Nashville sits at 36.12 degrees north, and on the longest day of the year its sky holds 14 hours and 28 minutes of daylight. That number, not the heat, is what tells an onion whether to bulb. At this latitude, Tennessee gardens fall into intermediate-day territory, with a handful of long-day and short-day varieties reaching in from either side.
The short answer for Tennessee
Onions don’t respond to soil temperature or rainfall when they decide to stop growing leaves and start building a bulb. They respond to how many hours of light hit their leaves each day. At 36.12N, the summer ceiling is 14 hours 28 minutes, and that places most of Tennessee squarely in the intermediate-day zone, the group of varieties bred to bulb somewhere between 12 and 14 hours of daylight rather than waiting for the long, northern summer evenings.
Nine intermediate-day varieties have published ranges that cover 36.12N: Cabernet, Calibra, Candy, Elsye, Expression, Monastrell, Red Long of Tropea, Scout, and Sierra Blanca. Two long-day varieties, Bridger and Walla Walla Sweet, list ranges wide enough (35-50N and 35-55N) to reach down into middle Tennessee as well. On the short-day end, White Castle’s range of 30-38N also brushes this latitude, though it has no dedicated page on this site.
| Day-length group | Varieties that fit 36.12N | Published range |
|---|---|---|
| Short-day | White Castle | 30-38N |
| Intermediate-day | Cabernet, Calibra, Candy, Elsye, Expression, Monastrell, Red Long of Tropea, Scout, Sierra Blanca | 32-48N (varies by variety) |
| Long-day | Bridger, Walla Walla Sweet | 35-55N |
Six more sit in what’s really a continuum rather than a clean cutoff: Gabriella, Madalyn, Red Rock, Red Sensation, Texas Grano 1015Y, and Vidora are all short-day types whose published ranges top out within a degree and a half of 36.12N. They may bulb here. They’re not guaranteed to, and a gardener planting them should treat the result as an experiment rather than a sure thing.
What will not work here
Thirteen varieties on this site’s lists have published ranges that start above 36.12N, and every one of them will fail in a Tennessee garden. Not “may struggle.” Fail. The mechanism is simple: these are long-day types (plus one intermediate, Red Long of Florence) bred for places where the summer sun hangs on well past 15 hours. Nashville’s ceiling of 14 hours 28 minutes never reaches the trigger these plants are waiting for. Without that trigger, the onion has no signal to stop making leaves and start making a bulb, so it just keeps growing top growth through the whole season. Then frost or humidity takes the plant down, and what’s left in the ground is a swollen, useless stem base, not a stored onion.
- Ailsa Craig (38-60N), needs day length this latitude never provides
- Barolo (38-55N), bred for a summer sun that stays up longer than Tennessee’s
- Blush (38-55N), same threshold problem, no bulb trigger reached
- Frontier (40-50N), needs latitudes well north of the state line
- New York Early (38-50N), day length here falls short of its requirement
- Patterson (38-55N), a classic storage onion, but not for this latitude
- Powell (43-47N), bred for a much shorter growing window further north
- Red Carpet (40-45N), well outside Tennessee’s daylight ceiling
- Red Long of Florence (38-50N, intermediate-day), the one exception to the long-day pattern, but still out of range
- Redwing (43-65N), a far-northern variety, no fit here at all
- Rossa di Milano (38-50N), the trigger point sits above what Nashville ever sees
- Safrane (42-50N), bred for latitudes several states north
- Talon (42-50N), same story, no bulbing signal reached
Plant any of these in Tennessee and the season doesn’t end with a small onion. It ends with rot.
What changes across Tennessee
Tennessee is a long, narrow state, stretching roughly from just above 35 degrees north at the Mississippi line to a little past 36.6 degrees north where it meets Kentucky along the northeast border. That’s not a huge spread, but it’s enough to nudge a gardener at either edge slightly out of Nashville’s exact numbers. All the figures on this page, the 36.12N latitude and the 14 hours 28 minutes of summer daylight, belong specifically to Nashville, the reference city for these climate normals.
| Location | Approximate latitude | What changes |
|---|---|---|
| Northeast corner (near Mountain City) | about 36.6°N | Slightly longer summer days than Nashville; the blurred-zone short-day varieties tilt less favorable, while long-day types bred for the low end of their range (35-38N) get marginally more plausible |
| Nashville (reference city) | 36.12°N | The baseline for every figure on this page |
| Southwest corner (near Memphis) | about 35.0°N | Slightly shorter summer days; blurred-zone short-day varieties like Red Rock (30-36N) sit more comfortably here than they would in Nashville |
A gardener near either edge shouldn’t read the Nashville numbers as their own. The difference is small in absolute terms, but for a crop that’s triggered by daylight rather than temperature, a degree and a half of latitude is enough to shift which blurred-zone varieties are worth the gamble.
Sweet now, or onions that last
Every onion variety carries a trade a gardener can’t negotiate away. The volatile sulfur compounds that make an onion sharp on the tongue are the same compounds that protect it from breaking down in storage. Cut those compounds back for a milder, sweeter bite, and you’ve also cut back the onion’s own defense against rot. The University of Missouri Extension puts it plainly: sweet onions do not store as well as the more pungent types.
Day length lines up with this chemistry almost exactly. Short-day and intermediate-day varieties, the ones best suited to Tennessee’s 14 hours 28 minutes of summer light, tend to run sweeter and mature with fewer of those protective sulfur compounds. Candy and Sierra Blanca are typical of this group. Walla Walla Sweet, despite being a long-day variety with a range that happens to reach into middle Tennessee, is also known for a mild flavor rather than long storage life.
The practical result shows up in the pantry, not the garden bed. A Tennessee harvest weighted toward intermediate-day varieties gives good onions to eat through late summer and fall, but by March the bin is likely to be empty or full of onions that have started going soft. A gardener who wants something left to cook with in late winter needs to deliberately favor whichever varieties in their mix run more pungent, and manage expectations that even those won’t match the six-month keepers grown much farther north, where the long summer days push sulfur compounds higher.
Checking this against your own garden
Nashville’s numbers are a starting point, not a guarantee for every yard in the state. A garden fifty or eighty miles away, especially toward either state line, is working with a slightly different day-length ceiling and a different frost calendar. Before ordering onion sets or transplants, it’s worth confirming a few things locally rather than assuming the Nashville figures apply exactly.
- Look up your own latitude if you garden well north or south of Nashville, most county GIS or extension sites list it for free.
- Confirm your average last spring frost date and first fall frost date through your county’s University of Tennessee Extension office rather than a generic national chart.
- Compare a variety’s printed day-length range against your own latitude, not Nashville’s, especially if you’re considering one of the blurred-zone short-day types.
- Ask your local extension agent which varieties other gardeners in your specific county have actually gotten to bulb, since microclimates along ridgelines and valleys can shift results from what the map alone suggests.