There are a few perennials that save their big show for the end of the summer. Among them: Rudbeckia (yellow flowers) and helenium.
The ultimate perennial garden. It's not difficult to bring that vision to mind. For me, it's a classic English border, with iris, lupine and poppies, that continues blooming right into the fall, with wave after wave of color and form: foxgloves, roses, delphinium, dianthus, hollyhocks, lilies and more.
The truth is, since I don't live in the British Isles or the Pacific Northwest, making that vision a reality in my own garden is all but impossible. Bloom time in most American perennial borders runs from late May through early July—a glorious but short 5 or 6 weeks. By late July, most perennial gardens are past their prime and look pretty dull for the rest of the season.
Fortunately, there are many perennials that bloom in late summer, including coneflower, asters, mums, Russian sage, cimicifuga, sedum, rudbeckia, and phlox. But I've found annuals are the real key to summer-long color.
Unlike many perennials, most annuals thrive in summer's heat and once they get started, will keep blooming right into early fall. With a little planning you can create annual-perennial partnerships that will keep your flower gardens looking terrific for a full 5 or 6 months.
I'll provide a list of my favorite perennial companions, but first, here are a couple things to keep in mind:
Heliotrope thrives in the cooler temperatures of late-summer and early fall.
1. Choose the right style of annuals. You don't want plants that are too billowy or too bold in flower or foliage. The best companions are vase-shaped, old-fashioned flowers with a casual form. The types of annuals that work best are those that weave among the foliage of your perennials. Good examples include verbena, nicotiana, salvia, diascia, and heliotrope.
2. Choose heat-tolerant annuals. Unless your perennial border is in the shade, you’ll want to select sun-loving, warm-weather plants such as ageratum, alyssum, cleome, morning glory, browallia, nicotiana, salvia, verbena and matricaria.
3. Don't neglect the tropicals. There are many fabulous heat-loving plants from zones 8 and 9 that can be used as annuals. Though many of these are quite large and possibly too bold to blend into the average perennial border, you may want to find space for them. Good candidates include canna, elephant ear, ginger lily, Mexican sage, and the many varieties of plectranthus.
I plant most of the annuals right into my perennial borders in the spring. Others get planted in the cutting garden and get moved over as needed to fill those inevitable holes that occur as the season progresses. You need to choose a rainy day to move these plants, which may already be in flower, but if you soak them well ahead, and keep the root ball intact, they should survive the move just fine.
Here are the annuals I plant in my perennial gardens:
Matricaria: I can't imagine my garden without this refreshing blast of white. I grow matricaria from seed since I've never found it offered in nurseries.
Mums: Some varieties will overwinter here in zone 4, but I usually treat them as annuals. Plant a couple fist-sized clumps in the spring. By fall each plant will give you dozens of long-lasting blooms. For good late season color, keep your mum plants pinched back until early July.
Coleus: There are hundreds of incredible colors of coleus to choose from. Pinch them back to keep them bushy and remove the flower heads as they appear.
Anise Hyssop: I've interplanted this for so many years that it now self sows and I just dig up and move the plants where I want them. It's not exceptionally showy, but has good foliage and form, with fluffy purple flowers in August and September.
Salvia horminium (Salvia viridis): This is a great plant with purple, pink or white bracts. Like a poinsettia, it's the top leaves that provide the show, not the flowers. Salvia horminium looks good right through late fall.
Impatiens: I plant pink ones along the front of my long perennial border. The border faces north, so the perennials provide them with some shade. Impatiens bloom until frost, and the dependable splashes of pink pulls the border together.
Cleome: I plant these at the back of the perennial border. The four-foot high plumes provide both height and color from July through September.
Ammi majus: Also known as Bishop's weed, this plant looks like an airier version of Queen Anne's lace. Sow it successively from June through July since the plants fade after a couple weeks of blooms.
Sanvitalia procumbens: This low, spreading plant looks like a miniature black-eyed Susan. It loves the heat, and works well in rock gardens or at the front of a sunny border.
Salvia farinacea: The purple 'Victoria' has rich green foliage and spiky blue flowers. Its form is a bit stiffer than most good perennial companions, but I find it makes a great mid-border filler.