Thrips do most of the damage to onions in Texas, and they do it in spring, just as the plants decide how big a bulb to make. Cutworms and maggots go after young transplants earlier, in cool weather. Wireworms and nematodes work from below. The calendar matters as much as the bug list, because an onion’s leaf count is locked in by day length, and every leaf a pest removes before then comes out of the bulb.
The short answer for Texas
At 29.98 degrees north, the latitude of the Houston station whose climate normals this site uses, the longest day of the year gives 13 h 56 of daylight. Short-day onions begin bulbing at about 12 hours of daylight, according to Clemson Cooperative Extension. That happens around the spring equinox, so Texas onions have a fixed deadline: grow as many healthy leaves as possible before the days pass 12 hours, then bulk up.
Each leaf your onion holds on that date becomes one ring of the bulb. Anything that chews, rasps or rots foliage before then makes the final onion smaller, and nothing you do afterward gives those rings back. That is why the order of attackers matters more than the list.
Winter is the quiet season, but not a safe one. Cutworms and armyworm caterpillars clip young plants at the soil line or chew the leaves at night. Seedcorn and onion maggots can tunnel into small plants in cool, damp weather. Wireworms and root-knot nematodes, the latter especially in sandy soil, work below ground and show up as stunted, uneven rows that no amount of fertilizer fixes.
Spring is thrips season. Onion thrips are tiny, pale insects that hide in the folds where leaves meet near the neck and rasp the surface, leaving silvery streaks and whitened tips. Their numbers climb as weather warms and dries, which lands right on the bulbing window. Thrips can also carry iris yellow spot virus, a disease that leaves straw-colored, diamond-shaped lesions on leaves. Exact spray thresholds and product choices are not published here, and they differ by county. Your extension office is the place to get them.
Fall planting is what makes this timeline work in the mild-winter parts of the state. For dates, see When to Plant Onions in Texas. For the broader picture, start with Onions in Texas.
What will not work here
Some choices fail no matter how well you manage bugs. The clearest is a variety whose day-length need the state cannot meet. A long-day onion waits for 14 to 16 hours of daylight before it bulbs. Houston peaks at 13 h 56, so the trigger never comes. The plant keeps making leaves all season, never forms a bulb, and eventually rots in the ground. It also sits in the garden through the whole thrips season, feeding whatever finds it.
- Long-day varieties (14 to 16 hours needed): the longest Houston day is 13 h 56, so they never bulb. Expect leaves, no onion, then rot.
- Any onion sold without a day-length rating: you cannot compare it to 13 h 56, so you are guessing. Skip it or check with your extension office before planting.
- Short-day onions set out late: planting late does not delay the harvest, it shrinks it. Bulbing starts when the days reach about 12 hours, and the plant has only the leaves it managed to grow by then.
- Waiting for thrips to appear before you act: by the time silvery streaks are obvious, leaf area has already been lost, and that leaf area was destined to become bulb.
The late-planting point surprises people. A gardener who sets transplants out a month behind schedule expects to harvest a month behind. In fact the harvest date barely moves, because the days lengthen on their own schedule. The onions simply arrive small, with a few weeks of growth missing. Pest damage works the same way. A plant that loses a third of its leaves to thrips in February has effectively planted late, without changing the calendar.
The variety side of this is covered in The Best Onion Varieties for Texas.
What changes across Texas
All the figures on this page belong to one place, the Houston station at 29.98 degrees north. Texas is enormous, running from the Rio Grande Valley at the southern tip to the Panhandle in the north, a span of many degrees of latitude. Latitude and day length for those far corners are not published here, so a reader near either edge should look up their own.
| Area | Latitude | Longest day | What it changes |
|---|---|---|---|
| Houston (reference) | 29.98 degrees north | 13 h 56 | Short-day onions, fall planting |
| Far south (Rio Grande Valley) | Lower than Houston | Shorter than 13 h 56 | Longer growing window and earlier warmth, so thrips and other pests can start sooner |
| Far north (Panhandle) | Higher than Houston | Longer than 13 h 56 | Harder winters push planting to late winter or early spring; check the variety’s day-length group |
The pest calendar shifts along with it. In the south, a mild winter lets insects stay active longer and warm weather returns early, so the thrips window opens sooner and lasts longer. Farther north, cold slows caterpillars and thrips in winter, and planting moves to late winter or early spring. Cool, damp spring weather there is when maggots and leaf diseases are most likely to hit young plants.
A reader near the northern edge should also confirm which day-length group their variety belongs to, since more summer daylight up north pushes the bulbing trigger for some onions. The reference city cannot answer that for you.
Sweet now, or onions that last
Southern gardens lean sweet and short-keeping. Northern gardens lean sharp and long-keeping. The reason is chemistry. The bite of an onion comes from volatile sulfur compounds, and those same compounds protect the bulb in storage. A sweet variety carries fewer of them, so it tastes better raw but will not last 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.” Short-day types, which suit Texas, tend to run sweeter. Long-day types keep longer.
Your pantry in March shows the difference. A Texas gardener growing short-day onions has mostly eaten them by then, or is cutting away soft spots from the last few. A northern gardener with pungent long-day bulbs is still cooking from the same braid. Never count on a sweet onion to store.
Storage still needs care, especially with damage from the season. Cure first: bulbs go into storage only after the necks are tight and the skins papery, never straight from the ground. Then keep them dry. Clemson Extension gives the ideal as “between 45 °F and 55 °F, and 50 to 60 percent humidity” (Clemson Cooperative Extension, Home & Garden Information Center). That is the opposite of carrots and other roots, and a single cellar cannot hold both well. Bulbs with bruised or wounded necks rot first, so eat those first.
Checking this against your own garden
Nothing here is a statewide rule. The numbers come from one station, and your garden may sit several degrees away. These checks take about a week:
- Look up your own latitude and compare it with 29.98 degrees north. If you are far from Houston, your longest day will differ from 13 h 56.
- Find your local first and last frost dates, and plan planting from those rather than a statewide date.
- Read the day-length rating on every onion variety you plan to plant. If it lists long-day, or no rating at all, set it aside.
- Walk the rows once a week and look inside the leaf folds near the neck for tiny pale insects and silvery streaks. Check the soil line for clipped seedlings.
- Call your county extension office (in Texas, Texas A&M AgriLife Extension) and ask which pests are active in your area right now and what treatments are labeled for onions.
- Put a thermometer and hygrometer where you plan to store onions, and see whether it holds 45 °F to 55 °F and 50 to 60 percent humidity.
Common questions
What is the worst onion pest in Texas?
Onion thrips. They feed on leaves during the spring weeks when the plant is deciding how large a bulb to make, and they can spread iris yellow spot virus.
When should I start watching for pests?
Right after planting. Cutworms and maggots hit young plants in cool weather, and thrips build as spring warms. Weekly checks from transplanting through bulbing are the safest habit.
Can I save onions that have thrips damage?
Leaves that are already scarred will not recover, and the bulb will be smaller for it. Protecting the leaves that remain still helps. Ask your county extension office which options are labeled for onions in your area.
Does planting late avoid the pests?
No. Planting late shrinks the harvest, because the bulb size is set by the plant’s size when day length triggers bulbing, and it can push growth into the thrips season without escaping it.