Kentucky · zones 6a–7a
When to plant onions in Kentucky.
Sow or transplant between Feb 7 to Mar 23 in a typical year, keyed to Kentucky's median last frost around Apr 17. A second, fall sowing fits Aug 23 to Oct 4. Dates below are state medians; your ZIP code shifts them by days to weeks.
The 2026 windows
Onions dates for a typical Kentucky season.
1 · Start indoors
Dec 27 to Feb 9
About 6 weeks before set-out. Warm spot, real light.
2 · Set out
Feb 7 to Mar 23
Keyed to the ~Apr 17 last frost. Sow 0.5" deep, 4" apart.
Fall round
Aug 23 to Oct 4
Counted back from the ~Oct 18 first frost.
Harvest
~95 days later
~0.5 lb per plant over ~1 weeks.
Kentucky spans more than one climate: Zone 6 is the most common zone, but the state runs 6a to 7a. That's why the planner works from your ZIP code's frost dates, not your zone: mountain and coastal gardens in the same state can be three weeks apart.
Kentucky growing notes
Bluegrass phosphate and an old tobacco barn make a Kentucky onion crop.
Phosphate-rich ground builds the bulb
Onions lean hardest on phosphorus early, while the plant is still building roots, and the Inner Bluegrass's limestone-derived soil supplies more of it naturally than most garden ground in the country, a genuine head start for anyone getting a bed established in that part of the state. Nitrogen takes over as the driver once leaf growth ramps up, since it is leaf count at the day-length trigger, not phosphorus, that ultimately sets bulb size. Kentucky's latitude, about 36.5 to 39 degrees north, falls in the intermediate-day band, where the safer variety picks are ones bred to bulb on 12 to 14 hours of daylight rather than the shorter or longer thresholds suited to other regions.
Final bulb size comes down to one thing: how many leaves the plant grows before day length reaches its bulbing trigger, since nothing that happens after that point adds any more. UK Extension has Kentucky growers setting transplants out in late March to early April, two to four weeks ahead of the typical mid-to-late-April last frost, since the plant shrugs off a hard frost without damage and only loses ground by sitting in the tray waiting for one that will not hurt it. Keep nitrogen flowing through that early leaf-building stage, then ease off once the base of the plant starts to swell, since by then the plant has stopped adding leaves and started converting what it already has into bulb.
The curing room hiding in plain sight
Many Kentucky farms still have an old tobacco barn or stripping room standing unused since the crop declined, and those buildings happen to be close to ideal for curing onions: shaded, well ventilated by design, and built specifically to move air across a hanging crop without direct sun ever reaching it. A home gardener without a barn can copy the same principle on a smaller scale with a covered porch or a garage with a box fan running and the door cracked.
Judge doneness by the neck rather than a date on the calendar: give it a squeeze at roughly the two-week mark, and if it still yields like a green stem, let it cure longer; once it resists like dry paper about to tear, the bulb has finished curing and is ready to come off the rack. Keep cured bulbs cool and ventilated afterward, since Kentucky's humid summer air lingering into early fall will soften anything stored in a sealed container faster than it would in a drier climate.
Maggot after a wet spring, thrips after a dry summer
Onion maggot sits at the edge of its range in Kentucky, more of a threat in a cool, wet spring than a certainty every year, tunneling into a transplant's base until the plant wilts and lifts free of the ground on roots gone rotted and mushy. Seedcorn maggot causes the identical damage under the identical cool-wet-spring conditions and is the more consistent culprit farther into the state, especially where fresh green manure or a heavy load of undecomposed compost went into the ground within the past few weeks. Lay row cover over the bed on transplant day so neither adult fly finds anywhere to lay eggs, and rotate away from any ground that grew onions, garlic, or leeks in the past three years to starve out whatever population is already there.
Thrips take over once summer turns hot and dry, leaving fine silvery streaks along the leaves. Thump several leaves over a shallow pan or dark-colored bowl rather than plain paper, since thrips show up better against a dark background; roughly three pale, wriggling specks on a single leaf means it is time to spray insecticidal soap into the crease where leaf meets neck, the one spot thrips tuck away from a straight shot.
In Kentucky: watch for
The pests that find onions here.
A middle-length season, about 184 frost-free days, means onion, bulb overlaps onion thrips for a stretch each summer. Check plants every week or two and treat early.
Silvery streaks and curled leaf tips in hot spells.
Organic fix: Insecticidal soap or neem; hang sticky traps.
Seedlings wilt; white maggots in the bulb base.
Organic fix: Float a row cover over the row; rotate beds each year.
Pale patches with fuzzy gray growth in damp weather.
Organic fix: Space for airflow, water early in the day, spray neem.
White rot
Yellowing and dieback, then wilt; fluffy white mold at the bulb base and roots so rotted the plant lifts straight out.
Organic fix: No cure once it is in the soil. Plant clean stock, never follow onions with onions, and keep infested soil off tools and boots.
Your season math: Kentucky gives onions about 184 frost-free days, and it needs roughly 95 days to a first harvest. It's a one-planting crop, so time that single sowing well.
Planting onions in Kentucky: questions
When should I plant onions in Kentucky?+
In a typical Kentucky year, sow onions or set out transplants between Feb 7 to Mar 23, keyed to the state's median last frost around Apr 17. There's also a fall window: Aug 23 to Oct 4. Your exact dates shift with your ZIP code and microclimate.
When do I start onions seeds indoors in Kentucky?+
Start seeds inside about 6 weeks before set-out: roughly Dec 27 to Feb 9 for a typical Kentucky season.
What USDA zone is Kentucky?+
Kentucky spans zones 6a–7a, mostly zone 6. But frost dates, not zones, set planting time. The free planner uses your ZIP code's actual frost dates.
How many onions plants do I need?+
Depends who you're feeding. The crop page has exact counts per household size, or the free planner does the math for your family in about 60 seconds.