California has no single indoor sowing date. Count backward from your last spring frost: tomatoes go in 5 to 6 weeks before planting out, peppers and eggplant 9 weeks, broccoli and cabbage 10 to 11 weeks, onions 14 to 15 weeks. Coastal and Southern California gardeners can skip the frost math, because no freeze occurs there in an average year. Northern California, the Central Valley and the Sierra do freeze, and their calendars run on the same weeks.
The date everything counts back from
Every indoor sowing date is a subtraction problem. Start with the day the seedling goes into the ground, subtract the weeks it needs indoors, and you have your seed-starting day. For tender crops, that planting-out day comes after the last spring frost. That is why the frost date anchors the whole calendar. Pick the wrong anchor and every date downstream is off. The California guides cover how much the state’s climates differ, and this is where the difference matters most.
Coastal and Southern California: no frost date to count from
At the Los Angeles reference station, NOAA’s 1991-2020 Climate Normals (from NCEI) could not compute a freeze normal at all (the data carries code -666). NOAA’s Regional Climate Centers ACIS daily records show zero days at or below 32°F in 30 years, and the frost-free season is listed as 365 days. So there is no last spring frost date to give you. Indoor sowing here is not timed by frost. What sets the schedule is the season: heat, drought and local water restrictions. Cool-season crops need to be established before summer heat, and warm-season crops go out when nights and soil have warmed. I won’t invent dates for that, because the data doesn’t support any.
Northern California, the Central Valley and the Sierra
Never generalize Los Angeles to the whole state. The USDA Plant Hardiness Zone Map (2023 edition, built on 1991-2020 normals) places California anywhere from 5a to 11a. The system rests on one variable, the average annual extreme minimum temperature, so it says how cold the worst night gets but not when the last frost lands. Colder parts of the state, meaning farther north or higher in elevation, have a later last spring frost, and every indoor sowing there moves later too. Look up your own range on the page for first and last frost dates in California.
The word “range” matters. Frost dates are averages with spread on both sides. Sow tomatoes at the early end of your range and you may end up with tall, pale seedlings waiting weeks in pots for the ground to be safe. Sow at the late end and you may lose weeks of growing season you can’t get back. Write down both ends of your last-frost range, then count back from the late end for tender crops and from an earlier planting-out date for cool-season ones.
The calendar, crop by crop
The weeks below come from the University of Minnesota Extension’s table “When to plant seeds and how long to keep them indoors,” which lists weeks indoors before planting out. Treat them as the usual extension guidance, not a law. The source also names calendar months, but those are Minnesota’s and don’t apply in California. The window for your garden comes from counting these weeks back from your own planting-out date.
| Crop | Weeks indoors before planting out | Sowing window where frost occurs | Frost-free coast and Southern California |
|---|---|---|---|
| Onions (and leeks) | 14-15 weeks | Earliest start of all. Count back from a planting-out date ahead of the last frost. | No frost-based window. Time so transplants are established in the cool season. |
| Broccoli | 10-11 weeks | Count back from planting out, which comes before or around the last frost. | No frost-based window. Time to finish growing before summer heat. |
| Cabbage | 10-11 weeks | Same logic as broccoli, with transplants going out in the cool end of spring. | No frost-based window. Time to the cool season. |
| Lettuce (head) | 10-11 weeks | Count back from planting out in cool weather, ahead of warm spells. | No frost-based window. Time to the cool season. |
| Eggplant | 9 weeks | Count back from the late end of your last-frost range, since eggplant dislikes cold. | No frost-based window. Time to warm nights and warm soil. |
| Peppers | 9 weeks | Count back from the late end of your last-frost range. | No frost-based window. Time to warm nights and warm soil. |
| Tomatoes | 5-6 weeks | Count back from the late end of your last-frost range. | No frost-based window. Time to warm nights and warm soil. |
Read the table from the bottom up and it makes intuitive sense. Tomatoes are quick, so they start last. Peppers and eggplant crawl through their first weeks, which is why they need 9 weeks. The cool-season crops at the top need the longest head start. Other entries in the same extension table follow the pattern: celery 12-13 weeks, Brussels sprouts and cauliflower 10-11 weeks, okra 9 weeks, sweet potato slips 3-4 weeks.
Head lettuce at 10 to 11 weeks surprises people, since leaf lettuce seems so quick. For the practical side of that, see starting lettuce. For the warm-season side, the walkthrough on growing tomato seedlings indoors covers pots, soil and spacing.
Not everything belongs on the indoor list. Root crops such as carrots, beets, radishes and parsnips resent having their roots disturbed, so they do better sown where they will grow. Beans and squash grow quickly enough that an indoor head start rarely pays, and in most cases they sulk after transplanting. Direct-sow them outdoors instead, using the California planting calendar for outdoor timing.
Pick your planting-out date first, then subtract the weeks in the table. Doing it in that order keeps seedlings from sitting indoors for weeks while the weather stays too cold.
Light, warmth and hardening off
A sunny windowsill looks like the obvious spot. It usually isn’t enough. Light through glass comes from one side and is weak compared with the outdoors, so seedlings stretch toward it. The result is a long, thin, floppy stem, a seedling spending its energy on height instead of roots. A grow light hung close above the seedlings and left on for long stretches each day fixes that, and a plain shop light does the job too. Raise the light as the plants grow. Window light can still carry seedlings in the sunniest rooms, but expect to rotate trays and watch for leaning.
Warmth is the other half of the equation, and it works differently. Seeds germinate best at steady warmth, not at whatever the room does between a sunny afternoon and a cold night. A windowsill can cool off sharply after sundown, and an unheated garage can stay cold for days. A heat mat under the tray, a warm spot on top of a refrigerator, or a closed room that stays at an even temperature all beat a drafty sill. Tomatoes, peppers and eggplant care most. Once seeds sprout, they want light right away and can come off the extra heat.
Hardening off is the step people skip, and it matters even where frost never visits. Seedlings raised indoors have never met direct sun, wind or a real temperature swing. Moved straight to the garden, they scorch and stall. The University of Minnesota Extension guidance is to harden off all plants over the last 1 to 2 weeks before planting out. Start with a couple of hours in shade, then lengthen the time, add morning sun, and finally leave the trays out overnight if the weather allows. In coastal and Southern California, the shock comes from strong sun and wind instead of cold, and the same gradual exposure protects against it. Block out the final 1 to 2 weeks before planting out for hardening off, and count those weeks inside the indoor time, not on top of it.
What goes wrong
Starting too early is the most common error, and it looks like a tray of seedlings that outgrew their pots. Stems are tall and pale, roots circle the container, and leaves yellow because the plants are out of room and nutrients. Transplants like these can recover, but they set back growth after planting. Prevention is arithmetic: use the weeks in the table, count from the correct end of your last-frost range, and resist the urge to start “a little early to be safe.” If a warm spell tempts you, pot seedlings up into larger containers rather than waiting for them to crowd.
Damping off is the other classic. Seedlings that looked fine yesterday are lying on the soil today, their stems pinched and brown at the soil line. It is caused by fungus-like organisms that thrive in wet, cool, stagnant conditions. Once it starts, you cannot save the tray. Prevent it with clean containers, fresh seed-starting mix instead of garden soil, moderate watering from below, and air moving gently across the trays. Thin crowded seedlings early, since a dense tray stays damp longer.
The third mistake happens at planting out. An average last frost date is the midpoint of a spread, so in many years frost still arrives after it. Planting tender seedlings the week after the average date bets on a mild year. Cold soil is a quieter problem, because even without frost it can stall peppers and eggplant. For tomatoes, peppers and eggplant, treat the late end of your last-frost range as the earliest safe planting-out date, and keep row cover on hand for any cold snap that follows.
Common questions
Do I need to start seeds indoors in coastal or Southern California?
Not for frost protection. At the Los Angeles reference station, NOAA records show no days at or below 32°F in 30 years. You can still start seeds indoors for a head start or to control spacing and germination, but you time it to the cool or warm season you are targeting, not to a frost date.
How early should I sow tomatoes indoors in Northern California or the Central Valley?
University of Minnesota Extension lists tomatoes at 5 to 6 weeks indoors before planting out. Find your local last-frost range, take the late end as your planting-out date for tomatoes, and count back 5 to 6 weeks.
Which USDA zone is California?
No single zone covers the state. The USDA Plant Hardiness Zone Map (2023) shows a range from 5a to 11a, based on the average annual extreme minimum temperature. Zone says nothing about when your last frost falls, so use your own local frost range to date your sowing.
How long should I harden off seedlings?
The guidance from University of Minnesota Extension is to harden off all plants over the last 1 to 2 weeks before planting out. Increase outdoor time a little each day, starting in shade, before they go into the ground.