Hardy unusual roots such as salsify, scorzonera and parsnip can go in around the last spring frost window, which at Portland runs April 22 to May 1. Tender ones, like oca and yacon, wait until that window has clearly passed. Those dates describe the mild coast. Inland and northern Maine run later in spring, so treat the Portland dates as the earliest reasonable starting line, not the rule.
The short answer for Maine
Both frost dates in this section are probabilities, not guarantees. NOAA’s 1991-2020 Climate Normals, measured at the Portland station, put the last spring frost at April 22 to May 1. That is the band in which the chance of a 32°F freeze still ahead sits between 30 and 70 percent. Inside that band, a hard freeze is about as likely as not. Before it, freezes are more likely than not. After it, they become the exception, though never impossible. The fall side works the same way: the first fall frost lands between October 9 and October 21. You can read the underlying numbers in the NOAA normals file for Portland.
What the dates tell you to do
A probability band means you choose your risk by root type. Roots that shrug off a light freeze can be sown at the front edge of the window. Roots that die at 32°F belong behind the back edge, plus a cushion. The frost-free stretch at the coast runs a little over five months if you wait for the late spring date and stop at the early fall date, and closer to six if you take the generous ends of both bands. That is workable for most of what this cluster covers, and tight for the slowest.
| Root type | Examples | Timing against the Portland window |
|---|---|---|
| Hardy seed-sown roots | Salsify, scorzonera, parsnip | Sow as soon as the soil is workable, at or just before April 22 to May 1 |
| Perennial roots set as pieces | Jerusalem artichoke, horseradish | Plant in early spring once the ground thaws and drains |
| Frost-tender tropical roots | Oca, yacon | Set out after May 1, with a week or two of margin, ideally as started plants |
This page does not have a Maine-published planting calendar for these specific crops to cite, so the timing column is my reading of the frost bands, not an official schedule. If you have not chosen what to grow yet, The Best Unusual Roots to Grow in Maine sorts them by how much season they need. For the wider picture of growing them here, see Growing Unusual Root Crops in Maine.
What will not work here
Planting tender roots on the first warm weekend in April. At Portland, the probability of a 32°F freeze is still well above zero until at least May 1, and a single night at 32°F finishes a yacon or oca shoot. Maine springs also tend to give a few mild days and then a hard snap, which is exactly when the band is most dangerous.
Counting on the long end of the fall. The first fall frost arrives October 9 to October 21 at the coast, and that is the generous number. Any root that needs most of the frost-free season to bulk up will be cut off by an early frost in a short-season spot. Tropical tubers are the clearest case: they form their crop as days shorten and cool, and a frost that blackens the tops ends the growing. Those are better treated as a gamble than a plan unless you have a cover or a protected site.
Sowing slow seed into cold, wet soil and walking away. Salsify and parsnip germinate slowly, and the same spring cold that makes the April 22 start tempting also keeps seed sitting in damp ground. A soggy start rots seed or yields a ragged stand. A crust of dried clay can finish what the cold started, so keep the seedbed evenly damp without drowning it.
Leaving the winter to chance. The mean daily minimum of the coldest month at Portland is 15.6°F. That is the figure that decides whether a plant survives winter in the ground at the coast, and it is mild by Maine standards. Hardy perennial roots like horseradish and Jerusalem artichoke handle it without fuss. Tender tubers do not, and they will not overwinter in the soil. Parsnip and salsify left in the ground for a late harvest are a different question: the coastal minimum is manageable with a deep mulch, but frozen ground can lock the crop away until it thaws, so lift what you want for midwinter before the freeze sets in.
Finally, copying a calendar from a milder state. A date that works for a gardener in southern New England is simply too early for most of Maine. The state’s frost dates and winter minimums are different enough that borrowed schedules fail in predictable ways.
What changes across Maine
Maine is not one garden. The USDA Plant Hardiness Zone Map (2023 edition, built on 1991-2020 normals) places the state in zones 3b to 6a. That map uses a single variable, the average annual extreme minimum temperature, so it says how cold winter gets, not how long summer lasts. A spread that wide means there is no single answer for the state. A coastal garden and a garden in the far north live in different growing worlds.
The Portland numbers in this article sit at the mild end of that spread. The frost data comes with a plain warning: inland and northern Maine run substantially later in spring and earlier in fall. If you garden in the north or at altitude, shift three things. Move your spring sowing later and wait for the soil to dry and warm before you commit. Pull your fall deadline earlier, which means any root that needs the whole season is a poor bet. And lean on the hardy types, parsnip, salsify, scorzonera, horseradish, rather than the tender tropicals, which lose the most from a shortened summer.
Winter shifts too. A zone 3b winter is colder than the coastal 15.6°F, so the mulch you can get away with at the coast becomes a requirement inland, and tender roots must be lifted and stored indoors without question. Find your own spot on the USDA map rather than guessing from a neighbor or a seed packet, since a hillside and a river valley a few miles apart can fall into different zones.
Where to check this for your own county
The best next stop is University of Maine Cooperative Extension, the state’s land-grant extension service. Look there for two things. First, the local planting calendar for your area, which uses frost dates from stations closer to you than Portland. Second, the varieties trialed in your county, meaning cultivars someone has actually grown in Maine soil and weather, with results rather than catalog promises.
Be ready to ask a human, too. County extension offices often have staff or Master Gardener volunteers who know the quirks of local soil, late-frost pockets and wet springs. If you call or write, bring your town, what you want to grow, and whether your soil is sandy, loamy or clay. Those three details turn a general answer into a useful one.
A county recommendation beats a state one, and this page is the state one. It gives you a coastal anchor and a way to reason about root types, but it cannot tell you whether your valley holds frost until mid-May. Where the extension service and this page disagree, go with the extension service.
One more caution: extension calendars are usually written one crop at a time, so you may need to look up each root separately, and some unusual roots may not have a dedicated Maine calendar at all. In that case, use the calendar for the closest common crop (parsnips for salsify, for example) and keep the frost-band logic from this page as your backup.
Checking this against your own garden
Your own observations outrank every date here. Three are worth watching, and none need equipment.
- Soil warmth by feel. Push a bare hand a few inches into the bed in the morning. If it feels cold enough to ache, hold off on tender roots and slow seed. If it feels merely cool and the soil crumbles instead of smearing, you are ready for the hardy ones. A bed that warms days before the one across the yard has a better microclimate: sun, drainage, or shelter from wind.
- The first true leaves. Sow a quick test row of something fast, such as radishes, and watch for the first true leaves, the ones after the rounded seed leaves. If they appear on schedule and stay healthy, your soil and air are cooperating. If they stall or yellow, wait.
- The first flush of male flowers. If you grow squash, the male blooms come before the female ones, and they only open in real warmth. Their arrival tells you the warm-season window has truly begun in your yard, so tender roots set in beside them are safe from late chills.
Write the dates down for a season or two. A garden notebook of last frost, first workable soil and first true leaves will beat any published band within a few years, because it describes your hill, your wall, your low spot.
Common questions
Can I sow parsnips in April in Maine?
At the coast, yes, as soon as the soil can be worked, which is around the April 22 to May 1 last-frost window at Portland. Parsnips tolerate light frost, but cold, waterlogged soil can rot the seed, so wait for the bed to drain. Inland and northern gardens should expect to start later.
When is it safe to plant oca or yacon outside?
After the Portland last-frost window ends on May 1, with a week or two of margin, since the dates are probabilities and a late freeze remains possible. Further inland, wait longer. Starting them indoors and transplanting gives them the best use of a short season.
When does fall frost end the season?
At Portland, the first fall frost falls between October 9 and October 21, according to NOAA’s 1991-2020 normals, with the same 30 to 70 percent freeze-probability band. Inland and northern Maine generally see frost earlier.
Which hardiness zone is Maine?
The state spans zones 3b to 6a on the USDA Plant Hardiness Zone Map (2023), so there is no single answer. Check the USDA map for your location instead of assuming, since the zone is based on average annual extreme minimum temperature only.