Three things eat onions in Vermont, and they show up in order: onion maggots in spring, thrips through the dry stretches of summer, and storage rot from harvest into winter. The calendar sets the order, but the damage depends on how long the plant stays in the ground, and day length controls that. Burlington sits at 44.47 degrees north and gets 15 h 22 of daylight on the longest day. That makes Vermont long-day onion country, and it gives every pest a wide window.
The short answer for Vermont
Those 15 h 22 are a calculated figure, not a measured one. They come from the latitude of the Burlington station, using the standard sunset formula with the June solstice sun angle of 23.44 degrees. It uses a geometric horizon, so real daylight runs about seven minutes longer. The number matters because an onion does not bulb when the soil warms up. It bulbs when the days reach a length its genetics recognize. A short-day variety starts at about 12 hours, a threshold published by Clemson Cooperative Extension. A long-day variety waits for 14 to 16 hours. Vermont clears that second bar easily in early summer, which is why long-day types are the ones that finish here.
The catch is the waiting. A long-day onion spends spring and the start of summer growing leaves, and those leaves are what the pests are after. The plant is exposed for months before it swells a bulb, so a pest that chews leaf or base early can cap the harvest before bulbing even begins.
The early window: onion maggot
The onion maggot is the larva of a small fly that lays eggs at the base of onions in spring. The larvae tunnel into the base and the young bulb. Seedlings and sets can be killed outright, and bigger plants wilt, yellow and pull out of the ground with a hollowed, mushy base. Cool northern springs suit this pest, and Vermont has plenty of them. The flight follows local warmth rather than a date on the calendar, so a fixed date for your town is not published here. Your extension office can tell you when flies are active nearby.
The late window: thrips, leaf disease, storage rot
Thrips are tiny insects that rasp the leaves and leave silvery streaks. Their numbers tend to build in warm, dry weather from midsummer on. Because bulb size follows leaf area when bulbing starts, heavy thrips pressure turns into smaller bulbs. Wet, humid weather brings a different trouble: fungal leaf diseases such as downy mildew and botrytis leaf blight. The last feeder arrives after harvest. Fungi and bacteria cause rot in storage, and they favor onions that went into the cellar uncured or damp.
What will not work here
A few common moves fail in Vermont, and they fail for predictable reasons. Some of them feed the pests you are trying to beat.
- Short-day varieties. They start bulbing at about 12 hours of daylight, which Vermont reaches early in the season. The plant is still small when the trigger fires, so the bulb is small too, and no amount of feeding changes that.
- Late planting to dodge the maggot. Waiting does not delay the harvest, it shrinks it. Whatever size the plant has reached when day length triggers bulbing is the size of the bulb. A plant put out late is simply a smaller plant on the trigger date.
- Onions after onions. Maggot pupae overwinter in the soil. Planting alliums in the same bed, or leaving cull bulbs and old tops lying around, gives the next flight a head start.
- Row cover laid after the flies arrive. Cover keeps flies off only if it goes on at planting with the edges sealed. Laid over a bed where eggs are already in place, it traps the damage underneath.
- Sweet onions for winter. They will not last. The reason is chemical, and the storage section below explains it.
- Uncured bulbs in the cellar. Bulbs go into storage only when the necks are tight and the skins papery. Straight from the ground, they rot.
The first item is worth a second look if you are choosing seed or sets. Matching the variety group to the latitude comes before any pest plan, and The Best Onion Varieties for Vermont covers that choice in detail. Timing is its own topic: When to Plant Onions in Vermont lays out the calendar, and the point above explains why shaving a few weeks off it costs bulb size rather than saving time.
What changes across Vermont
Vermont is not one latitude. It runs from the Massachusetts line in the south to the Canadian border in the north, and the 44.47 degrees north used here belongs to Burlington, in the low Champlain Valley. A garden in the far southern counties sits at a lower latitude. A garden near the border sits at a higher one. The exact latitude of your own town is not published here, so look it up if you are far from Burlington.
| Location | Latitude | Longest day | What it changes |
|---|---|---|---|
| Burlington (reference) | 44.47 degrees north | 15 h 22 | Long-day onions bulb; maggot flight in spring, thrips from midsummer |
| Far southern Vermont | Lower than 44.47; check yours | Slightly shorter, a matter of minutes | Same long-day group; spring starts earlier, so pest windows open earlier |
| Near the Canadian border | Higher than 44.47; check yours | Slightly longer, a matter of minutes | Same long-day group; later spring, later planting, shorter season for the plant to size up |
Notice what does not change. Across the whole state, the longest day differs by minutes rather than hours, so every Vermont gardener stays in the long-day group. What moves is the calendar. Elevation matters as much as latitude here. A hillside garden can thaw weeks after a valley one, which pushes planting back and shifts the maggot flight with it. The same cold delay also shortens the time the plant has to build leaf before bulbing starts. Treat the Burlington numbers as a starting point, then adjust for your own hill or valley.
That is also why the broader picture on Onions in Vermont is a better guide than any single date. Pests follow warmth, and warmth in Vermont varies a lot over short distances.
Sweet now, or onions that last
Every onion grower in a northern state makes the same trade, and pests are only part of it. Sweet onions taste better raw and go soft in weeks. Pungent onions bite, and they keep. The University of Missouri Extension puts it plainly in its guide to planting, growing and harvesting onions (G6355): “As a rule, ‘sweet’ onions do not store as well as the more pungent types.”
The reason is chemistry, not luck. The bite of an onion comes from volatile sulfur compounds, and those same compounds help protect the bulb while it sits in storage. A sweet variety carries fewer of them. It is better on a burger in August and has little defense against rot by November.
Day length lines up with this. Short-day types run sweeter, and long-day types keep longer. A southern garden hands you onions to eat soon. A Vermont garden, growing long-day types, can give you onions that last into March, if you cure and store them properly.
Picture your pantry in March. A gardener who grew sharp, long-day bulbs and cured them has a mesh bag of firm onions on the shelf. A gardener who grew a sweet type for storage has a soft spot in every third bulb, with the rot spreading to its neighbors. Clemson Cooperative Extension gives the target: “Ideal conditions are between 45 °F and 55 °F, and 50 to 60 percent humidity” (Clemson HGIC). Onions want dry air, the opposite of carrots and other roots, so one damp cellar cannot hold both well. Eat the sweet ones first, and save the sharp ones for winter.
Checking this against your own garden
The Burlington figures are a reference point, not a verdict for every Vermont yard. A few checks this week will tell you how closely your garden follows them:
- Find your latitude. If you are far from Burlington, look up the latitude of your town. The day-length gap is small, but knowing it tells you where you sit relative to the reference.
- Look up your frost dates. This page holds no frost dates, and none should be borrowed from another town. Ask your local extension office for the figures that match your elevation.
- Call or visit your extension office. In Vermont, that means UVM Extension. Ask when onion maggot flies are active near you, and whether newer pests like the allium leafminer have been reported in your area.
- Check your bed history. If onions, garlic or leeks grew in a bed last year, pick a different bed.
- Measure your storage space. Put a thermometer and a humidity gauge where you plan to keep cured onions. Compare the readings to 45 °F to 55 °F and 50 to 60 percent humidity.
- Walk the rows. Look for silvery streaks on leaves and wilting plants with soft bases. Catching either early gives you time to act.
A county or regional extension office knows things no statewide figure can, including which pests are active this season in your valley.
Common questions
Are short-day onions safer from pests because they finish early?
No. They start bulbing at about 12 hours of daylight, well before a Vermont plant has grown much leaf. The result is a small bulb, not a pest-proof one, and maggots and thrips can still damage it.
Does row cover stop onion maggot?
It can, if it goes on at planting with the edges sealed, and only in a bed that held no onions, garlic or leeks the year before. Flies emerging from soil under the cover would otherwise be trapped with your crop.
Can I store onions in the same cellar as carrots?
Not well. Onions want 45 °F to 55 °F and 50 to 60 percent humidity, and they must be cured first, with tight necks and papery skins. Carrots and other roots want damp air, which invites rot in onions.
Where can I find out which pests are active near me?
Your local extension office. In Vermont, UVM Extension tracks what is active region by region. This page does not publish pest dates for individual towns, because those shift with elevation and with each spring.