Wyoming onions get eaten by a short list of attackers: onion maggots and cutworms in spring, thrips through summer, and rot once the bulbs are in storage. The timing matters more than the list. An onion sets its bulb size from the leaves it has when day length flips the switch, so damage before that point costs you pounds, and damage after it mostly costs you shelf life. Here is how the calendar works.
The short answer for Wyoming
Cheyenne sits at 41.15 degrees north, and on the longest day of the year it gets 14 h 58 of daylight. That number is the clock for everything below. Onions do not bulb because it is warm. They bulb when days reach a set length, and until then every leaf is a solar panel feeding the bulb to come. Anything that eats those leaves, or the roots under them, is taking from the final harvest.
A short-day variety starts bulbing at about 12 hours of daylight (a threshold published by Clemson Cooperative Extension). A long-day variety waits for 14 to 16 hours. Cheyenne’s 14 h 58 falls inside that long-day range, so Wyoming is long-day country. That is good news for bulb size and awkward news for pest exposure, because long-day plants spend a long stretch as leafy green targets before they bulb.
Four attackers, four windows
- Onion maggot: the flies lay eggs at the base of young plants in spring, and the larvae tunnel into the stem and bulb. Seedlings and early sets are the most exposed.
- Cutworms and wireworms: cutworms chew seedlings off at the soil line at night in spring. Wireworms bore into roots and bulbs, and they are most common where sod was recently turned into a bed.
- Thrips: tiny insects that rasp the leaves and leave silvery streaks. Their numbers build as summer warms, right while the plant is still making the leaves it needs for bulbing.
- Storage rot: neck rot and other fungal decay take bulbs that were cured poorly or stored damp. This one arrives after harvest.
Calendar dates for when each of these shows up in Wyoming are not published here, and they shift with elevation and with each year’s spring. Your county extension office can tell you when the flies and thrips were active locally. For the broader picture of growing the crop, start with Onions in Wyoming.
What will not work here
The failure in Wyoming runs opposite to the one in the Gulf states. A long-day onion in Florida makes leaves all season, never gets the day length it wants, and rots without a bulb. A short-day onion in Wyoming does bulb, but it bulbs early and small, because the 12-hour trigger arrives before the plant has grown much top. Bulb size equals leaf size at that moment. There is no fixing it afterward.
Timing mistakes work the same way. Planting late does not delay the harvest, it shrinks it. A plant that has reached only a few leaves when the day length triggers bulbing will make a bulb to match, no matter how well you feed it in July. Pests do the same math. A thrips outbreak that strips the foliage before bulbing costs bulb size, and a maggot that kills a seedling in May costs the whole plant.
These are the types to leave out of a Wyoming bed:
- Granex-type sweet onions (the Vidalia style): short-day, they bulb at about 12 hours and stay small this far north, and they do not store.
- Texas-type sweets (Grano style): the same short-day trigger, so the same small bulbs.
- Anything labeled “short-day” or sold for southern gardens: the label tells you the trigger is far below what Wyoming days deliver.
- Late-planted sets of any type: too little top growth at bulbing, so a small bulb whatever the variety.
The varieties that do suit the state are covered in The Best Onion Varieties for Wyoming, and the planting window is in When to Plant Onions in Wyoming. Planting early also shortens the time before the first pest wave hits, and it puts more leaf on the plant by bulbing day.
What changes across Wyoming
Wyoming runs from roughly 41 degrees north at its southern border to roughly 45 degrees north at its northern one. That is about four degrees of latitude, enough to matter. The 41.15 degrees comes from the coordinates of the NOAA NCEI station in Cheyenne, and the 14 h 58 was calculated from that latitude with the standard sunset formula, using a geometric horizon. Atmospheric refraction adds about seven minutes of real daylight that this figure does not count.
| Location | Latitude | Longest-day length | What it changes |
|---|---|---|---|
| Cheyenne (reference city) | 41.15 degrees north | 14 h 58 | Inside the 14 to 16 hour long-day range; the figure used throughout this page |
| Far northern Wyoming | roughly 45 degrees north | longer than 14 h 58 (exact figure not published here) | Even more clearly long-day country; short-day types are a worse bet still |
| High mountain valleys, any latitude | varies | set by latitude, not elevation | Day length is unchanged but the season is shorter, so planting and pest windows shift later |
The north gets more summer daylight, which is why northern gardens grow long-day onions and southern ones cannot. Inside Wyoming, the more practical difference is the season. Elevation moves frost dates and the pest calendar more than a degree of latitude does, so a Sheridan bed and a Cheyenne bed may be planted on different weeks and meet thrips on different weeks. If your garden sits well away from Cheyenne, treat 14 h 58 as a starting point, not as your own number.
Sweet now, or onions that last
The bite of an onion comes from volatile sulfur compounds, and those same compounds are what protect the bulb in storage. A sweet onion has fewer of them. That makes it better raw and worse in the cellar. The University of Missouri Extension puts it flatly: “As a rule, ‘sweet’ onions do not store as well as the more pungent types.” No variety gets around this, so never count on a sweet onion to see you through winter.
Day length lines up with the trade. Short-day types run sweeter, long-day types keep longer. A southern garden gives you onions to eat soon, and a northern one gives you onions that feed you into March. Wyoming is the second kind. Check your pantry in March and the difference shows. The pungent bulbs are still firm, with tight necks and papery skins. Any sweet ones you tried to hold are soft, sprouting, or already in the compost.
Storage is where the last pest, rot, gets its chance. Clemson Extension gives the target: “Ideal conditions are between 45 °F and 55 °F, and 50 to 60 percent humidity” (see its onion factsheet). Onions want dry air, the opposite of carrots and other roots, so one cellar cannot hold both well. Cure first. Bulbs go into storage only once the necks are tight and the skins are papery, never straight from the ground. Maggot-scarred or bruised bulbs go in the kitchen pile, not the storage crate.
Checking this against your own garden
The figures above belong to Cheyenne. A state-wide number would hide a four-degree spread and a lot of elevation, so a few quick checks will tell you how much of this applies to your bed. Your county extension office is the right place to ask, since the staff there know what has actually shown up in local gardens.
- Look up your latitude. Enter your address in any mapping tool. If it is more than a degree or two from 41.15 degrees north, expect longer summer days than 14 h 58.
- Get your local frost dates. Ask the extension office for the last spring and first fall frost for your town, and plant against those, not against Cheyenne’s.
- Ask which pests were active last year. Find out when maggot flies and thrips showed up near you, so you can protect seedlings and watch leaves at the right moment.
- Read the label on what you buy. Look for “long-day” on the packet or set bag, and put back anything marked short-day.
- Measure your storage spot. A basic thermometer and hygrometer will show whether it holds 45 °F to 55 °F and 50 to 60 percent humidity.
Common questions
What is the most common onion pest in Wyoming?
Onion maggots and thrips are the usual suspects, but which one dominates depends on your elevation and the year. Your county extension office can say which has been worst near you.
Does damage before bulbing matter more than damage after?
Yes. Before bulbing, every lost leaf shrinks the bulb the plant can build. After bulbing starts, leaf loss matters less, though damaged necks and scarred bulbs still invite rot in storage.
Can I grow sweet short-day onions in Wyoming?
You can plant them, but they will bulb early and stay small, since Cheyenne’s 14 h 58 of longest-day light is far past their roughly 12-hour trigger. Eat them fresh, because sweet onions do not keep.
When do I stop worrying about pests?
Not at harvest. After the bulbs are cured, the threat becomes rot. Keep the bulbs dry, at 45 °F to 55 °F and 50 to 60 percent humidity, and check the pile every few weeks, because one soft bulb can spoil its neighbors.