Onion thrips are the insect most Tennessee growers end up fighting, and they arrive as the weather turns warm and dry, just when bulbs are swelling. Maggots are a cool, wet spring problem. Rot takes over in storage. Exact pest dates for your county are not published here, and they shift with the weather each year. What stays fixed is the onion’s own clock. Day length decides when bulbing starts, and that decides when the crop is most exposed.
The short answer for Tennessee
Nashville sits at 36.12 degrees north, and its longest day, at the June solstice, brings 14 h 28 of daylight. Those two numbers set the schedule for everything that eats or damages an onion here. A short-day onion starts bulbing at about 12 hours of daylight, a threshold published by Clemson Cooperative Extension. Tennessee passes that mark around the March equinox, long before summer heat arrives. Bulbing begins in spring, the crop finishes in early summer, and the insects show up on a plant that is already committed to its final size.
That sequence is the useful part. The pests do not set the calendar; the plant does. Thrips are tiny insects that rasp the leaves and suck out the sap, and they build up as temperatures climb. Every leaf they scar is a leaf that cannot feed the bulb. An onion has only the foliage it grew before bulbing began, so early damage during the swelling weeks shows up directly as smaller bulbs at harvest. Damage after the tops fall over matters far less.
Maggots work on the opposite end of the season. The adult flies lay eggs near young plants in cool, damp spring weather, and the larvae tunnel into the base of the plant. Seedlings and young sets are the ones that suffer. Storage rot is a third kind of loss. It starts in the garden with bulbs that were wounded or never dried properly, then finishes the job in the pantry.
So the answer has a rhythm: maggots early, thrips as bulbs swell, rot afterward. Each one lines up with a stage the onion reaches on a timetable that day length controls. If you plant a variety suited to Tennessee’s day length, you keep that timetable short and predictable. Getting the variety right is the first pest-management decision, and it happens before any seed or set goes in the ground. The best onion varieties for Tennessee page covers which ones fit, and when to plant onions in Tennessee covers the timing. The broader picture lives in Onions in Tennessee.
One caution about the numbers. The 14 h 28 figure is the geometric daylight, calculated from the latitude with the standard sunset formula. Atmospheric refraction adds roughly seven minutes of real daylight that the figure does not count. It does not change the conclusion, but it explains why a sunrise-sunset chart may show a slightly longer day than the one quoted here.
What will not work here
Long-day onions. They wait for 14 to 16 hours of daylight before they start a bulb, and Nashville reaches 14 h 28 only for a short stretch around the solstice, at the very bottom of that range. In the southern part of the state the day is shorter still. A long-day variety planted here does not bulb on schedule. It keeps making leaves through spring and into summer, forms a small bulb late or none at all, and then the plant rots in the heat.
Leaves all season and no bulb is exactly what a thrips colony wants. A plant that stays in leaf through July and August gives insects and leaf diseases weeks of extra feeding that a properly timed onion never offers. A crop that was supposed to be finished by early summer is still standing in the wettest, warmest weather of the year, and the damp base of a bulbless plant is where rot starts.
These are varieties and types that catalogs written for northern gardens often recommend, and that do not belong in a Tennessee bed:
- Walla Walla: a long-day sweet onion, so it needs day lengths Tennessee does not deliver in time, and it gives leaves instead of a bulb.
- Ailsa Craig: a long-day exhibition type that holds its leaves while waiting for a trigger that arrives late or not at all.
- Copra: a long-day storage onion, and the day-length requirement defeats the very keeping quality it is grown for.
- Stuttgarter: a long-day type, so it wants the northern day length and gives a poor bulb here.
- Yellow Sweet Spanish types: long-day onions, so they overgrow in leaf and risk rotting before they finish.
Growers sometimes try these because the catalog photo is tempting or because the same variety did well for a relative in Michigan or Oregon. Both are far north of 36 degrees, where summer days are much longer. The variety did not fail here because of bad soil or poor care. It failed because the signal it needs never arrives on time.
What changes across Tennessee
The figures above belong to Nashville, the city whose station supplied the latitude of 36.12 degrees north. Tennessee stretches from roughly 35 degrees north along the Mississippi line to about 36.7 at the Kentucky border, so the state covers more than a full degree and a half of latitude. That is a modest spread, and it does not move anyone out of the short-day group. The longest day differs by minutes across the state, not hours.
| Part of the state | Latitude | Longest day | What it changes |
|---|---|---|---|
| Far south and southwest (near the Mississippi line) | Roughly 35 degrees north | Slightly shorter than Nashville’s 14 h 28 | Long-day types fall even shorter of their trigger. Short-day onions bulb a bit earlier. |
| Nashville, the reference city | 36.12 degrees north | 14 h 28 | The baseline for every figure on this page. |
| Far north (near the Kentucky line) | About 36.7 degrees north | Slightly longer than 14 h 28 | Still short-day territory, with a little more room on the long end. |
Latitude tells only half the story. Elevation in East Tennessee pushes frost dates later in spring and earlier in fall, so a garden in the mountains may plant a couple of weeks apart from one in the Mississippi lowlands even at the same latitude. Day length does not change with altitude, but soil temperature and frost do, and those govern how big the plant is when bulbing starts. The pest calendar follows the same pattern, since insect activity tracks warmth. A warm lowland garden sees thrips sooner than a cool hillside one.
If you live near the edge of the state, do not assume Nashville’s numbers describe your yard. Check your own latitude and treat the table as a range, not a verdict.
Sweet now, or onions that last
Short-day onions run sweet and short-keeping. Long-day onions run sharp and long-keeping. The difference is chemistry, not luck. The bite of an onion comes from volatile sulfur compounds, and those same compounds protect the bulb in storage. A sweet variety has fewer of them, which makes it better raw and worse over the winter. The University of Missouri Extension puts it plainly: “As a rule, ‘sweet’ onions do not store as well as the more pungent types.”
Tennessee’s day length puts most home gardens on the sweet side of that trade. Short-day bulbs come early, taste mild, and are best eaten within weeks of harvest. No variety promises a sweet onion that also stores, and any description that does should be read with suspicion.
Look at what this means for your pantry in March. A gardener who grew sweet short-day onions will mostly be out of them by then, or down to a few soft, sprouting bulbs. A gardener who grew the more pungent short-day and intermediate types, and cured them properly, may still have firm bulbs on the shelf. Neither is wrong. The question is whether you want onions to eat in July or onions that last into spring, and most Tennessee gardens end up growing a bit of both.
Storage depends on how you handle the bulbs. Curing comes first: bulbs go into storage only once the necks are tight and the skins are papery, never straight from the ground. Bulbs that were scarred by thrips or wounded at the neck are the first to rot, so set them aside for the kitchen. Clemson Cooperative Extension’s Home and Garden Information Center gives the target for what remains: “Ideal conditions are between 45 °F and 55 °F, and 50 to 60 percent humidity.” Onions want it dry, not damp, the opposite of carrots and other roots. A single cellar cannot hold both well.
Checking this against your own garden
The Nashville figures are a starting point, not a verdict on your yard. Nothing on this page should be read as a state-wide number that applies to every garden. Here is what you can verify this week:
- Look up your own latitude, using a map app or the coordinates on a weather station near you. If you are far from 36.12 degrees north, your longest day differs from 14 h 28.
- Find your local frost dates through your county extension office. They vary with elevation and terrain, and the state-wide picture hides those differences.
- Read the day-length label on every variety you own or plan to buy. Discard anything marked long-day.
- Take a photo or a sample of any damaged plant to your county extension office for identification before you spray anything. Thrips scars, maggot damage, and leaf disease can look alike from the garden path.
- Measure the temperature and humidity where you plan to store onions. Compare them against the 45 °F to 55 °F and 50 to 60 percent range.
Your county extension office can also tell you what insects have been active locally this season, which no general guide can do. Pest dates move with the weather, and the people who track them work in your county.
Common questions
What is the main insect pest of onions in Tennessee?
Onion thrips are the one most growers deal with. They feed on the leaves and build up as the weather warms and dries, usually while bulbs are swelling. Onion maggots are the other insect to watch, mainly on young plants in cool, wet spring weather.
Do long-day onions work in Tennessee?
No. They need 14 to 16 hours of daylight to bulb, and Nashville’s longest day is 14 h 28, reached only around the solstice. They grow leaves all season, form little or no bulb, and then rot.
When do onions start bulbing in Tennessee?
Short-day varieties start at about 12 hours of daylight, which Tennessee passes around the March equinox. Bulbing begins in spring, well before the longest day in June.
Can I store sweet onions through winter?
Generally not. The sulfur compounds that give onions their bite also protect them in storage, and sweet types have fewer. Cure them first, then keep them at 45 °F to 55 °F and 50 to 60 percent humidity, but plan to eat them first.