Q: I have a tough time putting together container plantings that remain colorful all through the summer. Would you recommend some long-lasting but unusual flowers that I’ll be able to find at the garden center? — Mindy Beltramo, Santa Rosa, Calif.
A: My recommendation: Do not rely on flowers alone. Long-lasting container combos tend to partner plants chosen for extended bloom time (usually annuals) with plants that add attractive foliage colors, shapes, and textures to the mix (often tender perennials). If one plant goes into a slump, another will pick up the slack. To keep things simple, I often restrict each pot to only three kinds of plants. Though all three should require similar sun or shade conditions, it’s best if no two members of the trio are of the same height or shape. One may be tall and upright; another, medium-high and mounding; and the last, low and spreading, to soften the container’s edge. Besides giving you aesthetic contrast, this arrangement avoids competition for light and space.
To compose a three-part harmony, you need a container that’s at least 12 inches across — the larger the vessel, the more plants you’ll be able to include in each height range. If the container is going to stand against a wall or fence, put tall plants in the back, where they won’t block light and air from shorter neighbors. If the container will be out in the open, place the tallest plants in the center. The same principles apply when you group smaller containers, each holding only one or two plants. For extra height, you can set some of these containers on overturned empty pots and mask the improvised “pedestals” with lower containers in front.
Many of the best tall accents for pots have variegated foliage, especially cordylines, yuccas, phormiums, and Canna ‘Pretoria.’ Vertical specimens with handsome reddish leaves include other canna cultivars, purple basil, and varieties of fountain grass. For silver leaves, try Plectranthus argentatus. For the middle, mounding layer, the range of annuals is almost limitless. Dependable choices for shade are browallia with amethyst, violet, or white flowers; hypostes, grown for its green leaves splashed with white or pink; torenia with snapdragon-like flowers in purple, blue, or pink; and the ubiquitous impatiens. For sun, Swan River daisy, lantana, coleus, dwarf morning glory, and shimmering osteospermum extend your options beyond the familiar petunias, geraniums, and nasturtiums (but don’t write these off just because you see them a lot: they turn up in so many pots because they are tried-and-true). Virtually any free-flowering or handsomely leaved annual that stays between 6 and 18 inches high is a good midsize container prospect. Regularly snip off spent flower stalks and errant branches and administer a dose of dilute liquid fertilizer every week or two, and the parade of color will go on as long as the mild weather holds.
Appealing spreaders for the lowest layer of your container are licorice plant (Helichrysum petiolare), creeping Jenny (Lysimachia nummularia), ornamental sweet potato vine (either Ipomoea batatas ‘Blackie’ with deep purple leaves or the cool chartreuse I. batatas ‘Margarita’), blue-flowered scaevola, white bacopa (Sutera cor-data), ‘Tapien’ verbena in lavender or pink, and blue trailing lobelia. Combining colors harmoniously is a subjective enterprise, and in practice you’ll probably limit your palette to what’s available at your local garden centers. But here are a few guiding principles for spinning the flower color wheel: pinks, blues, and purples are nearly always compatible; hot yellows and oranges work well with reds that verge on purple or brown. Of course, white and silver go with everything, as do very pale yellow and amazingly versatile chartreuse.
Remember, too, that it’s easy to add and subtract colors, even in midseason. Using a long knife, cut a circle around the root mass of the plant you want to remove, pull out the cylinder of roots and potting mix (toss this on the compost pile), and plug in your newfound beauty. In a week or so, the replacement will look as if it had been in your container from the start.