Hardy unusual roots such as parsnips, salsify and horseradish can go in as soon as the soil is workable in spring, ahead of the last frost. Tender ones wait until after May 18, and even then they want cover. Those dates come from NOAA’s 1991-2020 normals for a single Wyoming station, and the state is far bigger than one station. Here is how to read them and what to adjust.
The short answer for Wyoming
Sow hardy roots as early as the ground allows, and hold tender ones until after May 18, the late end of the last-spring-frost window of May 7 to May 18. Plan to have the tender ones dug before September 24, the early end of the first-fall-frost window of September 24 to October 5.
Those windows are probabilities, not guarantees. They come from the NOAA NCEI 1991-2020 Climate Normals for station USW00024018 (NOAA’s data service), and they mark the stretch where the odds of a 32°F freeze run between 30 and 70 percent. A date inside that band is a coin flip. Even a date just past the band’s late edge leaves a smaller but real chance of a freeze.
The season is short. The reference figure is roughly 130 frost-free days at 6,000 feet, and at higher elevations frost can arrive in any month. Season extension is not optional here. Row cover, low tunnels and raised beds are part of the plan, not extras.
How the window sorts the crops
Frost-tolerant crops treat that window as a suggestion. Cold-hardy ones such as parsnips, salsify and scorzonera germinate slowly in cool soil, so they gain from an early start, and a light frost on their seedlings is survivable. Perennials like horseradish and Jerusalem artichoke go in as soon as the ground thaws. Frost-tender roots are the opposite case: their tops die at the first freeze, so they live entirely inside the frost-free stretch.
| Type | Examples | Planting window | Harvest timing |
|---|---|---|---|
| Hardy, cool-soil sowing | Parsnip, salsify, scorzonera | When the soil is workable, without waiting for the May 7 to May 18 band | After fall frost is fine; they can stay in the ground |
| Hardy perennials | Horseradish, Jerusalem artichoke | As soon as the ground thaws | Late fall, or the following spring |
| Frost-tender, long season | Sweet potato, oca, yacon | After May 18, with protection | Before the September 24 to October 5 band |
Where to go deeper
This page covers timing only. For the broader picture, see Growing Unusual Root Crops in Wyoming. For which varieties suit these conditions, read The Best Unusual Roots to Grow in Wyoming.
What will not work here
A short window punishes the crops that need the longest warm season. Wyoming’s numbers explain why.
Slow tender roots in open ground
Sweet potatoes, oca and yacon spend their early weeks building vines and foliage, and they pack on most of their root size late in the season. That late stretch is exactly what Wyoming cuts off. With roughly 130 frost-free days at 6,000 feet, and a first fall frost that can arrive between September 24 and October 5 (earlier at higher elevations), these crops get a short, uncertain run. A single freeze blackens the tops and ends the growing. This page has no variety-level data showing any of them reliably finishing in open Wyoming ground, so treat them as experiments that need covers, not as staples.
Planting them on May 7 because a calendar says “last frost” is the quickest way to lose them. That date sits at the early edge of the band, where the chance of a 32°F freeze is still high.
Late sowings and weekly watering
July sowings of slow hardy roots fail for a related reason. A parsnip seeded in midsummer has too few weeks left before the first fall frost window to make a root worth digging. Spring is when those crops get their window.
Watering habits also fail here. Wyoming is semi-arid, with dry air and steady wind, and a seedbed can dry out between weekly soakings. Parsnip and salsify seed are slow to sprout, and a crust that forms and dries kills the seedlings before they break through. These crops need the top inch of soil kept evenly damp until they emerge. A weekly deep soak is not enough.
Leaving hardy roots in the ground over winter is possible but not carefree. Cheyenne’s coldest-month mean daily minimum is 18.1°F. That is an average, so real cold snaps go lower, and an unmulched bed will freeze hard. A thick mulch layer keeps the ground workable for digging and moderates the freeze-thaw swings that can rot roots.
What changes across Wyoming
Wyoming is not one garden. The USDA Plant Hardiness Zone Map (2023 edition, built on 1991-2020 normals) places the state in zones 3a to 6a, a spread of several zones. So there is no single answer for the whole state. The USDA system rests on one variable, the average annual extreme minimum temperature, so it measures winter cold and says nothing directly about how long your summer lasts.
The dates on this page come from one reference station, and the 18.1°F coldest-month figure comes from Cheyenne. They describe that part of the state. Other parts differ, sometimes a lot. Someone in a high basin or a mountain valley can expect a later last frost, an earlier first frost, and frost in months where Cheyenne’s numbers would not predict it.
If you are in the north or at altitude, shift your plan in three ways:
- Push spring sowing later than the dates here, and watch the soil rather than the calendar.
- Pull harvest of anything tender earlier, ahead of the first fall frost window or earlier.
- Lean on hardy roots and perennials such as horseradish and Jerusalem artichoke, and treat frost-tender crops as covered-bed projects.
To find your own zone, use the USDA Plant Hardiness Zone Map and enter your ZIP code. Do not guess from a neighbor’s tag at the garden center. Hardiness tells you whether a perennial root survives winter. It does not tell you how many frost-free days you have, so you need both pieces of information.
Where to check this for your own county
A county recommendation beats a state one. This page is the state one, built from a single station and a statewide zone range, so it gives you a starting point and not a verdict. The University of Wyoming Extension is the next stop, and its county offices work from local conditions.
This page does not have a county-level planting calendar, and it does not claim to know which of your county’s microclimates you sit in. Extension can fill that gap. When you contact them or browse their site, look for two things:
- A local planting calendar, ideally with spring and fall frost dates for the town nearest you and guidance on when to sow cool-season crops.
- Varieties trialled in your county, or the closest one with similar elevation, so you are choosing roots that have already been tested in comparable cold and wind.
Ask directly whether anyone there has seen unusual roots grown locally. Extension staff often know which gardeners nearby have tried parsnips, salsify or horseradish and what happened. If the answer is “not much,” you are working in territory with little local data, and that tells you to start small, sow a few feet of several types, and see what holds up.
One caution on sources: seed catalogs and national guides give dates written for milder places. When a package says “sow after danger of frost,” use the Extension county information to decide what that means for you.
Checking this against your own garden
Your own observation outranks every date above. Three things are worth watching, and none need equipment.
Soil warmth by feel. In the morning, push your bare hand a few inches into the bed. If it feels cold enough to ache, tender roots should wait and hardy seed will sprout slowly. Do this in different beds. A south-facing bed or a raised bed that feels noticeably warmer than a shaded one has just shown you a microclimate you can use.
First true leaves. On parsnip, salsify or scorzonera, the first pair of leaves is the seed leaves, and the first true leaves come after. Note the date you sowed and the date true leaves appear. A long gap means the bed is cold, or that it dried out and the seed sat waiting. Write that gap down. It is your own number for next year.
Where frost lingers. On a clear, cold morning, look at where frost sits longest on the grass and where it never shows up. Low spots collect cold air and frost there longest. A spot near a south-facing wall may stay clear. Those clear places are where tender crops have the best odds, and the white places are where they do not.
Keep a simple notebook for a season. A single year is one draw from the odds, and with a 30 to 70 percent band, your garden can land outside the dates on this page and still be normal.
Common questions
Can I sow unusual roots before the last frost date in Wyoming?
Hardy ones, yes. Parsnips, salsify, scorzonera, horseradish and Jerusalem artichoke can go in once the soil is workable, before the May 7 to May 18 window. Frost-tender roots such as sweet potato, oca and yacon should wait until after May 18 and still need protection.
When should tender roots be harvested?
Before the first fall frost window of September 24 to October 5. That is a probability band, not a promise, and at higher elevations frost can come earlier. Keep covers or cloth handy so you can protect plants during an early cold night and keep growing.
Is the season long enough for sweet potatoes?
It is a stretch. The figure is roughly 130 frost-free days at 6,000 feet, and sweet potatoes need a long warm stretch. This page has no Wyoming variety trial data for them, so check with University of Wyoming Extension before investing much space or money.
Do these dates apply across the whole state?
No. They come from NOAA data for one station, USW00024018, and Wyoming spans USDA zones 3a to 6a. Mountain and northern gardens generally run colder and shorter, so treat these dates as a starting point and confirm them with your county Extension office.