In New York City, start onions and leeks first, between December 8 and December 26. Broccoli, cabbage and head lettuce follow from January 5 to January 23. Peppers and eggplant go in from January 19 to January 30, and tomatoes from February 9 to February 27. These dates count back from a last spring frost of March 23 to April 3. Upstate and in the Adirondacks, where spring frost comes later, every one of them moves later too.
The date everything counts back from
Every indoor sowing date starts from one number: the last spring frost. For New York City, NOAA’s U.S. Climate Normals for 1991-2020 put that date in a band of March 23 to April 3, the stretch when a 32°F freeze falls somewhere between 30 and 70 percent likely. Write that range down and count backward from it. It is the New York City figure, taken at Central Park. Pavement and buildings hold heat there, so the park frosts less than the countryside around it.
A frost date is an average over thirty years of weather. In some springs the last freeze comes before the band opens, and in others it lingers past the end. That is why the band gets stated as a range rather than a single day, and why it works better as a planning tool than as a promise.
Hardiness zones answer a different question. The USDA Plant Hardiness Zone Map (2023 edition, built on 1991-2020 data) runs from 4a to 7b across New York State. It is built on one variable, the average annual extreme minimum temperature, so it tells you which perennials survive a winter. It says nothing about when to sow a tomato. For seed starting, frost dates do the work.
Why does the frost range matter for sowing dates?
Because the two ends of the range pull in opposite directions. Count back from the early end, March 23, and you finish with seedlings ready to go out while the garden is still too cold. They wait in their pots, outgrow their containers, and turn tall and thin. Peppers are especially prone to this, sulking in cramped cells and sometimes setting a flower before they ever see a garden bed.
Count back from the late end, April 3, and the opposite problem shows up. You lose weeks of growth, and a tomato that could have been sturdy and well rooted at planting time ends up small. The practical middle is to sow inside the window for each crop. Lean toward the early edge if you have strong light and room, and toward the late edge if you have a cool windowsill and a small space.
Gardeners in the colder parts of the state add a second adjustment. Farther north and at higher elevation, the last spring frost arrives later, so the whole calendar slides later with it. In the milder parts of the metro area it can slide a little earlier. The First and Last Frost Dates in New York page lays out how the dates shift between places, and the New York guides collect the rest of the regional timing.
The calendar, crop by crop
The weeks below come from the University of Minnesota Extension’s guidance on starting seeds indoors. They give the usual number of weeks a seedling spends indoors before planting out. They are general horticultural practice, not a law, and good gardeners shade them a week either way. Minnesota’s own calendar months do not carry over to New York. The sowing windows here come from counting those weeks back from New York City’s March 23 to April 3 last frost.
| Crop | Weeks indoors before planting out (usual guidance) | Sowing window, New York City |
|---|---|---|
| Leeks, onions | 14-15 weeks | December 8 to December 26 |
| Celery | 12-13 weeks | December 22 to January 9 |
| Broccoli, Brussels sprouts, cabbage, cauliflower, head lettuce | 10-11 weeks | January 5 to January 23 |
| Eggplant, okra, peppers | 9 weeks | January 19 to January 30 |
| Tomatoes | 5-6 weeks | February 9 to February 27 |
| Sweet potatoes | 3-4 weeks | February 23 to March 13 |
The pattern is easy to see once you look at it. The slowest crops go first, and the onion and leek window of December 8 to December 26 is easy to overlook, because it falls in the holiday rush. Onions and leeks grow slowly from fine seed, and the long head start is what lets them size up before summer heat. Celery follows close behind.
Broccoli, cabbage, cauliflower and Brussels sprouts share one window, from January 5 to January 23. Head lettuce sits in that same band in the extension table. For loose-leaf types and the practical details of germinating them, see Starting Lettuce.
Peppers and eggplant come next, from January 19 to January 30. They are slow to sprout and demand warmth, which is why they start well before tomatoes. A common calendar mistake is sowing tomatoes alongside the peppers because the packets sit side by side. Tomatoes grow fast, and with a window of February 9 to February 27, they are ready at about the time the garden is. The Growing Tomato Seedlings Indoors guide covers what those weeks look like in practice.
Sweet potatoes bring up the rear, from February 23 to March 13, since they grow quickly and need only three to four weeks indoors. Gardeners starting from slips rather than seed should follow the same logic and count from planting out.
If you plan to set hardy crops like onions and brassicas outside ahead of the frost range, move their sowing dates earlier by the same amount. Hardy crops tolerate a cold snap that would flatten a tomato.
Which crops should you not start indoors?
Some crops resent being moved, and a head start indoors costs more than it gains:
- Root crops such as carrots, beets, radishes, turnips and parsnips grow a taproot that forks or stunts when disturbed.
- Beans and peas sprout fast in warm soil and rarely gain anything from a pot.
- Squash, cucumbers and melons grow so quickly that a seedling can outgrow its container within weeks, and their roots dislike transplanting. Direct sowing is the better choice in most cases.
Those crops belong in the ground once soil has warmed, and the New York Planting Calendar sets out when that happens for direct-sown vegetables.
Light, warmth and hardening off
Winter sun is weak and the days are short, and the seeds you sow in December and January sprout into exactly that. A windowsill gives light from one side only, so seedlings stretch toward the glass, with long pale stems that flop over. Rotating the pots helps a little, but it does not fix the shortage. Most gardeners who start onions or peppers in midwinter end up using a simple fluorescent or LED shop light hung close above the seedlings and raised as they grow. A cheap outlet timer keeps it on a steady daily schedule. Sturdy stems and dark green leaves are the sign you got it right.
Warmth matters just as much for germination. Seeds sprout best when the mix stays evenly warm day and night, and a cold sill, a drafty basement or a spare room with the heat turned down all slow things. Peppers and eggplant are among the most sensitive to this. Many growers put the tray on a seedling heat mat until the sprouts show. Once the green is up, take the mat away and let light do the work, because most seedlings no longer need bottom heat once they are up, and extra warmth can encourage stretching.
How do you harden off seedlings?
Extension guidance says to harden off all plants over the last 1-2 weeks before planting out. Indoor seedlings have never felt wind, direct sun or a cold night, and moving them straight to the garden scorches leaves and can set growth back for weeks.
Start with a sheltered, shady spot for a short stretch on a mild day. Each day, lengthen the time outside and shift the tray into more sun and breeze. In the last days, leave the plants out through the day and, if the nights are mild, overnight as well. Bring everything inside if a frost or a cold rain threatens, and keep watering, since small pots dry out fast in wind. The process is tedious, but it takes the shock out of the move, and it is cheaper than replacing a flat of cooked seedlings.
Tomatoes, peppers and eggplant are the crops to be most careful with. Broccoli and cabbage can handle a chillier step outdoors, which is why they can go out earlier in spring.
What goes wrong
Starting too early is a common problem, and the signs are plain. Seedlings grow tall and floppy, their lower leaves yellow, roots circle the bottom of the cell, and peppers may flower while still in a pot. Plants like that take a long time to recover after transplanting. The fix is to sow inside the window, not before it. If your light is limited, favor the later end of each range. A smaller, stocky seedling usually beats a large leggy one.
What does damping off look like, and how do you prevent it?
Damping off is a fungal disease. Seedlings come up healthy, then pinch at the soil line, thin to a thread and fall over, often in a patch that spreads across the tray. Once you see it, there is no cure for those plants. Prevention is simple: use clean containers, fresh seed-starting mix rather than garden soil, and water from the bottom so the surface stays drier. Keep air moving over the tray with a small fan, and thin crowded seedlings early. Overwatering and stagnant, cool air make the problem worse.
The third mistake happens in April, when the garden looks ready. The average last frost is a middle point, and plenty of springs still bring a freeze after it. Even without frost, soil warms more slowly than air, so tomatoes, peppers and eggplant set into cold ground stall and turn purple-tinged. Hold tender transplants until the last-frost range of March 23 to April 3 has passed in the city, and later upstate. Then watch the soil and the ten-day forecast. A row cover or a few plastic jugs covers a surprise cold night, and the New York Planting Calendar gives a fuller picture of when each crop goes out.
Common questions
Can I start everything at the same time in January?
You can, but the results suffer. Onions and leeks need 14 to 15 weeks indoors, while tomatoes need only five to six weeks. Sowing them together means tomato seedlings that outgrow their pots before the garden is ready, or onions that never size up. Following the staggered windows in the table keeps each crop at the right size on planting day.
What if I garden upstate or in the Adirondacks?
Sow later. The dates here are New York City’s, and the colder parts of the state have a later last spring frost, so every window moves later with it. Find your local last frost date, then count back the same number of weeks from it. A county Cooperative Extension office can give you the local average. Hardiness zones in the state run from 4a to 7b on the 2023 USDA map, which shows how much the winters differ, though zones guide perennials rather than sowing dates.
What if I miss a sowing window?
A few days late costs little. A few weeks late means smaller plants at transplant time, which matters most for onions, leeks and peppers, the slowest crops. Tomatoes recover best from a late start because they grow so fast. If you are badly behind on a slow crop, buying transplants is a sensible shortcut.
Do I need grow lights?
For the December and January starts, yes in practice. Daylight is short and weak then, and a windowsill seedling will stretch. For tomatoes sown in February, a bright south-facing window can work, though lights still produce stockier plants. Brightness at the leaf matters more than the fixture: set a light close above the seedlings and raise it as they grow.