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On the cusp of fall, the direction a flower garden is headed may not be the stuff of viral social media images. And it may not inspire a weary gardener’s delight.

But don’t stop now. As Jenny Rose Carey, a longtime flower gardener and vocal believer in the possibilities of the weeks ahead, puts it: “September is the new May.”

Of course, in the garden, May is a moment neither short on color nor marred by what she calls “the crispy bits.” (First action item: Tease those out with your snips and push ahead.) But Ms. Carey offers some encouraging words. Think fresh, she advises us, and think vivid — even as autumn comes on, and right through to frost.

Ms. Carey, a popular lecturer and author, is the former senior director of the Pennsylvania Horticultural Society’s Meadowbrook Farm, a public garden in a historic estate setting. So it is no surprise that her recipe for a design with staying power relies on strategic plant choices.

It also involves something that gardeners too often skip: proactive maintenance tactics that help coax the maximum output from flowering plants — even annuals planted months ago. At her garden in Pennsylvania, the nasturtiums are still at it, welcoming fall as enthusiastically as they do at Giverny, the painter Claude Monet’s garden in France, where they lead visitors down the literal garden path into the next season. You just have to know the trick.

As the August heat begins to wane, Ms. Carey, who taught horticulture at Temple University for years, is in back-to-school mode. That means taking field trips: visiting gardens in search of ideas, especially during the seasons that often stump us. She has taken inspiration from public gardens like Wave Hill in New York City, where fall has long been a celebrated season, and Chanticleer Garden in Wayne, Pa., where this month and next are still prime time, too.

How do you keep your garden going strong until the frosty end? Start by looking at landscapes like those for clues.

Flower Shape and Scale (and Color, Too)

The title of Ms. Carey’s new book makes her approach clear: “The Ultimate Flower Gardener’s Guide: How to Combine Shape, Color, and Texture to Create the Garden of Your Dreams.”

Color is just one element of her strategy — and it’s not even the first thing on her list. Although it may be what many gardeners focus on when they’re plant shopping, a color palette alone will not ensure cohesion in the garden.

And strong color elicits strong opinions: Hot-colored gardens have their detractors as well as their adherents, and so do pastel ones.

“But in general, everyone agrees that a garden that looks really pretty, nice, joyful,” Ms. Carey said, “has a variety of shapes of flowers, textures of leaves and heights.”

She advises us to look more closely at form in the garden — including the forms of flowers. How often do we gardeners consciously consider that?

The pollinators do. A diversity of insects responds to a range of shapes. Each species’ evolution alongside particular native plants has engineered intimate relationships; flowers are not one size fits all.

Think about the forms that inflorescences take: As summer winds down, Patrinia (Patrinia scabiosifolia) and the taller sedums are still going strong with their flat-topped blooms. Spherical choices include globe amaranth (Gomphrena globosa) and rattlesnake master (Eryngium yuccifolium). Great blue lobelia (L. siphilitica) offers up vertical spikes, as does obedient plant (Physostegia virginiana).

Other options include the later-blooming varieties of the surprise lilies (Lycoris), with their trumpets, and lots of flowers shaped like daisies.

Ms. Carey’s reminder to choose diverse forms is particularly important from high summer onward, when so many daisylike flowers come on. Among them: the sneezeweeds (Helenium), various Rudbeckia and Silphium, asters and perennial chrysanthemums.

Visualize what even something as common as cleome (Cleome hassleriana), with its explosive, spidery blooms, could do to punctuate such a scene, she said.

Play with scale, too. “Some of the asters are very cute, little, tiny daisies, and then you get the bigger, bolder ones,” Ms. Carey said. “Get your eye in a little bit more and be a little more critical of your compositions.”

Make the most of flowers with subtle differences, like Rudbeckia Henry Eilers, with its quilled petals. Here’s where color can really help, at a time when there is often too much yellow: Mix it up with the orange-and-yellow flowers of Rudbeckia hirta Irish Eyes, with their showy green centers, or the red-and-yellow petals of the annual blanket flower (Gaillardia pulchella).

Annuals That Can Go the Distance, With Care

The first plants we may think of to bolster the garden’s late show are classic fall perennial combinations, like asters and goldenrods.

Two goldenrods that behave better and spread more slowly than others in mixed plantings, Ms. Carey said, are Solidago rugosa Fireworks and the compact cultivar S. sphacelata Golden Fleece. Also recommended: aromatic aster (Symphyotrichum oblongifolium Raydon’s Favorite) and white wood aster (Eurybia divaricata), which “makes a good mixer and mingler,” she said.

But Ms. Carey is equally fond of the annuals grown mostly from seed and planted in spring: Zinnias, spiky Celosia, flowering tobacco (Nicotiana) and orange Cosmos sulphureus, with its bonus of ferny-textured foliage. All are still going strong in late summer, thanks to the regular trimming of stems that held spent blooms. So are cleome, Gomphrena and various salvias, with their tubular flowers that invite hummingbirds to make a pit stop on the way south.

And those nasturtiums, which most people would have pulled when they became leggy: Ms. Carey’s received rejuvenating haircuts, as did her snapdragons (Antirrhinum majus). Both are staging a September reprise, proving that the extra effort pays off.

Some later-blooming perennials — including asters, goldenrods, garden phlox (Phlox paniculata), sneezeweed and even her largest sedums — got the Chelsea chop as spring wound down. That severe haircut, when a third to half of the height is cut back, receives its name because it is performed in England at the end of May, around the time of the Chelsea Flower Show.

The result is less flopping and slightly later blooming — good news for the fall garden.

Don’t Forget the Dahlias

Ms. Carey’s garden, called Northview, is cottage-like in style, befitting her 1887 Arts-and-Crafts home. Her flower-packed designs aren’t heavy on ornamental grasses or bold-foliage tropical plants like elephant ears (Alocasia and Colocasia), bananas and cannas, two plant groups that extend the season until the first frost at Wave Hill and Chanticleer.

Dahlias are Ms. Carey’s “bold beauties,” her late-season mainstay. Their range of shapes is an object lesson in the mix-it-up advice she gives about flower forms. There are spiky cactus types and frilly anemones, some with a single row of petals like a daisy and others positively overstuffed.

Her favorites are the tight pompon shapes with their preposterous geometry. “How can they be so symmetrical and just so simple, but gorgeous?” she said.

Similarly spherical flowers are an element she consciously adds to the garden every season, from spring alliums to the globe thistle (Echinops ritro), rattlesnake master, globe amaranth and more.

She lets frost blacken and knock down the dahlias — “a very sad day in the garden” — and waits a week before cutting back the stems to about six inches above ground and digging the tubers. The uprooted plants rest on crates for a few days, stem ends down, so excess moisture drains out. Then they are rolled up in newspaper and stashed in the cellar for winter.

The Finale: Chrysanthemums and Colchicums

The last act of the season will be the perennial chrysanthemums, which will last until frost takes them. These are not those less-hardy, potted, garden-center mums of the moment in autumnal shades, but silvery-pink Emperor of China, pink-and-white Clara Curtis and Sheffield Pink. Their rosy tones provide a cooling foil to fall’s fiery colors.

These plants, too, get some tough love at Ms. Carey’s place. Pinching them back several times before midsummer reduces their eventual height and promotes more blooms, as the Chelsea chop does earlier, on other perennials.

There is another way that September is like May, Ms. Carey noted: It is a time when we are busy working, not just admiring. Those faded bits are trimmed back, and then back again — although care should be taken to leave behind anything that birds (or people) may enjoy come winter.

Seed is saved; perennials are divided. Bulbs are bought and planted, including perhaps more autumn crocus (Colchicum), another of the flower garden’s last gasps.

Their robust foliage, which emerged in spring, disappeared as they went dormant in summer. Now what look like miniature waterlilies in lilac and white seem to appear suddenly, out of nowhere, enlivening the edges of some beds.

It’s one more hopeful effort in the race against the inevitable — almost convincing us that the procession will never end.

Margaret Roach is the creator of the website and podcast A Way to Garden, and a book of the same name.

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