Two frost dates, thirty years of NOAA averages behind each one, decide every tomato date you’ll set in Maine. Nothing about seed-starting, transplanting, or cultivar choice matters until you know when that spring window closes and when the fall one opens again in October. Zones 3b to 6a stretch across this state, so Portland’s milder edge and a colder inland garden are never running the same calendar, even when the month says otherwise.
What this comes down to in Maine
For a Portland-area garden, the frost window from NOAA’s 1991-2020 normals is the anchor for every tomato date you calculate. Nothing goes into open ground with confidence until that window has closed, and nothing stays productive past the point where the first fall frost window opens in October. Everything else, seed-starting dates, transplant timing, which cultivar you even bother with, gets measured against those two ranges.
The USDA Plant Hardiness Zone Map places the state across zones 3b to 6a, and that spread is the reason a single planting date can’t cover Maine. Portland sits toward the milder end of that range. A garden further north or at higher elevation is working with a shorter frost-free season even though the calendar looks identical.
The practical takeaway is that transplant date and harvest date are two separate calculations, both built off the same frost data. You need transplants to go in after the spring frost risk has genuinely passed, and you need a cultivar whose days to maturity leave enough runway before the fall frost risk begins. Skip either check and the plant either freezes as a seedling or freezes with a vine full of green fruit. If you haven’t settled on which tomatoes make sense for this length of season, the variety-by-variety breakdown is worth reading before you buy seed, since the arithmetic below depends on which cultivar you’re growing. This page assumes you’ve already covered the basics of growing tomatoes in Maine and just need the dates themselves.
Working it out for your own garden
The method is the same regardless of which cultivar you’re growing or how the frost dates shift from year to year. Start from NOAA’s fall frost window for Portland, October 9 to October 21. Count backward by the cultivar’s days to maturity, converted to weeks, to find the latest point a transplant could reasonably go in and still ripen fruit before frost risk climbs. Then check that this latest date is no earlier than the spring frost window has already closed. A range minus a number of weeks gives you another range, not a single day, so resist the urge to pick one date out of it.
Running the numbers on two real cultivars
Take San Marzano II, a paste tomato at 78 days from transplant. Catalog guidance for this region puts transplant in the early May to mid-May window, which sits just after Portland’s spring frost risk has cleared. Add 78 days and first ripe fruit lands in early August. Measured against Portland’s fall frost window, that leaves roughly 69 days of margin between first ripe fruit and frost risk, which is enough room for a full paste tomato harvest before the season closes.
Now take a cultivar with even more days stacked on top, something in the 85-day range like Amish Paste. Run the same arithmetic from the same early-May transplant window and first ripe fruit still lands in early August, with about 62 days of margin before frost. That margin holds for Portland. It thins fast the further you move from the coast: in a shorter, colder inland or northern season, an 85-day cultivar started at the same calendar point may not reliably clear frost with a full harvest, which is exactly why margin, not just days to maturity, is the number worth watching.
| Variety | Days to maturity | First ripe (early-mid May transplant) | Margin before Portland’s fall frost window |
|---|---|---|---|
| Sakura | 55 | Early July | 92 days |
| Sun Gold | 57 | Mid July | 90 days |
| Jasper | 60 | Mid July | 87 days |
| Cherokee Purple | 72 | Late July | 75 days |
| Amish Paste | 85 | Early August | 62 days |
Notice the pattern: every extra week of days-to-maturity eats directly into your margin, and margin is what buffers you against a cold snap, a slow August, or a fall frost that arrives at the early end of NOAA’s window instead of the late end.
What changes across the state
Portland’s frost figures come from a coastal weather station, and the ocean is doing real work there. Large bodies of water release heat slowly, which delays the first fall frost and can nudge the last spring frost earlier than an inland location at the same latitude would see. Move even a short distance from the coast and that moderating effect drops off.
Elevation matters just as much. Cold air sinks, so low-lying valleys and hollows collect it on clear, calm nights and frost there before the surrounding higher ground does, regardless of what the regional average says. A garden in a frost pocket can run noticeably behind a garden a few hundred feet up the same hillside.
Then there’s the north-south split. Maine’s USDA hardiness zones span 3b to 6a, and that gap between the mildest and coldest zones is wide enough that a planting calendar built for Portland is only a starting point for Aroostook County or the western mountains. Both the spring frost window and the fall frost window run substantially later in spring and earlier in fall as you move north and inland, which shortens the whole season on both ends at once, not just one side of it. That’s the reason a 78-day tomato with 69 days of margin near the coast can be a genuinely risky choice a couple of hardiness zones colder.
The mistakes this state punishes
Maine’s short, variable season doesn’t forgive a few specific errors, and they show up every year in the same form.
- Planting into cold soil after a warm April stretch, which stalls root growth even when the air feels fine.
- Choosing a long-season cultivar, something past 78 or 80 days, without checking its margin against the fall frost window for your part of the state.
- Skipping the second frost check entirely and assuming that once transplants are in, the season is settled.
- Missing the point in the season when a known local pest arrives, since a late response costs a lot more fruit than an early one.
The fix for the first two is the arithmetic in the section above: work from the frost window, not the weather forecast, and know your cultivar’s margin before you buy the seed packet. The fix for the fourth is timing, not chemistry. If you don’t already know which insects and diseases actually turn up on Maine tomatoes and when, the pest-by-pest rundown for this state lays out the sequence so you’re watching for the right thing at the right point in the season, rather than reacting after the damage is done.
The most expensive version of all these mistakes is compounding two of them at once, planting a long-season variety early into cold soil on the assumption that an early start buys back the days a slow-maturing cultivar needs. It doesn’t. Cold soil delays the plant regardless of the calendar date, so the extra days you thought you gained never materialize, and you’re left with the same tight margin you started with.
Checking this against your county
The Portland figures in this article are a state-wide starting point, not a substitute for a local check. Confirm your own frost risk with your county’s extension office before committing to a planting date, since inland counties and anything north of the midcoast run on a meaningfully different schedule than the coastal station this page is built from. UMaine Cooperative Extension maintains county-level offices and tomato-specific guidance that accounts for local elevation and microclimate in a way a single state figure can’t.
Common questions
Can I go by the average last frost date instead of the range?
Not reliably. NOAA’s figure for Portland is a range, April 22 to May 1, built from 30 years of freeze probability data, and picking a single day out of it invents precision that doesn’t exist. Treat the whole range as the window your soil and weather need to clear before transplants go in.
What if I want to plant earlier than the frost window using row cover?
Row cover and other protection can buy you some cushion against a light frost, but it doesn’t change the underlying soil temperature that tomato roots need to grow. A covered transplant set out well ahead of the window can still sit stalled in cold soil even if it survives the air temperature.
Is one planting date right for the whole state?
No. The frost figures on this page come from a Portland-area weather station, and Maine’s USDA hardiness zones run from 3b to 6a, a wide enough spread that inland and northern gardens need their own, later window. Use this page’s method with your own county’s frost data rather than the Portland numbers directly.
Can I still get tomatoes if I miss the early-May transplant window?
Often yes, depending on the cultivar. A short-season tomato like Sakura or Sun Gold, at 55 to 57 days to maturity, carries enough margin against Portland’s fall frost window to absorb a late start. A long-season paste tomato has much less room to give.