What Eats Onion in Washington, and When

Three things eat onions in Washington: onion maggots working on young plants in cool spring soil, thrips rasping the leaves once summer warms up, and rot organisms that move into bulbs in storage. Which one you meet, and when, depends less on the calendar than on the onion’s own clock, and day length sets that clock. Exact emergence dates vary by county and by year, and this page does not publish them. It can show where in the plant’s life each pest finds an opening.

The short answer for Washington

At 47.44 degrees north, the latitude of the station near Seattle that the climate normals here come from, the longest day of the year runs 15 h 45 of daylight. That number decides what kind of onion you grow, and the onion’s schedule in turn decides when pests can hurt it. Long-day onions wait for 14 to 16 hours of light before they start forming a bulb, and Washington clears that bar easily. Short-day types begin bulbing at about 12 hours, a threshold published by Clemson Cooperative Extension. So Washington gardeners plant long-day varieties, set out in late winter to early spring, and the plants spend months making leaves before anything happens underground.

Three windows, one clock

All those leafy months are the exposure. In spring, young plants and fresh sets are small and tender, and onion maggots (the larvae of a fly) tunnel into the base of seedlings while the soil is still cool. Through early and midsummer, the plant is at full leaf, and thrips, tiny insects that scrape the leaf surface and drink the sap, build up in warm, dry spells. After harvest, the bulbs move indoors and the threat becomes rot. Each window opens at a different point in the plant’s life, which is why no single spray date or single calendar week covers them all.

Why leaf loss costs you bulb size

The plant has a fixed job before the long days arrive: build as many healthy leaves as it can. Each leaf roughly corresponds to one ring of the finished bulb, and whatever size the plant has reached when day length triggers bulbing is the size the bulb will be. A maggot that kills a seedling in April removes the plant entirely. Thrips that strip leaves in June shrink what the plant can pass downward. Timing your planting, choosing the right variety group, and protecting young plants all feed the same outcome. For dates, see When to Plant Onions in Washington, and for the named long-day choices, The Best Onion Varieties for Washington.

What will not work here

The classic failure in the South is a long-day onion that never gets the light it needs: leaves all season, no bulb, then rot. Washington gets the opposite problem, and it is just as firm. Plant the wrong group here and you get a bulb, but a bulb the size of a golf ball at best, because the trigger arrives before the plant is big enough to use it. Planting late does not delay the harvest. It shrinks it.

• Short-day varieties: they start bulbing at about 12 hours of daylight, which arrives in Washington while plants are still small, so the bulbs stay small.
• Southern-market sweet onions in the short-day group: same early trigger as above, and they also keep poorly, so a small harvest goes soft early.
• Any onion planted late: whatever the variety, fewer leaves exist when bulbing begins, so the bulb is smaller than a spring-planted one would have been.
• Seedlings left unprotected against maggots: not a variety, but a stand thinned in April cannot be rebuilt in June, because the bulbing clock keeps running.

The late-planting rule catches more gardeners than the variety rule does. A package label that says “sweet” looks harmless, and a garden cart in May looks like plenty of time. It is not. The onion does not care when you got around to it; it cares about how much leaf it had when the days stretched past the threshold.

Pest damage works the same way as late planting. A plant that loses leaves early behaves like a plant that started late, and the bulb pays for it.

What changes across Washington

The reference figures belong to Seattle, but Washington is not one latitude. The state runs from roughly 46 degrees north along the Columbia in the south to about 49 degrees at the Canadian border. The day lengths below come from the same sunset-angle formula, rounded, and they leave out the few minutes of real daylight that atmospheric refraction adds.

Nobody in Washington crosses into the short-day group, since even the southern edge is well above the 14-hour floor. The differences are practical instead. A gardener in the far north gets the most light of anyone in the state but often has to wait longer for workable soil, so the plant starts its leaf-building later with the same clock running. A gardener near the Columbia can plant earlier, and the pests tied to soil warmth, such as the maggot fly, will also move on a different schedule than they do on the Sound. Eastern and western Washington differ far more in rainfall and summer heat than in day length, which matters for thrips, since they favor hot, dry weather.

Sweet now, or onions that last

Every onion gardener in Washington eventually runs into the trade-off. The bite of an onion comes from volatile sulfur compounds, and those same compounds are what protect the bulb in storage. A sweet variety has fewer of them. The University of Missouri Extension puts it plainly: “As a rule, ‘sweet’ onions do not store as well as the more pungent types.” Short-day types run sweeter and long-day types keep longer, which is why a southern garden hands you onions to eat soon and a northern one hands you onions that feed you into March.

Washington’s long-day onions lean toward the keeping side. Come March, a well-grown, well-cured crop should still be firm in the pantry, sharp when raw, and mellow once cooked. A sweet variety, if you grow one for summer salads, will be gone or soft well before that, so eat it first. Do not expect a sweet onion that stores; you will not find one.

Storage is also where pest damage comes due. Bulbs scarred by maggots or weakened by thrips-stressed leaves break down first, so sort them out for the kitchen and keep only sound ones. Clemson Cooperative Extension gives the target: “Ideal conditions are between 45 °F and 55 °F, and 50 to 60 percent humidity.” Onions want it dry, the opposite of carrots and other roots, so one cellar corner rarely suits both. And curing comes first. Bulbs go into storage only when the necks are tight and the skins papery, never straight from the ground. More on the state as a whole is at Onions in Washington.

Checking this against your own garden

The figures above describe one station near Seattle. A garden in Bellingham, Yakima or Walla Walla sits at a different latitude and a different elevation, and the pest calendar shifts with both. Your county extension office can tell you what is active locally. Treat any state-wide number as a starting point, not a verdict. These are checks you can finish this week:

• Look up your own latitude on a map or through NOAA, and compare it with 47.44 degrees north.
• Find your local last spring frost and first fall frost dates from your county extension office, since this page does not publish them.
• Ask the extension office which onion pests are active in your county and in what weeks.
• Check the variety tags on your seed or sets and confirm each is a long-day type.
• Put a thermometer and a hygrometer in your intended storage spot and compare them with 45 °F to 55 °F and 50 to 60 percent humidity.
• Walk the bed weekly once leaves are up, looking at the base of the plants and at the leaf surface.

Common questions

What are the main pests of onions in Washington?

Onion maggots, thrips, and storage rots cover most of the trouble. The maggot damages the base of young plants in spring, thrips feed on leaves in summer, and rot shows up in bulbs that were cured badly or stored damp.

When do onion pests show up?

The exact week depends on soil and air temperature in your county, and this page does not publish dates. As a rough order, expect maggots in spring, thrips in warm summer weather, and rot after harvest.

Does planting time affect pest damage?

It affects how much damage the plant can absorb. A plant set out late has fewer leaves when day length triggers bulbing, so the same loss costs more bulb. Planting in late winter to early spring gives it the most time to build leaf.

Can I store sweet onions through winter in Washington?

Not reliably. Sweet types have fewer of the sulfur compounds that protect bulbs, so eat them first. Keep the pungent long-day onions for winter, cured and held at 45 °F to 55 °F and 50 to 60 percent humidity.