Across most of Arizona’s low desert, there is no last spring frost to count back from. Phoenix reached 32°F in only 6 of 30 springs, so seed starting there is timed by summer heat instead. In the high country around Flagstaff, frost is real and hard, and the standard method applies: take the weeks each crop spends indoors and subtract them from your local last spring frost. Which method you use depends on where you garden.
The date everything counts back from
Standard seed-starting advice begins with a last frost date. Find it, count backward by each crop’s indoor weeks, and you have a sowing date. That works in Minnesota. In the Phoenix area, the date mostly does not exist. NOAA’s 1991-2020 Climate Normals for the Phoenix station (USW00023183) list the frost-free season as 365 days, and NOAA’s daily records show 32°F in only 6 of 30 springs. When a freeze does arrive, it is a rare cold snap, landing somewhere between late December and mid January.
Low desert: the anchor is a season, not a frost
Without a frost to dodge, the limiting factor is summer heat, which stops fruit set on tomatoes and peppers for months. So the date you count back from is the week you want transplants in the ground: either early enough that plants set fruit before the heat arrives, or after it eases for a fall crop. Pick your planting-out week first, then count back from it using the weeks in the next section.
The range matters because the planting-out week is never exact. Sow at the early end of your range and seedlings wait in their pots, outgrowing them and stretching toward the light. Sow at the late end and you lose weeks of garden time, which in the low desert can mean fruit that never sets before the heat closes the window. Aim for the middle, then adjust once you see how fast your seedlings grow.
High country: a real last frost
Flagstaff and the rest of the high country are a different climate and freeze hard. The direction is simple. Colder and higher means a later last spring frost, so every indoor sowing date moves later too. Seedlings that would be ready in the low desert while you are still shoveling snow off a raised bed are a mistake there, not a head start.
Hardiness zones will not settle the question. The USDA Plant Hardiness Zone Map (2023 edition, built on 1991-2020 normals) places Arizona across 4b-10b. That system rests on one variable, the average annual extreme minimum temperature. It tells you how cold winter can get, not when the last freeze falls. For a dated frost range in your town, use First and Last Frost Dates in Arizona, and keep in mind that other parts of the state differ from either end of this contrast. The wider Arizona guides cover those regional differences.
The calendar, crop by crop
The weeks below come from the University of Minnesota Extension table “When to plant seeds and how long to keep them indoors,” which gives weeks indoors before planting out. Treat them as the usual extension guidance, not a law. A cool windowsill and a warm grow shelf will not produce seedlings of the same size on the same day.
That table also names calendar months. Those are Minnesota’s, and they do not transfer. The Arizona window comes from counting these weeks back from your own planting-out week (low desert) or your own last spring frost (high country). No month is borrowed.
| Crop | Weeks indoors before planting out | Resulting sowing window in Arizona |
|---|---|---|
| Onions (and leeks) | 14-15 weeks | The earliest sowing of the list. Low desert: count back from your cool-season planting-out week. High country: count back from the local last spring frost. |
| Broccoli and cabbage | 10-11 weeks | Cool-season crops. Low desert: aim to plant out once summer heat has eased. High country: count back from the last spring frost, so sowing falls later at higher elevations. |
| Head lettuce | 10-11 weeks | Same count as broccoli. Low desert: time it to the cool stretch, well clear of the heat. High country: count back from the last frost. |
| Eggplant and peppers | 9 weeks | Low desert: plant out early enough to set fruit before the heat stops it, or after it eases for a fall crop. High country: count back from the last frost and wait for warm nights. |
| Tomatoes | 5-6 weeks | The shortest lead of the main crops, so the latest sowing. Low desert: same heat logic as peppers. High country: count back from the last frost. |
Read the table top to bottom and you can see the practical order of a seed-starting season: onions first, then the cabbage family and lettuce, then peppers and eggplant, with tomatoes last. Anyone who sows everything on the same weekend ends up with tomatoes that are tall and pot-bound long before the broccoli is big enough to move.
The other starts on the extension list
UMN Extension gives celery 12-13 weeks and okra 9 weeks, the same as peppers. Sweet potatoes get 3-4 weeks. For a deeper walk through the tomato routine, Growing Tomato Seedlings Indoors goes step by step. If lettuce is on your list, Starting Lettuce covers that crop on its own terms.
Crops that should stay out of the seed tray
Root crops such as carrots, beets, radishes and turnips are best sown where they will grow. They form a single taproot that resents disturbance, and a transplanted carrot tends to fork. Beans rarely gain anything from indoor starts either. They germinate fast in warm soil, and their seedlings sulk when roots are handled. Squash and its relatives are the same in most cases: they grow so quickly that a seedling outgrows its pot in a short time, and the transplant check can cost more than the head start saves.
Start onions, brassicas, lettuce, peppers, eggplant and tomatoes indoors; sow roots, beans and squash in the ground. That one sentence covers most seed-starting decisions. Once you know your planting-out week, the Arizona Planting Calendar shows what the season allows around it.
Light, warmth and hardening off
A sunny window looks like enough light. It usually is not. Glass filters sunlight, and a window delivers it from one side only, so seedlings lean and stretch toward it. The stems get long, thin and weak, which is what gardeners mean by “leggy.” A legged-out tomato can still be planted, but a leggy broccoli or onion seedling often never catches up. Lights hung a short distance above the plants and kept on for much of the day give stockier growth. The closer the light, the less stretching.
Steady warmth for germination
Seeds want steady warmth more than peak warmth. A room that swings from warm afternoons to cold nights slows germination and gives uneven stands. That is a common problem in Arizona homes, where a garage or spare room can drop sharply overnight in winter and heat up fast in the afternoon. A shelf in an interior room, a heat mat under the tray, or a spot on top of a warm appliance holds temperatures more evenly. Once seedlings emerge, many crops prefer a slightly cooler spot with strong light, which also discourages legginess. Peppers and eggplant are the sulkiest about cold, and their 9-week lead means they spend a long time in those trays, so the warmth matters more for them.
The week of hardening off
Seedlings raised indoors have never met wind, direct sun or temperature swings. UMN Extension advises hardening off all plants over the last 1-2 weeks before planting out. Plan on 1-2 weeks of gradual outdoor exposure, starting in shade, before any seedling goes into the ground. In Arizona, the sun is the thing to ease into. A tray moved straight into full desert light can scorch leaves in an afternoon. Begin with a few hours in open shade, add morning sun, then lengthen the stay each day. Bring trays in if the forecast turns harsh, and cut back on water gradually rather than all at once. By the last day, plants should be handling a full day outside without wilting.
What goes wrong
Starting too early
Too-early seedlings look like long, pale stems, roots circling the pot, and plants already flowering at transplant time. It happens when the sowing date came from enthusiasm instead of a count. In the low desert, early sowing also tempts people to plant out before the weather settles. The fix is to follow the weeks per crop and sow in the middle of your range. If seedlings do get ahead of you, move them to a bigger pot and keep them cool and well lit rather than rushing them into the ground.
Damping off
Damping off is a fungal collapse. A seedling that looked fine yesterday is pinched and thin at the soil line, falls over, and does not recover. It strikes in cold, wet, crowded conditions, and trays sitting in standing water are the usual setting. Prevention is mostly hygiene: clean containers, fresh seed-starting mix, thin sowing, gentle airflow from a small fan, and watering only when the surface begins to dry.
Planting out into cold soil
In the high country, the average last frost is not a safe planting date. It is the midpoint, which means roughly half of years still have a frost after it. Soil also lags behind air, so a transplant set out the week after the average last frost can sit in cold ground and stall. Wait for warmer soil, and have row cover or buckets ready for a late cold night. In the low desert the equivalent mistake runs the other way: do not plant out tomatoes, peppers or eggplant so late that summer heat arrives before the plants can set fruit.
Common questions
Is there a “last frost date” I should use in Phoenix?
Not for sowing dates. NOAA lists the frost-free season at 365 days for the Phoenix station, and 32°F showed up in only 6 of 30 springs. A rare cold snap can bring it between late December and mid January, so keep frost cloth handy for tender transplants, but plan your calendar around summer heat.
How many weeks before planting out should I start tomatoes, peppers and eggplant?
The usual extension guidance from UMN is 5-6 weeks for tomatoes and 9 weeks for peppers and eggplant. Count back from the week you plan to set plants in the garden, which in the low desert is set by the heat, and in the high country by the last spring frost.
Can my hardiness zone tell me when to start seeds?
No. The USDA Plant Hardiness Zone Map is built on the average annual extreme minimum temperature, and Arizona spans 4b-10b on the 2023 map. A zone says how cold winter gets, not when the last freeze arrives or when summer heat shuts fruit set down.
How long should I harden off seedlings?
UMN Extension says to harden off all plants over the last 1-2 weeks before planting out. In Arizona, start in open shade and add sun slowly, since seedlings raised under lights or behind glass have never taken direct desert sun and scorch quickly when moved straight into it.