Lowland Hawaii has no last spring frost, so there is no frost date to count back from. Count back from the day you want transplants in the ground instead. Onions and leeks go in pots 14-15 weeks ahead of that day. Broccoli, cabbage and head lettuce need 10-11 weeks, eggplant and peppers 9, and tomatoes 5-6. Rainfall, elevation and exposure decide that planting-out day, not a thermometer.
The date everything counts back from
On the mainland, seed-starting calendars hang on one number: the average last spring frost. Pick that date, subtract the weeks each crop needs indoors, and the sowing window falls out. The method is sound. The anchor just doesn’t exist in most of Hawaii.
NOAA’s 1991-2020 Climate Normals for station USW00022521 show why. The data carry a not-computable flag for every freeze percentile at both 32°F and 36°F, because the station records zero days per year at or below 32°F. No freeze occurs in inhabited areas of the state. Frost shows up only on the high volcanic summits, Mauna Kea, Mauna Loa and Haleakala, and nobody is gardening up there. You can read the underlying data in the NOAA NCEI normals for that station, and the longer explanation in First and Last Frost Dates in Hawaii.
What replaces the frost date
The anchor becomes your own planting-out day, and you choose it by looking at three things: how much rain a bed gets across the year, how high your garden sits, and whether it faces the windward or leeward side of its island. A windward bed that stays soggy through a rainy stretch is a poor place to set out a tomato transplant. A leeward bed that bakes dry in summer is a poor place for lettuce. Higher-elevation gardens warm up later, so warm-season crops move later there, and cool-season crops tolerate that setting better.
Don’t lean on the USDA zone either. The 2023 USDA Plant Hardiness Zone Map (built on 1991-2020 normals) puts the state in zones 9a-13a. That system rests on a single variable, the average annual extreme minimum temperature. It tells you which perennials survive a cold night, and it says nothing about when a seedling should go into a pot. Hawaii also spans that whole range, so one zone label can’t describe a coastal yard and a cool hillside lot at once.
A range still matters here, just a different one. Your planting-out window will be several weeks wide, and the same trap applies as anywhere. Sow to the early edge of it and seedlings sit in pots getting leggy and root-bound while they wait for a good planting day. Sow to the late edge and you lose weeks of growing season. Choose your planting-out day first, then subtract the weeks in the table below to get each sowing date. If you want the season-by-season picture of what to put in the ground once you have transplants, the Hawaii Planting Calendar covers it.
The calendar, crop by crop
The weeks below come from University of Minnesota Extension’s table on when to plant seeds and how long to keep them indoors. Treat them as the usual guidance, the figures extension horticulturists have long used, and not as a law. The extension table also lists calendar months, but those are Minnesota’s and mean nothing here, so they’re left out. For Hawaii, the sowing window is always “this many weeks before the day you plant out.”
Weeks indoors and the resulting window
| Crop | Usual weeks indoors before planting out | Sowing window in Hawaii |
|---|---|---|
| Onions and leeks | 14-15 weeks | Sow 14-15 weeks before your planting-out day. Earliest of all the crops here. |
| Broccoli | 10-11 weeks | Sow 10-11 weeks before planting out, timed for the cooler part of your local year. |
| Cabbage | 10-11 weeks | Sow 10-11 weeks before planting out, same cool-season timing as broccoli. |
| Head lettuce | 10-11 weeks | Sow 10-11 weeks before planting out, away from your hottest stretch. |
| Eggplant | 9 weeks | Sow 9 weeks before planting out into warm, well-drained soil. |
| Peppers | 9 weeks | Sow 9 weeks before planting out, same as eggplant. |
| Tomatoes | 5-6 weeks | Sow 5-6 weeks before planting out. Latest of the group. |
Read the table from the bottom up when you plan a bed. Tomatoes are the last seeds in pots and the first to feel rushed if you start them with the onions. Anyone who has watched a tall, floppy tomato seedling lean across a shelf for a month knows how that goes. The extension table also gives 12-13 weeks for celery, 9 weeks for okra and 3-4 weeks for sweet potatoes (grown from slips), if you want to plan those alongside. For tomato-specific handling, see Growing Tomato Seedlings Indoors.
Lettuce deserves a note. The 10-11 weeks applies to head types, and the real question in Hawaii is where your planting-out day lands relative to heat. Leaf lettuce is often sown directly and cut young, which skips the indoor stage entirely. If you do want transplants, Starting Lettuce goes deeper on the sowing itself.
Crops that should stay out of the pots
Some crops resent transplanting, and starting them indoors costs you more than it gains. Root crops such as carrots, beets and radishes form their edible part below ground, and a disturbed taproot comes out forked or stunted. Beans germinate fast and sulk when their roots are handled. Squash and its relatives can be started indoors in a few cases, but in most gardens direct sowing is simpler and the plants establish faster. In a place with no frost to dodge, there’s even less reason to get a head start on them.
Rule of thumb for the calendar: sow onions and leeks first, tomatoes last, and put roots, beans and most squash straight into the ground. Write the planting-out day on a calendar, count backward from it, and mark each crop’s sowing window as a stretch of days rather than a single date. A few days either side hurts nothing.
Light, warmth and hardening off
Leggy seedlings are a light problem more often than a water or feeding problem. A seedling that can’t find enough light stretches toward whatever it can find, and the result is a pale, thin stem that flops over. Hawaii’s bright sun can trick people into thinking a windowsill is enough. Glass filters some of the light, deep eaves and lanai roofs cut more, and the light comes from one side, so the seedlings bend toward it. A simple shop light hung close above the tops, raised as the plants grow and run on a timer, usually does better than any window.
Germination wants steady warmth more than peak warmth. Seeds in a pot that swings from a hot afternoon to a cool, damp night sprout unevenly or not at all. Pots on a shelf in a stable indoor spot are fine in most lowland homes. In cooler, higher-elevation houses, the warmth may need help, and a seedling heat mat under the tray is the usual fix. Once seeds have sprouted, the trays can come off the heat, and the light matters more than the warmth.
Then comes hardening off, which is the step people skip and regret. Indoor seedlings grow soft, with thin leaf surfaces that have never met direct sun, wind or a trade-wind shower. Put them straight into a bed and you may see bleached, papery patches within a day or two, which is sunburn on leaves that were never exposed. University of Minnesota Extension advises hardening off all plants over the last 1-2 weeks of the indoor period. Start with a sheltered, shaded spot for an hour or two, then lengthen the time outdoors, then move the trays into brighter light and eventually into the weather they’ll live in.
On an exposed windward site, wind does as much damage as sun. Shelter the trays from steady gusts for the first days. Begin hardening off 1-2 weeks before your planting-out day, and treat that window as part of the indoor weeks, not extra.
What goes wrong
Starting too early
Seedlings kept in pots past their window get tall, thin, and pot-bound. Roots circle the container, leaves yellow, and the plant stalls after transplanting while it recovers. Peppers and eggplant are the worst offenders because they were slow starters to begin with, and an early sowing leaves them more weeks to stew. Prevention is just the table: count back from your planting-out day, and resist sowing early because the weather feels right. If a heavy rainy stretch pushes your planting day later, a leggy tomato can be buried deeper at transplant time, but a root-bound pepper rarely recovers fully.
Damping off
You water a flat of seedlings on Monday and they look fine. By Thursday several have pinched at the soil line and fallen over. That is damping off, a fungal problem that thrives in wet, stagnant conditions, and warm, humid Hawaii homes can set the stage. Keep trays clean, use fresh seed-starting mix instead of old garden soil, water only as needed, and give the air some movement with a small fan. Thin out crowded seedlings. Once damping off appears, it spreads quickly, so remove affected plants and let the tray dry a bit.
Planting out into the wrong soil
On the mainland, the classic mistake is planting out the week after the average last frost, when the air has warmed but the soil is still cold. In lowland Hawaii, there’s no frost, so that exact trap doesn’t apply. Something similar does. Heat-loving transplants set into cool soil at higher elevation sulk, and any transplant set into soil soaked by days of rain may rot or stall. Cool-season crops set out into the hottest part of the year bolt or stop growing. Wait for a stretch of weather when the soil drains and suits the crop, and set transplants then, even if it means a short delay. If you must wait, move the pots into brighter light and keep them lightly watered rather than letting them dry out or sit wet.
Common questions
Is there a last frost date I can use in Hawaii?
Not in inhabited areas. NOAA’s 1991-2020 normals for station USW00022521 show zero days per year at or below 32°F, so no freeze percentile can be computed. Frost occurs only on the summits of Mauna Kea, Mauna Loa and Haleakala. Pick your planting-out day by rainfall, elevation and windward or leeward exposure, then count back.
How early should I start tomatoes indoors?
University of Minnesota Extension gives 5-6 weeks indoors before planting out. That is the shortest lead time of the common crops, so tomatoes go into pots after onions, brassicas, peppers and eggplant. Start them too early and they grow tall and weak before the bed is ready.
Does my USDA hardiness zone tell me when to sow?
No. The USDA Plant Hardiness Zone Map 2023 uses one variable, the average annual extreme minimum temperature, and the state falls in a range of 9a-13a. That helps you choose perennials that survive a cold night. It doesn’t set sowing dates, which depend on your planting-out day and local conditions.
Which crops can I skip starting indoors?
Root crops, beans and, in most cases, squash do better sown directly where they will grow. Everything else on the table benefits from an indoor start, with the weeks counted back from planting out. More local advice is collected in the Hawaii guides.