In coastal Maine, sow turnips for a spring crop from mid-April through early May, and for a fall crop from mid-July to mid-August, timed against the region’s short, cool growing season. The exact window shifts by weeks depending on how far you are from the coast, which is where the rest of this page earns its keep.
The short answer for Maine
Turnip seed germinates in soil as cool as 40°F, which means Maine gardeners don’t have to wait for settled summer weather the way tomato growers do. The number that actually matters is the state’s last spring frost window, and at the Portland reference station, that runs from April 22 to May 1. That date marks roughly a 30 to 70 percent chance of a 32°F freeze, not a guarantee, so it describes probability, not a hard line you can circle on the calendar and trust completely.
For a spring turnip crop, sow two to three weeks before that window closes, since turnip seedlings shrug off a light frost even if the adult plant would not. Rushed too early into cold, wet ground, seed rots before it sprouts, so waiting until the soil crumbles rather than clumps is the better trigger than a fixed date.
For a fall crop, work backward from the first fall frost, which arrives at Portland between October 9 and October 21. Turnip roots need roughly 40 to 60 days to bulk up depending on variety, so counting back from the early edge of that window puts the last reasonable sowing date in mid-August at the coast. Inland and northern growers need to move that earlier, sometimes by two to three weeks, because their own frost dates run later in spring and earlier in fall than the reference station this page relies on.
Turnips also tolerate, even benefit from, a touch of frost once mature. Cold snaps convert starches to sugars in the root, so a fall crop that catches the leading edge of frost often tastes better than one pulled in warm weather. That’s a reason to lean toward the later end of the sowing window rather than rushing the fall planting out of caution.
What will not work here
Rutabaga is the crop that fails first in Maine’s timeline, and it fails for a simple reason: it needs closer to 90 days to bulk properly, nearly double what a fast turnip variety asks for. Sown at the same mid-August date that works for turnips, rutabaga runs straight into the first fall frost window before it has finished sizing up, especially away from the coast where that frost arrives sooner. A gardener who wants both crops needs to sow rutabaga a full month earlier than turnips, not on the same schedule.
Slow-maturing turnip varieties run into the same wall from the other direction. A crop that ordinarily takes 55 to 60 days can stretch longer once day length shortens in September, since cooler light-starved days slow leaf growth even before frost touches the plant. Anyone counting on the label’s “days to maturity” number as a fixed countdown, without building in a buffer for shortening days, ends up with immature roots when the frost finally lands.
Close spacing is the other quiet failure. Maine summers run humid enough that crowded turnip beds hold moisture against the leaves overnight, and that dampness is exactly what lets clubroot and other brassica diseases take hold in garden soil. Spacing rows for airflow rather than squeezing in a few extra plants matters more here than in a drier climate, where crowding is mostly a nutrient competition problem rather than a disease one.
Winter storage in the ground is the last trap. With the coldest month’s average daily minimum sitting at 15.6°F at the Portland reference point, any turnip left unmulched in open soil past the first hard freezes is exposed to conditions well below what an unprotected root survives. Leaving a fall crop in the ground “for later” without heavy mulch is a bet against a very cold month, not a storage strategy.
What changes across Maine
Maine spans USDA hardiness zones 3b to 6a, and that is not a small spread. Zone 3b winters bottom out dozens of degrees colder than zone 6a winters, which tells you the frost dates used above, drawn from the Portland reference station, describe the mild end of the state, not the whole of it.
A gardener working land in the state’s northern reaches or at higher inland elevation should expect the last spring frost to land noticeably later than the April 22 to May 1 window, and the first fall frost to arrive noticeably earlier than October 9 to October 21. In practice, that squeezes the frost-free season from both ends, which is exactly the situation that punishes rutabaga and slow turnip varieties hardest, since they have the least room for error to begin with.
The practical shift is straightforward even without knowing an exact zone: move spring sowing later, move fall sowing earlier, and lean toward the faster end of any variety’s maturity range if you’re inland or at altitude rather than on the coast. Someone gardening near the water can afford to gamble on a 60-day variety for a fall crop; someone gardening in the state’s colder interior is better served treating 45 days as the safer ceiling.
This page can’t tell an individual reader which zone their own yard falls into, since microclimates shift block to block, let alone county to county. The USDA Plant Hardiness Zone Map is the tool built for that exact question, and checking it against your own address is worth five minutes before committing seed to ground.
Where to check this for your own county
The University of Maine Cooperative Extension runs the state’s official county-level guidance, available at extension.umaine.edu. That’s the place to look next, because a county recommendation beats a state-wide one, and everything above this section is, by design, a state-wide answer built from a single reference station.
Two things are worth hunting for once you’re there. First, a county or regional planting calendar, since Maine extension offices frequently publish sowing windows tuned to local frost records rather than the Portland-based dates used on this page. Second, variety trial results specific to Maine growing conditions, since not every turnip or rutabaga cultivar sold nationally has been tested against the state’s shorter season and cooler summer nights.
Extension offices also field direct questions from local gardeners, which is useful for anything this page can’t answer, like a specific soil type’s drainage behavior or a particular valley’s tendency to hold frost longer than the surrounding hills. That kind of hyper-local knowledge rarely makes it into a statewide climate normal, but it often makes it into an extension agent’s working notes.
Treat this article as the starting estimate, not the final word. It’s built from thirty years of climate averages at one station, which is a solid foundation for planning but not a substitute for what your own county extension office has recorded about your own soil and your own microclimate over time.
The Moon calendar is a separate, older custom: see when to plant turnips by the moon, written as tradition rather than fact.
Checking this against your own garden
Every date on this page is an average pulled from three decades of weather records, and your own garden is free to disagree with it in either direction. Three things you can check by hand outrank any calendar entry.
Soil feel comes first. Turnip seed wants ground that crumbles easily and feels cool rather than cold, not the sticky, clay-wet soil of an early spring thaw. If your soil still clumps in your hand in late April, it’s telling you your particular plot runs behind the regional average, whatever the calendar says.
Germination speed comes second. In properly warmed soil, turnip seedlings typically break the surface within a week. A much slower emergence is a direct signal that your soil is colder than the season’s calendar date suggests, and a reason to hold off on a second sowing rather than assuming the first one simply had bad seed.
Early bolting is the third signal, and it points the opposite direction. A turnip that throws up a flowering stalk unusually fast, before the root has bulked at all, is often responding to a cold snap early in its life or an unusual heat spike later on. Either way, it’s telling you that your particular spot runs less predictably than the averages on this page account for, which is exactly the kind of detail worth writing down for next year’s sowing.