Count back from the last spring frost. Near Portland, NOAA’s 1991-2020 climate normals put that frost between March 6 and March 21. That sets onion and leek sowing from November 21 to December 13, peppers and eggplant from January 2 to January 17, and tomatoes from January 23 to February 14. Colder parts of Oregon sow later and milder ones earlier. The calendar below shows the weeks behind every date.
The date everything counts back from
The anchor is Portland-area data in the NOAA NCEI U.S. Climate Normals for 1991-2020. The linked file is the Portland airport station, which runs somewhat later. The March 6 and March 21 dates are the 30 percent and 10 percent chances of a later 32°F freeze at the Portland KGW-TV station. Across that stretch, the chance of a later freeze falls from about 30 percent to about 10 percent. Inside that band, some years still freeze and others have already warmed up. The last frost is therefore a range of dates, and the indoor calendar built on it is a range too.
The width of that range changes how you sow. Start at the early end and the seedlings finish growing before the garden is ready for them. They sit in their pots, stretch toward the light, circle their roots and turn pale as the potting mix runs out of nutrients. Start at the late end and you are betting on a cold spring. If it turns mild, you have lost weeks of growth, and your transplants go out small next to anything a neighbor started earlier. Write March 6 to March 21 at the top of your calendar and count back from both ends.
Each crop’s window comes from the weeks indoors that University of Minnesota Extension lists in its guidance on starting seeds indoors. The longest stretch counted back from the earliest frost date gives the first day of a window. The shortest stretch counted back from the latest date gives the last day. The weeks are the usual extension guidance for how long a seedling needs before it is ready to move outside. A seedling’s pace depends on its conditions, so check your own plants and your local extension office for timing.
Does one last frost date cover all of Oregon?
No. Portland sits west of the Cascades, where winters are mild and wet. Central and eastern Oregon is high desert with a far shorter growing season, and higher ground in the mountains is colder still. The colder the spot, the later the last spring frost, so every sowing date shifts later there. Low, mild ground closer to the coast shifts the other way, earlier.
The USDA Plant Hardiness Zone Map (2023 edition, built on 1991-2020 normals) runs from roughly 5a to 10a across the state. That system rests on one variable, the average annual extreme minimum temperature. It tells you which shrubs and perennials will survive winter, but it says little about when to sow tomatoes. Portland is the reference for the dates here. For your own town, check First and Last Frost Dates in Oregon, or call your county extension office, then slide each window later or earlier by the same amount your frost dates differ.
The calendar, crop by crop
The table pairs each crop’s usual weeks indoors with the resulting sowing window for the Portland area. Treat the weeks as standard extension guidance and a good starting point, not a fixed rule. A seedling that grows fast on a warm shelf may be ready a few days sooner.
| Crop | Weeks indoors before planting out | Indoor sowing window (Portland area) |
|---|---|---|
| Leeks, onions | 14-15 weeks | November 21 to December 13 |
| Celery | 12-13 weeks | December 5 to December 27 |
| Broccoli, Brussels sprouts, cabbage, cauliflower, head lettuce | 10-11 weeks | December 19 to January 10 |
| Eggplant, okra, peppers | 9 weeks | January 2 to January 17 |
| Tomatoes | 5-6 weeks | January 23 to February 14 |
| Sweet potatoes (slips sprouted from tubers) | 3-4 weeks | February 6 to February 28 |
Onions and leeks head the list because they grow slowly and need the longest head start. Brassicas and head lettuce come next. All of these tolerate cool weather, so they can go out near the last frost once they are hardened off. Lettuce has its own sowing details in Starting Lettuce.
The warm-season crops need a different reading. The weeks count back to the last-frost range, but tomatoes, peppers, eggplant, okra and sweet potatoes want warm soil and mild nights, and in western Oregon those usually arrive well after the last freeze. For these crops, sow toward the late end of the window, or a little after it, and keep them in a bigger pot if the weather stalls. Peppers and eggplant at January 2 to January 17 will be sturdy plants by mid-March, but they will still be waiting on warmth. Tomatoes, a crop many people start in a hurry, get more detail in Growing Tomato Seedlings Indoors.
Colder gardens shift the whole table later. A high-desert garden with a later last frost can push every window back by the same gap, and that is easier than starting early and fighting leggy plants. Once seedlings are ready, the Oregon Planting Calendar covers the outdoor side of the schedule.
Which crops should not be started indoors?
Some crops do better sown straight into the ground. The usual exceptions are these:
- Root crops such as carrots, beets, radishes and parsnips, which grow a single taproot that dislikes being moved and often forks or stunts after transplanting.
- Beans, which sprout quickly in warm soil and gain little from an indoor head start.
- Squash and pumpkins, which grow fast and resent root disturbance. A few gardeners in very short-season spots start squash in pots just ahead of planting out, but in most gardens direct sowing wins.
Skipping these frees up shelf space and lamp time for the crops that really need it.
Light, warmth and hardening off
A sunny windowsill looks like enough, but a winter window delivers weak, one-sided light, and western Oregon winters add long stretches of cloud. Seedlings respond by stretching toward the glass. The stems grow thin, pale and floppy, and the plant spends its energy on height instead of roots. The usual fix is a fluorescent or LED grow light hung just above the seedling tops and raised as they grow, running on a timer for much of the day. Light from above keeps stems short and thick, and a timer saves you from guessing.
Warmth matters just as much at germination. Seeds sprout best in steady warmth, and the soil temperature counts more than the air in the room. A cold garage or a drafty sill slows sprouting and leaves flats wet and slow to dry, which invites mold. A waterproof seedling heat mat under the tray gives even bottom heat. Tomatoes, peppers and eggplant want the most warmth and are the slowest to sprout in a cool house. Once the seedlings are up, you can take the mat away or move them somewhere a little cooler, which keeps growth compact.
How do you harden off seedlings before planting out?
Indoor plants have never felt wind, direct sun or a chilly night. University of Minnesota Extension advises hardening off every plant over the last 1 to 2 weeks before planting out. The process is gradual:
- Start with a few hours in a sheltered, shady spot, such as the lee of a wall or fence.
- Add a little more time and more sun each day.
- Leave the plants out overnight only near the end, and only when no frost is forecast.
- Pull the trays back inside during hard rain, strong wind or a cold snap.
Water the flats a little less as the week goes on, but never let the mix dry out completely. Plants that skip this step often show bleached or scorched leaves from the sun and wind, and the damage sets them back for weeks.
What goes wrong
Most indoor-start failures fall into three patterns, and each has a clear cause and a simple fix.
What happens when you start seeds too early?
Early seedlings stretch, crowd and run out of nutrients. They have long, weak stems, yellowing lower leaves and roots wound tightly around the pot. Plants in that shape often stall after transplanting. Peppers and tomatoes started too soon may even set a few flowers while still in the pot, which drains energy from the plant. Prevention is mostly arithmetic: use the table, stay inside your window, and if the weather holds you back, move plants into larger pots instead of leaving them in cramped cells.
What is damping off, and how do you stop it?
Damping off is a fungal disease that strikes young seedlings. The stem pinches at the soil line, turns thin and brown, and the seedling falls over, often within a day. Cool, soggy mix and stagnant air cause it. Clean containers, a fresh sterile seed-starting mix, water from below, a small fan for air movement and a mix that dries slightly between waterings all reduce the risk. If one tray gets it, remove the affected seedlings and let the rest dry down.
Can you plant out the week after the last frost dates?
You can, but tender crops often struggle. The March 6 to March 21 range marks when a freeze is still possible in some years, so it does not mean the garden has warmed up. Soil that is still cold and wet stalls tomatoes, peppers and eggplant, and one cold night can set them back for weeks. The sturdier fix is to wait for warm nights and soil, well beyond March 21, and keep warm-season plants in bigger pots until then. Cool-season crops like broccoli, cabbage and lettuce handle the cold much better, so they can go out on the earlier schedule once hardened off.
Common questions
Do these dates work for Bend, Ontario or other places in central and eastern Oregon?
No. The dates are built on the Portland station’s frost range, and central and eastern Oregon has a later last frost in the high desert. Find your own last frost range, then shift each window later by the same gap. Your county extension office can confirm local dates, and more local timing is collected in the Oregon guides.
Is January too early to start tomatoes?
The tomato window runs from January 23 to February 14, which reflects 5 to 6 weeks of indoor growth before the last frost. Tomatoes go out later than the frost date because they need warm soil and nights. Starting at the late end of the window, or a little after it, keeps seedlings from outgrowing their pots while they wait.
Why do the weeks come from a Minnesota table?
Seedlings need time indoors that varies with the crop and the conditions, so check your local extension office. The University of Minnesota Extension gives the weeks as usual guidance: 14 to 15 for onions and leeks, 10 to 11 for brassicas and head lettuce, 9 for peppers and eggplant, and 5 to 6 for tomatoes. Its calendar months belong to Minnesota and do not apply here.
How long should hardening off take?
Plan on the last 1 to 2 weeks before planting out, giving seedlings a little more outdoor time and sun each day. Cool-season crops like broccoli and cabbage tolerate the process well. Tomatoes and peppers are touchier, so give them the full two weeks and skip any day with cold wind or a freeze in the forecast.