Hardy unusual roots such as salsify, scorzonera, parsnips and burdock go in as soon as the soil can be worked in spring. Tender ones, the oca and yacon crowd, wait until the last spring frost has passed. The dates below come from Billings, one of the state’s milder valleys. Elsewhere in Montana they slide by weeks. For the full picture of what grows here, start with Growing Unusual Root Crops in Montana.
The short answer for Montana
At the Billings reference station, the last spring frost falls between April 29 and May 10, and the first fall frost between October 1 and October 14. Both ranges come from NOAA’s 1991-2020 Climate Normals, and both are probabilities of a 32°F freeze, not dates the weather has promised to keep. They mark the band where the odds sit between 30 and 70 percent. A freeze after May 10 is less likely but possible, and the same logic runs in reverse in the fall.
Billings is the mild case. Mountain valleys elsewhere in Montana can frost in any month, so a gardener there should treat these dates as the friendliest version of the state’s calendar.
The coldest month at Billings has a mean daily minimum of 17.9°F. That is an average, not a record low, but it is the number that decides whether a root can sit through winter in the ground. Roots split into three groups against these dates:
| Type | Examples | When to plant, against the Billings dates |
|---|---|---|
| Hardy, cool-soil sowers | Parsnip, salsify, scorzonera, burdock | Direct sow as soon as the soil is workable, before April 29 |
| Hardy tubers | Jerusalem artichoke | Plant tubers in early spring; they tolerate the frost band |
| Slow starters | Celeriac | Start indoors, set out around the May 10 end of the band, with cover on hand |
| Tender, warmth-demanding | Oca, yacon, sweet potato | Nothing outdoors until after May 10, ideally under plastic or row cover |
Hardy sowers can start early because their seed germinates in cool soil, and a late freeze mostly slows the seedlings rather than killing them. Tender roots have no such margin. One 32°F night after transplanting and the season is over. Fall sowing is also possible for the hardy group: seed goes in after the soil is cold, so it sleeps through winter and sprouts in spring. Trial data for that is thin, which makes it a gamble worth a small test row, not a whole bed. For variety choices within each group, see The Best Unusual Roots to Grow in Montana.
What will not work here
The slowest tender roots lose the arithmetic. Count from the later spring date (May 10) to the earlier fall one (October 1) and the frost-free stretch is a little under five months. Yacon wants a long warm season to size up. Oca makes its tubers late, as days shorten, which is exactly when the October 1 to October 14 frost band arrives. A freeze at the early end cuts off the bulking just as it begins. Sweet potatoes ask for heat as well as time. They are possible under plastic in a good year and a poor bet in the open.
Leaving tender roots in the ground over winter fails for a different reason. With a coldest-month mean minimum of 17.9°F at Billings, and colder winters in the higher and northern parts of a state rated 3a to 6a, yacon crowns, oca tubers and sweet potatoes do not come back. Lift them before the fall frost or treat them as annuals. Parsnips, salsify and Jerusalem artichokes are the ones that can stay put, ideally under a mulch layer.
Early planting is the third trap, and the most common. A warm stretch in late April feels like permission. The numbers say otherwise. April 29 is the early edge of the band, where a later freeze is still likely, not a safe date for anything tender.
Dryness is the fourth. Much of Montana is dry, and this page has no rainfall figure to quantify it, so check local totals before building an irrigation plan. The pattern to avoid is the weekly soak with dry soil in between. Long taproots respond to swings in moisture by splitting or turning woody, and parsnip and salsify seed that dries out mid-germination simply never shows. Steady light moisture and a mulch layer beat one heavy watering a week. Crowding is a problem for the same reason: rows planted too close compete for a limited water supply, so give roots the full spacing on the seed packet.
What changes across Montana
There is no single answer. The USDA Plant Hardiness Zone Map (2023 edition, built on 1991-2020 normals) places Montana in zones 3a to 6a, four numbered zones in one state. That spread is wider than the gap between Minnesota and Tennessee. Zones are built on one variable only, the average annual extreme minimum temperature, so they speak to winter survival and say nothing about when the last spring frost lands.
That distinction matters for planting dates. A gardener at altitude or in the north faces both problems: a colder winter and a shorter frost-free season. The practical shifts all point the same way:
- Start hardy roots as soon as the ground thaws, since the cool-soil window is the one thing a short season cannot take away.
- Push tender crops later than the Billings dates, or skip them outdoors.
- Start slow crops like celeriac indoors and use row cover on both ends of the season.
- Favor the earlier-maturing varieties in each group.
A gardener in a warmer, lower valley can lean the other way and try the borderline crops. Neither should assume the Billings band applies to them. To find the zone for your own address, use the USDA map itself and enter your ZIP code. Zone lines in mountain country run through very short distances, so a neighbor two valleys over may sit in a different one.
Where to check this for your own county
Start with Montana State University Extension, the state’s land-grant extension service, and look for your county office. A county agent knows which low spots hold frost, which soils warm slowly and which varieties have survived a local winter. Two things are worth looking for specifically: a local planting calendar with sowing dates for your area, and any variety trials run in your county. A trial result for a named variety beats any general recommendation, including this one.
A county recommendation outranks a state one, and this page is the state one. It is built on a single reference station, Billings, which the data itself flags as one of the milder valleys. This page does not carry county-level frost dates or trial results for unusual roots, and it does not claim to. Treat the Billings window as an orientation, then let your county office correct it.
If the county office has no calendar for these crops, ask for the nearest one for a comparable vegetable. Parsnips follow the carrot schedule closely enough to borrow it, and celeriac can follow the celery or leek timetable. The county’s frost-date guidance matters more than whether a crop is named.
Checking this against your own garden
Your own observations outrank every date above. Three are worth tracking, and none needs an instrument.
First, soil warmth by feel. Push a bare hand a few inches into the bed in the morning. Soil that feels cold and clumps wet is still in hardy-root territory only. Soil that feels neutral and crumbles is ready for slower starters. If one bed warms a week before another, you have mapped your microclimate: the warm one gets the tender crops.
Second, the first true leaves. Seedlings first show a pair of seed leaves, then the true leaves, which look like the adult plant. Note the date a test row, even a few radishes, reaches that stage. A slow date against your neighbors’ tells you the bed is cold or shaded. A fast one tells you it is a good spot for the risky crops.
Third, your own last frost. Watch a frost-tender plant, a pot of basil or the first squash, and note the night its leaves blacken. If that night lands outside the April 29 to May 10 band, you have learned something no statewide table can tell you. A low corner of the yard may frost a week after the rest. Write the dates down for two or three seasons and you will have a calendar that beats any on this page.
Common questions
Can I sow salsify or parsnips in fall in Montana?
It is possible. The seed goes in after the soil is cold enough that it will not sprout, then germinates in spring. This page has no Montana trial data on it, so try a small row and ask your county extension office whether local gardeners have had success.
Is April 29 safe for planting tender roots?
No. April 29 is the early edge of the Billings band, where the odds of another 32°F freeze are still high. Tender roots such as oca, yacon and sweet potato should wait until after May 10, and later at altitude or in the north.
Does my USDA zone tell me when to plant?
No. The USDA zone rests on the average annual extreme minimum temperature, so it tells you what survives winter, not when spring frost ends. Montana runs from 3a to 6a, so use the zone for overwintering decisions and your local frost dates for timing.
Which Montana frost dates should I use?
The Billings dates (April 29 to May 10 in spring, October 1 to October 14 in fall) are the ones this page has, and they come from the milder end of the state. Use them as a starting point, then confirm with your county extension office, because mountain valleys can frost in any month.