What Is the September Flower of the Month?

The September flower of the month is the aster, a daisy-like bloom that comes alive across meadows and gardens just as summer fades. Many birth-flower lists also recognize the morning glory as a secondary September flower. Together, these two plants capture the transitional mood of the month: asters carpeting roadsides in purple and pink, morning glories spiraling up fences and opening fresh each dawn. The pairing is more than decorative tradition, though, because both flowers have genuinely interesting biology worth exploring.

What Asters Actually Are

Asters belong to the family Asteraceae, one of the largest flowering plant families on Earth. What most people think of as a single aster “flower” is actually a composite structure called a capitulum, made up of dozens or even hundreds of tiny individual flowers packed together. The outer ring consists of ray florets, which look like petals and serve to attract pollinators, while the center is filled with disc florets that handle most of the reproduction.¹ This is the same basic architecture you see in sunflowers and daisies, which are close relatives. The disc florets often open in sequence from the outside in, so a single aster head can remain attractive for pollination over a surprisingly long stretch of time.

The genus name comes from the Greek word for “star,” a reference to the starburst pattern the ray florets make. In common garden use, the word “aster” gets applied loosely to several related genera, including Symphyotrichum (the New England and New York asters native to North America) and Callistephus (the China aster popular in florist arrangements). These plants share the characteristic composite flower head and the late-season bloom habit that made them the obvious choice for September’s birth flower.

Why Asters Wait Until September

Asters are not just casually late bloomers. Their flowering is tightly regulated by day length, a phenomenon called photoperiodism. Many aster species are short-day plants, meaning they initiate flower buds when nighttime hours exceed a certain threshold. As September arrives in the Northern Hemisphere, the days shorten past that critical point, and aster buds that have been developing all summer finally open.

Research on chrysanthemums, which are close relatives in the same family, shows how precisely this works. Chrysanthemum cultivars each have a variety-specific critical day length, and flowering only begins when the photoperiod drops below that threshold. The molecular trigger involves a balance between genes that promote and suppress flowering, with the suppressor weakening and the promoter strengthening once short days arrive.² Asters respond to essentially the same cue. That is why you rarely see asters blooming in June no matter how warm it gets: without the right ratio of darkness to daylight, the molecular switch stays off.

This photoperiodic control has a practical side effect. If you grow asters under artificial light that extends the day beyond the critical threshold, you can delay their bloom. Commercial growers sometimes use this trick in reverse, giving plants artificially long nights earlier in summer to push flowering ahead of the natural September schedule.

What Asters Smell Like

Asters are not famous for their fragrance the way roses or jasmine are, but they do produce a complex cocktail of volatile compounds. A chemical analysis of nine different aster species identified 52 distinct volatile compounds in the flowers, dominated by terpenoids.³ Most species had monoterpenes as the main component, giving them a light, herbal, slightly resinous scent. One species, Aster koraiensis, stood out by having sesquiterpenes as the dominant group instead, which tend to produce earthier, woodier notes.

The specific compounds identified include β-myrcene (also found in hops and lemongrass), D-limonene (the citrusy compound in orange peels), and caryophyllene (a spicy note found in black pepper and cloves). The researchers found that each species had a distinctive enough scent fingerprint to be told apart by an electronic nose device. So while asters may not perfume an entire room, they carry a subtle aromatic signature that varies meaningfully from one species to the next. If you lean in close to a wild aster and catch a faintly herbal, green smell, that is the terpenoid blend doing its work.

Asters as Pollinator Magnets

Blooming when most other wildflowers have already set seed gives asters an outsized ecological role. They become one of the last reliable nectar and pollen sources before winter. A study examining pollinator visits to asters found that hoverflies (syrphids) made up about 58% of all visitors, while bees of various species accounted for close to 40%.⁴ That hoverfly dominance is worth noticing. Hoverflies are important but underappreciated pollinators that are active later in the season than many bee species, and they seem particularly drawn to the open, flat flower heads that asters provide.

For gardeners thinking about pollinator support, this makes asters one of the most valuable late-season plantings. A patch of New England asters or aromatic asters in September and October can sustain bee and hoverfly populations at a time when forage is scarce. The disc-floret structure helps here too: because dozens of tiny flowers open in sequence across a single head, a visiting insect can feed efficiently without flying between widely spaced blooms.

Asters in Traditional Medicine

Several aster species have a long history of medicinal use, particularly in East Asian traditions. Aster tataricus, known as “zi wan” in Chinese herbal medicine, has been used for centuries as a remedy for coughs and respiratory complaints. Chemical analysis of the plant has identified a range of compounds including quercetin and kaempferol (both flavonoids with antioxidant properties), as well as shionone, a triterpenoid that gives the plant its distinctive pharmacological profile.⁵

Another species, Aster scaber, is eaten as a vegetable in Korea and has also attracted research attention for its polyphenol content. Its leaf extracts contain 23 identified polyphenolic compounds, with caffeic acid as the dominant phenolic compound and myricetin as the most abundant flavonol.⁶ The practical takeaway is that asters are not just ornamental. In parts of Asia, they sit at the intersection of food, medicine, and garden plant in ways that Western gardeners rarely encounter.

Morning Glories as September’s Other Flower

The morning glory (genus Ipomoea) is September’s secondary birth flower, and it earns its name honestly. Each bloom opens at dawn and typically withers by the afternoon, a lifespan so brief it has made the flower a symbol of the fleeting nature of beauty in Japanese culture, where it has been cultivated and bred intensively for centuries. The Japanese morning glory (Ipomoea nil) has been a subject of genetic research for so long that it serves as a model organism for understanding flower development.

That daily opening-and-closing cycle is controlled by light. Research shows that morning glory flowers need a dark period of at least eight hours before opening normally. When exposed to only four hours of darkness, or to continuous light, the flowers open extremely slowly or not at all.⁷ The mechanism involves genes tied to the circadian clock, the same internal timekeeping system that governs sleep cycles in animals. So when a morning glory opens at sunrise, it is not simply responding to light hitting its petals. It has been counting the hours of darkness and preparing its petals to unfurl at the right moment.

How Morning Glories Climb

Morning glories are vigorous twiners, and their climbing habit is more mechanically sophisticated than it looks. A morning glory vine does not simply drape itself over a support. It actively generates tension as it wraps, pulling itself tight against whatever it is climbing. Measurements of the common morning glory (Ipomoea purpurea) show that the geometry of the helix the vine forms around a pole directly affects the forces involved. On thinner supports, the vine curves more tightly, which produces greater contact pressure per unit length even though the overall tension in the stem is lower.⁸

To put some numbers on it, a typical morning glory stem twining around a small dowel sustains a tension equivalent to roughly 100 grams, balanced by contact forces of about 30 grams per centimeter of vine length. That is remarkable considering the stem itself weighs only about 40 milligrams per centimeter.⁹ The vine is generating contact forces hundreds of times its own weight. This explains why morning glories can grip surprisingly well even on smooth surfaces, and why they can be difficult to remove once they have wrapped around a structure. The vine essentially puts itself into tension, like a coiled spring pressing against the inside of a tube, except the spring is on the outside.

What Gives Morning Glories Their Colors

Morning glories come in a striking range of colors, from deep violet and electric blue to pink, red, and white. The pigments responsible are anthocyanins, the same class of compounds that color blueberries, red cabbage, and autumn leaves. What makes morning glory color science especially interesting is the role of vacuolar pH. Inside the petal cells, the acidity of the fluid-filled vacuoles determines whether anthocyanins appear red (in acidic conditions) or blue (in more alkaline conditions).¹⁰

Japanese breeders have exploited this for centuries, selecting for mutations that alter the pigment pathway or the pH control machinery. Some mutations affect the structural genes that build anthocyanin molecules, while others tweak regulatory genes that control how much pigment is made. Still others affect a completely different part of the process: the enzymes that attach sugar molecules to the anthocyanin, which changes the pigment’s stability and shade. The result is an astonishing variety of colors and patterns from what is, at its core, a single pigment system being dialed in different directions. If you have ever seen a Japanese asagao exhibition, the sheer range of morning glory colors and petal shapes on display is a testament to how much variation can be coaxed out of one flower’s genetics.

The Weedy Side of Morning Glories

For all their beauty, morning glories have a problematic reputation in agriculture. Several Ipomoea species are serious weeds in warm-climate cropping systems. Three-lobe morning glory (Ipomoea triloba), for instance, is an aggressive annual weed with vigorous growth, a long growing cycle, and tolerance to glyphosate, one of the most widely used herbicides globally. Its seeds can remain viable in the soil for well over a year, and burying them deeper actually increases their longevity, making tillage an unreliable control strategy.¹¹

This dual identity as beloved garden flower and despised agricultural weed is not unusual in the plant world, but it is worth keeping in mind if you plant morning glories. In many parts of the southern United States, Australia, and the tropics, certain Ipomoea species are classified as invasive. The same traits that make them charming in a garden setting, rapid growth, prolific seed production, and the ability to climb over and smother other plants, make them a real problem when they escape cultivation. If you grow morning glories, deadheading spent blooms before they set seed is one of the simplest ways to keep them from spreading beyond where you want them.

How Climate Change Is Reshaping September Bloom Seasons

The September flower calendar that most people grew up with is not fixed. Climate change is measurably shifting when flowers bloom, and the effects are not uniform across seasons. A study spanning roughly a century of records found that phenological shifts have increased flowering durations by an average of about 11 and a half days across species, with 94% of species experiencing greater overlap in their bloom times with other plants. The changes were particularly pronounced in autumn, where late-season species tended to push the end of their flowering later into the year.¹²

Regional data supports this pattern. In mountainous areas of southwest China, researchers found that while spring-flowering species bloomed about three days earlier than historical averages, autumn-flowering species delayed their bloom by roughly a day.¹³ Meanwhile, studies on herbaceous plants show that warmer temperatures and higher humidity during summer and autumn can extend bloom periods and delay the end of the flowering season.¹⁴

For asters specifically, the emergence of what researchers describe as an “autumn co-flowering mode” means that more species are blooming simultaneously in September and October than they did a century ago. That is a mixed bag. It could mean richer foraging for pollinators at a time when resources are typically thinning out. But it also means more competition among plants for those same pollinators, and potentially more pollen in the air for allergy sufferers. The September flower garden of the future may look noticeably different from the one we know now, with blooms from species that historically would have finished flowering in August lingering well into autumn alongside the asters.

Keeping Cut Asters Fresh

If you buy or cut asters for a vase, their longevity depends on a few straightforward practices. China asters (Callistephus chinensis) are the type most commonly sold by florists, and they respond well to clean water, a touch of floral preservative, and cool room temperatures. Research on China aster vase life has found that solutions combining an acidifier like citric acid, a biocide to suppress bacterial growth, and a sugar source to feed the flowers can meaningfully extend how long the blooms last. Stems that sit in plain water tend to clog with bacteria within a few days, cutting off water uptake and causing the flowers to wilt prematurely.

A few practical tips: cut stems at an angle to increase the surface area for water absorption, strip any leaves that would sit below the waterline (they rot and fuel bacterial growth), and change the water every two to three days. Asters are not as finicky as some cut flowers, but they do benefit from being kept out of direct sunlight and away from fruit bowls. Ripening fruit releases ethylene gas, which accelerates aging in many cut flowers, asters included. With decent care, cut asters can last a week to ten days in a vase, which is a solid run for an autumn flower.

Symbolism and the Birth Flower Tradition

The association between specific flowers and calendar months has roots in several cultural traditions, though the lists most people encounter today were largely standardized by the floral industry in the early twentieth century. Asters have long carried associations with patience, elegance, and wisdom, qualities thematically tied to the transition from summer’s abundance to autumn’s more reflective mood. In the Victorian language of flowers, asters could also express a sentiment along the lines of “I will never forget you,” making them a common choice for memorial arrangements.

Morning glories, meanwhile, tend to symbolize affection and the brevity of life, a connection drawn directly from their one-day blooming habit. In Japan, where morning glories have been cultivated since at least the Edo period, they carry connotations of summertime and the beauty of impermanence. The pairing of asters and morning glories for September is more poetically apt than most birth-flower assignments: one flower that endures for weeks through the cooling days, and another that burns bright for a single morning and is gone.