Window box on the deck
I've got a similar situation--I grow plants in window boxes in full sun on the top deck of my houseboat at Saylorville Lake, just a few miles north of Des Moines. I've experimented a lot. First, start with the biggest window boxes you can get. The length is up to you, but the wider and deeper the window box is, the more soil volume it will hold. Plants will do much better when they have plenty of room to root. I've had terrific success with succulent plants like Purslane and Portulaca (moss rose). I think Purslane is more beautiful and robust. I've also been impressed with the showiness and durability of ornamental sweet potato (Ipomoea) and Malabar Spinach. (I had to grow the spinach from seed, but it's easy and fast growing. It has bold burgundy stems and a look that may be too coarse for a small deck.) I grow purple heart plant (Setcreasea pallida 'Purple Heart') every year. It adds a lot of color and its pubescent leaves (fine hairs) help it hold up in hot, dry conditions. It's a vigorous grower and I end up pruning it back so that it doesn't overtake the window box. It does get brittle by late summer, so if it gets bumped, sometimes stems will fall off. Mine are always so full by then that they still look terrific regardless. Finally, I'll tout one last favorite--Nasturtium. Decades ago, I poked some Nasturtium seeds into a tiny pocket of soil surrounded by concrete on three sides and a garage wall on the fourth. It was a hot, miserable site with lousy soil. The Nasturtiums were FABULOUS. Their bluish-green saucer-shaped leaves catch droplets of rainfall and are every bit as eye-catching as the crinkly blooms. Nasturtiums don't transplant well, so you won't often find them as plants in garden centers. You've got to start them from seed right in the window boxes. But they're big seeds, easy to handle, fast growing, and outstanding.
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