August is late summer in Illinois — the prairie reaches its tallest and most colorful bloom, the harvest peaks, and the first hints of fall migration appear. Blazing star and early goldenrod set the grasslands purple and gold, and warm, settled nights make for some of the year's best stargazing.
Week by week in August
July 30 – August 5
- The tallgrass prairie nears its peak — big bluestem grows head-high and the first goldenrods begin to turn the grasslands gold.
- Monarchs build through their summer broods, and common buckeyes and giant swallowtails increase in the south.
- Sow fall lettuce, spinach, radishes, and a final round of beans now so they mature before the autumn cold.
August 6–12
- The Perseid meteor shower peaks around August 12, throwing dozens of bright, fast meteors an hour from a dark site after midnight.
- The prairie reaches peak bloom in purple and gold — blazing star, ironweed, goldenrod, and towering compass plant.
- The richest market weeks of the year: sweet corn, tomatoes, peppers, melons, and peaches at their peak.
August 13–19
- Monarchs are abundant on the blazing star and ironweed as the migrating generation is born across the prairie.
- Shorebird migration peaks at Emiquon, and the first southbound warblers trickle through in drab fall plumage.
- Set out fall transplants of broccoli, cabbage, and kale, and watch tomatoes for disease as the humidity lingers.
August 20–26
- Common nighthawks begin their evening migration flights, loose flocks coursing over towns at dusk.
- The Milky Way arches directly overhead on warm, settled nights — best seen from the dark skies of the Shawnee in the far south.
- Prime preserving season — shoppers buy sweet corn, tomatoes, and peaches in bulk for canning and freezing.
August 27 – September 2
- Ruby-throated hummingbirds peak as local and northern birds fuel up on nectar before crossing the Gulf — keep feeders full.
- Goldenrod washes the prairie gold and the first New England asters open as the fall display begins.
- The first black gum and sumac leaves flame scarlet, the earliest hints of fall color in the woods.
Birds This Month
August is the quiet turn toward fall. Songbirds fall largely silent as the breeding season ends and molt begins, but migration is underway beneath the calm. Shorebird migration peaks on the mudflats — least, pectoral, and semipalmated sandpipers, lesser yellowlegs, stilt sandpipers, and others stop at Emiquon, the Illinois River backwaters, and drained ponds. The first southbound warblers trickle through, harder to identify now in their drab fall plumage.
The most dramatic August movement is at the feeders and flowers: ruby-throated hummingbirds peak as both local birds and northern migrants fuel up on nectar before crossing the Gulf — keep feeders full and fresh. Common nighthawks begin their evening migration flights, loose flocks coursing over towns at dusk in the last days of the month. Chimney swifts gather in growing numbers, and purple martins stage in large pre-migration roosts. Grassland birds at Midewin and Nachusa are winding down their season.
What's Blooming
August is the peak of the tallgrass prairie bloom in Illinois — the grasslands are at their tallest and most spectacular. Prairie blazing star sends up dense purple spikes that draw migrating monarchs, the early goldenrods turn the prairie gold, and tall compass plant, prairie dock, yellow coneflower, and sawtooth and sneezeweed sunflowers tower overhead. Purple ironweed and dusty-rose joe-pye weed glow in the wet swales, wild bergamot finishes, and the first asters open.
The big bluestem grass — the iconic 'turkeyfoot' of the tallgrass prairie — is now head-high and beginning to flower. This is the month to walk Midewin, Nachusa, or Goose Lake Prairie for the full late-summer sweep of purple and gold humming with bees and butterflies. In gardens, sunflowers, zinnias, black-eyed Susans, and the first fall asters carry the color. The prairie won't be more impressive than it is right now.
Garden This Month
August is peak harvest and the pivot to fall planting. The summer garden pours out tomatoes, peppers, eggplant, beans, cucumbers, summer squash, sweet corn, melons, and the first winter squash and pumpkins sizing up. Keep picking frequently to keep plants producing, and keep up the deep, consistent watering — late-summer heat and dry spells stress crops and split tomatoes.
This is the window to plant the fall garden: sow lettuce, spinach, arugula, radishes, turnips, and a final round of beans, and set out transplants of broccoli, cabbage, kale, and collards for autumn harvest. Watch for and manage tomato diseases, squash bugs, and Japanese beetles. Pull spent crops, and plant a cover crop or sow more greens in the gaps. Keep harvesting and preserving the abundance — and start saving seed from your best open-pollinated plants.
Zone 5b (Chicago metro & northern Illinois): the harvest is in full swing — pick tomatoes, beans, and squash daily, and sow quick fall crops of lettuce, spinach, radishes, and arugula now so they mature before the autumn cold.
Zone 6b (south-central Illinois): peak harvest continues — keep watering through late-summer heat, plant fall brassicas and greens, and watch for the first signs of disease on tomatoes as humidity lingers.
What's at the Farmers Market
August is the richest market month of the Illinois year. The stands overflow with sweet corn at its peak, vine-ripe tomatoes of every color, peppers, eggplant, green beans, cucumbers, summer and the first winter squash, melons, okra, and new potatoes and onions. Southern Illinois peaches are at their juicy best, and blackberries and the first apples appear.
This is prime preserving season, when shoppers buy in bulk for canning and freezing. Cantaloupes and watermelons from the sandy river-bottom farms are sweet and heavy now. Buy sweet corn the day you'll eat it and keep it in the husk, refrigerated; store tomatoes at room temperature, stem-side down, never the fridge; and choose peaches and melons that are fragrant and give slightly to gentle pressure, ripening firm fruit on the counter. The markets are at their most abundant, colorful, and crowded of the year.
Night Sky This Month
August offers some of the year's best stargazing — warm, settled nights and the Milky Way arching directly overhead. The Summer Triangle of Vega, Deneb, and Altair rides high, and the galaxy's glowing band runs from Sagittarius and Scorpius low in the south up through Cygnus the Swan, its richest star fields packed with clusters and nebulae for binoculars.
The marquee event is the Perseid meteor shower, peaking around August 12 — one of the most reliable and beloved showers of the year, throwing dozens of bright, fast meteors an hour from a dark site in the hours after midnight. For the Perseids and the full Milky Way, the dark skies of the Shawnee National Forest in far southern Illinois are far better than the Chicago-washed northern sky.
The printable Illinois night-sky guide lists this year's exact Perseid peak date, the moon phase, and planet positions for your location.
Butterflies & Pollinators
August is a premier butterfly month in Illinois, with the blooming prairie at peak bloom. Monarchs are abundant now — the generation that will migrate to Mexico is being born, and adults pile onto the blazing star, ironweed, and early goldenrod. Big eastern tiger and black swallowtails, swarms of great spangled fritillaries, painted ladies, and the last of the regal fritillaries work the grasslands at Nachusa and Midewin.
Open ground and gardens are busy with cabbage whites, clouded and orange sulphurs, pearl crescents, common buckeyes (which build through late summer), red admirals, and many skippers. The huge giant swallowtail and the variegated fritillary turn up in good numbers in southern Illinois. With so much in bloom, any sunny prairie or pollinator garden is alive with wings on a warm afternoon — and the monarchs gathering now are tuning up for September's great southward flight.
What to plant for them is in the Illinois native plants guide
Trees This Month
August keeps Illinois's trees in full, mature green, though late-summer drought stress can bring an early yellowing or scorch to leaves on dry sites. The growing energy is in ripening the year's crop: oak acorns are sizing up, shagbark hickory and black walnut nuts swell in their husks, and the wild black cherry drops its dark fruit for the birds.
A few late bloomers persist — the buttonbush in wetlands and the very last basswood flowers — but the show is largely over. The earliest hints of fall appear at the margins: a few black gum (tupelo) and sumac leaves turn red, and stressed or early-turning trees show scattered color by month's end. In the southern swamps, the bald cypress hold their deep green over the Cache River sloughs, weeks from their autumn rust. The long green plateau of summer is beginning, almost imperceptibly, to tip toward fall.
The Illinois native plants guide covers what grows with them
Weather & Daylight
Across Illinois, a typical August day averages a high near 84°F and a low near 63°F, with about 3.6 inches of precipitation — statewide averages from the 1991–2020 U.S. climate normals.
Still thick with summer heat and humidity, highs in the mid-80s and sticky nights near 63F under frequent showers.
| Average high | 84°F |
|---|---|
| Average low | 63°F |
| Precipitation | 3.6 in |
| Sunrise (mid-month) | 6:07 CT |
| Sunset (mid-month) | 19:55 CT |
Figures are statewide averages over the 1991–2020 U.S. climate normals; local conditions vary with elevation and distance from the coast. Sunrise and sunset are computed for a midstate point on the 15th (CT).
Next month's version of this page, by email
One short note when Illinois's next month turns — what's arriving, blooming, and worth a night outside.
Go deeper with the Illinois guides
The complete Illinois birding, native-plant, and night-sky references — or the whole year, week by week, in one book.
Same month elsewhere: August in Indiana · August in Iowa · August in Kansas