Hardy roots such as salsify and scorzonera go into Minnesota soil in late April, close to the last spring frost. Tender ones, like oca and yacon, wait until nights turn mild. The honest answer is a window, not a date, and it moves by roughly three weeks between the Twin Cities and the northern counties. The rest of this page explains how to read it. For the wider picture, start with Growing Unusual Root Crops in Minnesota.
The short answer for Minnesota
In the Twin Cities, the last spring frost falls between April 17 and April 30, and the first fall frost between October 11 and October 23. These dates come from the NOAA NCEI 1991-2020 Climate Normals, and they are a probability of a 32°F freeze, not a guarantee. They mark the 30 to 70 percent band of those odds.
How to read the window
On April 17, another 32°F freeze is still more likely than not. By April 30 the odds have dropped to about 30 percent, which is still roughly one spring in three. Fall runs the other way: by October 11 there is about a 30 percent chance the first freeze has already come, and by October 23 about 70 percent.
The Twin Cities are the mild south of the state. Northern Minnesota runs two to three weeks tighter at both ends, so treat these figures as the generous end of the range.
Sowing by root type
Frost tolerance matters more than the root’s name. Sorting by it turns the window into a plan.
| Type | Examples | Timing against the frost window |
|---|---|---|
| Frost-hardy, seed-sown | Salsify, scorzonera, Hamburg parsley, burdock | Sow as soon as soil can be worked, from the early end of April 17 to April 30. Seedlings can handle a light freeze once up. |
| Long-season, started indoors | Celeriac | Transplant near the late end of the spring window, after April 30 if the weather is unsettled. |
| Perennial roots and tubers | Jerusalem artichoke, horseradish | Plant when the ground thaws. They return each year. |
| Frost-tender | Oca, yacon, mashua | Plant only after April 30 with mild nights. A hard freeze in the October 11 to October 23 band ends their season. |
Which varieties earn a spot in each row is a separate question, covered in The Best Unusual Roots to Grow in Minnesota.
What will not work here
Roots that cannot survive the winter outdoors
The mean daily minimum of the coldest month at the Minneapolis reference station is 8.8°F. That is the average low, so some nights run colder. It decides hardiness: whether a plant lives through the cold at all. Yacon, oca and mashua come from mild Andean climates, and they will not overwinter in a Minnesota bed. Treat them as annuals, or lift the roots and store them indoors.
Jerusalem artichoke and horseradish are another story. They are hardy roots and are better known for refusing to leave than for dying.
Slow roots on a short season
The fall band starts October 11. A root that needs nearly the whole season, sown late in the spring window, runs straight into it. The practical rule is to count backward from October 11, not October 23. Take the days to maturity on the seed packet and subtract them. If the sowing date that results has already passed, pick a faster root or start the plant indoors.
In the north, where both ends run two to three weeks tighter, the same arithmetic knocks out more crops. A root that finishes comfortably in the Twin Cities can fail to size up near the Canadian border.
Planting tender roots before April 30 also fails. Even at the late end of the band, about 30 percent of springs still bring a 32°F freeze, and one night is enough to kill a young yacon or oca plant. Waiting for the whole garden to be frost-free before sowing anything is the opposite mistake. Salsify and scorzonera are cool-season roots, and a late start wastes the spring weather they like best.
This page has no figures for summer heat or rainfall, so it makes no claims about either. For the timing questions above, the frost dates and the winter minimum are what decide the outcome.
What changes across Minnesota
Minnesota is not one garden. The USDA Plant Hardiness Zone Map (2023 edition, built on 1991-2020 normals) places the state in zones 3a to 5a. The system uses one variable, the average annual extreme minimum temperature, and the state spans two full zones. So no single answer exists, and the Twin Cities figures above are a reference point, not a statewide rule.
A reader in the north should shift everything later in spring and earlier in fall, by the two to three weeks noted above. In practice that means three adjustments. Sow hardy roots later, since the ground thaws later. Favor roots that mature quickly, because the fall freeze arrives sooner. Start long-season crops indoors rather than sowing them outside.
Tender roots deserve extra skepticism there. A reader near the colder edge of the range has the least margin for oca or yacon.
Zone is also not the same thing as frost timing. A zone tells you about winter survival of perennials, while the frost band tells you when annuals can start and must finish. Within a single county, a low spot where cold air pools can run noticeably cooler than a slope or a bed beside a south wall.
To find your own zone, use the USDA Plant Hardiness Zone Map and look up your location. This page will not guess it for you.
Where to check this for your own county
Your state extension service is the next stop. University of Minnesota Extension publishes gardening guidance written for Minnesota conditions, and its county offices know local soils and local frost patterns.
Look for two things there. The first is the local planting calendar for vegetables, ideally for your county or the nearest one. The second is any list of varieties trialled in your area, which tells you what has actually been grown nearby.
This page does not have an extension calendar written specifically for unusual roots, so it does not borrow one from elsewhere. Extension calendars are usually organized by crop. If your root is not listed, check its closest familiar relative, such as parsnip or potato, and note that you are extrapolating.
A county recommendation beats a state one. This page is the state one: a single reference station in the mild south, plus a warning that the north differs. When the extension calendar for your county disagrees with a date here, go with the county.
Checking this against your own garden
Your own observations outrank every date above. Three are worth watching, and none needs equipment.
- Soil warmth by feel. Dig a small hole and hold your bare hand in it for a minute. If it feels cold, wait. A bed that warms earlier than your neighbor’s is a sheltered, sunny microclimate. One that stays cold is probably low or shaded.
- First true leaves. Sow a pinch of radishes or lettuce early as a test row. The first true leaves, the ones after the seed leaves, show how fast that bed is waking up, and they are a better guide than the calendar.
- First male flowers on squash. If you grow squash or cucumbers, the first flush of male flowers means summer warmth has settled in. That is a fair cue for tender roots, and a late flush means a cooler spot.
Keep a note of the dates you see these signs, year after year. A few seasons of notes will tell you more about your garden than any regional table.
Common questions
Can I sow hardy unusual roots before April 17?
Hardy roots tolerate cold, but sowing earlier than the window carries more risk. Cold soil slows germination, so seed can sit for weeks. Sow when the soil can be worked and crumbles in your hand rather than clumping.
Do these dates apply in northern Minnesota?
No. They come from the Twin Cities reference station. Northern Minnesota runs two to three weeks tighter at both ends, with a later spring start and an earlier fall freeze.
Can I leave roots in the ground over winter?
It depends on the root. The Minneapolis reference shows a mean daily minimum of 8.8°F in the coldest month. Hardy perennial roots such as Jerusalem artichoke and horseradish cope with that. Tender roots such as oca and yacon do not, and need lifting before a hard freeze.
Is a frost date the same as a hardiness zone?
No. Frost dates describe the growing season, and a hardiness zone describes the average annual extreme minimum temperature. Two gardens can share a zone and still have different frost dates, because low spots and open exposures change when the last freeze arrives.