color as I am, you’ll also want to consider bloom and foliage color. I find this very challenging and end up spending too many hours (yes, hours) arranging the plants on the ground at the nursery to make sure I have just the right color combination. I’m always striving to create interesting, unexpected palettes that complement the color of our house (greyish teal).
Care. Shade plantings don’t dry out as much as plants in full sun, and I have made the mistake of over watering in the past. Make sure the soil is fairly dry before watering, and be sure to fertilize once a week. I use liquid miracle grow, but any fertilizer for annuals is fine.
Protection. Lastly, be sure that your window boxes are secure! We have one that is unsteady and has fallen off the house every spring onto our perennial bed. I will not elaborate on how maddening and disappointing this is. I whole heartedly suggest you employ nails, screws, shims—whatever you think necessary—to protect those precious babies, folks. Happy planting!