Garlic in Wisconsin is attacked on a schedule. Voles and wireworms go after cloves within weeks of planting. Bulb and stem nematodes work underground all winter and show themselves in spring, when onion maggots and slugs also arrive. Thrips peak in early summer, just before the bulbs come out of the ground. Most of that clock starts at the first hard freeze.
The short answer for Wisconsin
The reference date is October 17 to October 29. That is the band in which a 32 F freeze has a 30 to 70 percent probability, according to the NOAA NCEI 1991-2020 Climate Normals for station USW00014839. Garlic planted around then meets rodents and soil pests first. Everything else in the pest year follows in order.
What the date is anchored on
A frost date is a midpoint, never a safe date. In one year out of two the first freeze falls later than the figure, and in the other it falls earlier. For pests this matters more than for the calendar itself. A warm, late autumn keeps wireworms and slugs active near the surface longer. A sudden early freeze can leave mulch half-spread and cloves sitting in bare soil.
Garlic goes in during autumn and comes out the following summer, so a single bulb sits in the ground through every pest on the list. The planting calendar for Wisconsin explains how to count back from this date. Here the same date is used to count forward to what shows up and when.
Why it moves
One station’s record is not the whole state. Lake Michigan delays the first fall freeze at the lakeshore. Inland southern Wisconsin typically freezes in early October, and the north freezes far earlier. A gardener in the Northwoods and one in a lakeshore town are living through the same pest sequence on calendars that can sit weeks apart.
Working it out for your own garden
Start with your own first-freeze estimate and build the pest calendar from it. Elevation, a large lake and the coast can shift these dates by roughly two weeks either way. Take the October 17 to October 29 band and subtract 14 days for a cold inland or northern site. That gives roughly October 3 to October 15. Add 14 days for a sheltered lakeshore garden and you get roughly October 31 to November 12. These are working estimates, not published figures for your town.
Counting forward from the freeze
Once you have your date, the sequence runs in four steps:
- Around planting and the first freeze, voles, mice and wireworms go after the cloves.
- Through winter, rodents tunnel under mulch and snow, and nematodes sit in infested soil and seed.
- In spring, as growth resumes, onion maggots, slugs and nematode symptoms appear.
- In early summer, thrips build up in dry, warm spells before harvest.
A pest-by-pest reference
| Pest | When it hits | What you see | First move |
|---|---|---|---|
| Voles and mice | Autumn through winter | Missing cloves, tunnels in mulch | Mulch after the ground starts to freeze, not before |
| Wireworms | Autumn and spring, in soil | Narrow holes bored into cloves and bulbs | Avoid beds recently turned from sod |
| Bulb and stem nematode | Carried in seed, shows in spring | Stunted, twisted, swollen plants; soft bulbs | Buy clean seed; rotate out of alliums |
| Onion maggot | Spring | Wilting plants, rotted bases with larvae | Clean up debris; cover plants early |
| Slugs | Wet spring | Ragged leaves, slime trails | Pull mulch back from emerging shoots |
| Onion thrips | Early summer, hot and dry spells | Silvery streaks on leaves | Keep plants watered; don’t let weeds shelter them |
No published figure is attached to these windows in this article, and none should be assumed. The timing shifts with weather, so treat the table as an order of events. Local county data on emergence dates would need to come from extension.
What changes across Wisconsin
Lakeshore and the south
Along Lake Michigan, from Kenosha and Milwaukee up through Door County, the lake holds the first freeze back. Cloves planted there often sit in warm soil a little longer, which keeps wireworms and slugs active into late autumn. Mulch goes on later too. Spread it early in a mild October and mice settle in well before the ground is hard.
Inland southern Wisconsin, around Madison and the Rock River valley, typically freezes in early October. Planting and mulching happen sooner, and spring pests such as onion maggots wake up on a schedule that is easy to track against local bloom and leaf-out.
The north
The far north, including Vilas, Oneida and Iron counties and the Lake Superior shore, freezes far earlier than the reference band. Garlic there goes in sooner, and snow arrives early and stays. That works in the grower’s favor against many insects but not against voles, which travel under the snowpack for months and are rarely seen until the thaw. Check the beds in March and April, when damage becomes obvious.
Gardeners comparing notes with a drier, higher-elevation state can read the Wyoming planting calendar and the Wyoming variety guide. The contrast helps show how much of Wisconsin’s pest pressure comes from moisture and snow cover. The full state overview is in Growing Garlic in Wisconsin.
The mistakes this climate punishes
Planting and mulching errors
In a cold, snowy state the costly errors come before winter. Planting too late leaves cloves with little time to root before the ground locks up. Weakly rooted cloves are easier for rodents to dig out and slower to recover from wireworm damage. Planting too shallow leaves them exposed to frost heaving, which pushes cloves up and opens a path for voles and mice.
Mulching too early causes a different failure. A thick straw layer laid in a mild October is an invitation for voles to nest. Wait until the ground begins to freeze, then mulch. Then pull the mulch back from emerging shoots in spring, because a damp mat feeds slugs and holds moisture against the stems. Varieties matter here as well. Hardnecks suit Wisconsin winters, and the Wisconsin variety guide covers which ones to choose.
Seed and bed errors
Buying seed garlic from an unknown source is the way bulb and stem nematodes reach a garden. Once they are in the soil, no spray removes them, and replanting in the same bed keeps feeding them. Rotating out of alliums for several years, and buying clean seed from a trusted grower, is the only reliable defense.
Another trap is planting into a bed recently turned from lawn or sod. Wireworm larvae live in grass roots and will bore into cloves when nothing else is available. Starting garlic in a bed that has already grown vegetables for a season or two avoids much of that.
Checking this against your county
County-level frost dates and pest alerts are not published in this article. The University of Wisconsin-Madison Division of Extension runs county offices that can confirm local freeze timing and identify problem plants from a photo or sample. If a patch of garlic looks stunted and twisted, ask about nematode testing before replanting that bed.
Common questions
When should I check my garlic for pests in Wisconsin?
Check right after planting for rodent digging, again in March and April when snow melts, and weekly through early summer. Thrips and spring maggots tend to show before harvest.
Do deer and rabbits eat garlic?
Garlic is not a favorite, and rabbits and deer mostly leave the bulbs alone. Rabbits may nibble young green shoots in spring, but the real bulb losses come from voles, mice and soil pests.
Can I save seed from a plant with nematode damage?
No. Infested cloves carry the nematodes into the next planting. Discard affected plants and soil rather than composting them, and start over with clean seed in a different bed.
Is the October 17 to October 29 freeze date a safe planting deadline?
No. It is the band where a 32 F freeze has a 30 to 70 percent probability, and in one year out of two the first freeze comes later. Plan your pest protections around your own garden’s freeze, not this one station’s figure.