Edging a bed with low alyssum, wax begonia or impatiens is a lovely way to unify diverse inhabitants. Reminds me of a doily under a plate of cookies.
Just make sure you have matched the colors, unlike the year I dug up 48 impatients and moved them because they were salmon pink and the perennials inside the bed were purple/pink.
You could emulate Roger De Muth’s lovely choice of dusty miller in his Chenango Street garden. Silver is always soothing.
Unfortunately, growers introduce annuals that don’t match perennials, witness peach, hot and salmon-y pinks and purples that have too much blue. [I wanted peachy things so much that they got their own bed.]
Even those ‘Heavenly Blue’ morning glories and blue hydrangeas are the devil to find companions for. I hate blues and purples together. They both lose their power to sparkle when combined.
Foliage is part of every combination, whether upright coleus, towering canna or trailing sweet potato (and most likely all of them at once). Some great gardener coined the phrase, “thrillers, fillers and spillers”, which refers to the elements necessary for a great planter.
Getting more colors and textures from leaves, perhaps on flowering plants as well, stretches our gardening budget and makes more exciting combinations. Remember to limit the number of colors to three, so everyone gets to shine and share the spotlight.
Watering is a constant, so a pot large enough to grow into avoids having to do it every day, but that doesn’t mean it shouldn’t look full for a month.
Leave some breathing space between the residents up top, but a deep root run will pay off in August. Filling the bottom of a tall pot with packing peanuts or gravel helps no one. Fill the bottom with mulch or compost and the roots can penetrate, but it will never get waterlogged. This keeps the cost of potting soil and weight down, as well. Don’t forget a coffee filter or used softener sheet over the drainage hole to keep critters out and soil in.