When to start seeds indoors: a climate-zone guide for 2026.
Almost every seed-starting question reduces to two numbers: your USDA hardiness zone and your area's last frost date. Here's how to use them, with a crop-by-crop timing table and notes on what changed when USDA updated the zone map in late 2023.
Most gardening advice gets seed-starting almost right and then loses on the details. The problem isn't the basics, keep the soil warm, use a sterile mix, give the seedlings light, those are well-established. The problem is timing. Start seeds too early and you end up with leggy, root-bound transplants weeks before the weather can support them. Start them too late and the season is gone before plants can mature. The difference between a great vegetable garden and a frustrating one is often six to eight weeks of correct timing, and the timing math depends on where you live.
This guide walks through the timing logic from first principles, with the climate-zone framework that connects "when to start seeds" to "when can plants go outside" to "how many weeks of headstart does each crop actually want." It includes a crop-by-crop reference table for the most commonly grown vegetables and herbs, notes on the major USDA hardiness zone update from late 2023 (which shifted about half of the contiguous United States into a warmer zone), and the practical adjustments that follow.
The two numbers that determine everything
Seed-starting timing reduces to two numbers. One is yours. One is the crop's.
Your number is your average last frost date. This is the average date of the last spring frost in your area, calculated across decades of historical weather data. It varies dramatically by location, in southern Florida it's effectively never; in northern Maine it can be early June. Your last frost date is the anchor that the rest of the calendar swings around: most warm-season crops want to be transplanted outdoors after this date, and most seed-starting timing is expressed as "X weeks before last frost."
The most reliable source for your local last frost date is the National Centers for Environmental Information (formerly NCDC) freeze probability data, which is what most state extension services use to publish regional last-frost averages. Many extension offices publish a simple last-frost lookup for their region; some seed companies have built consumer-friendly zip-code lookup tools that draw on the same underlying NCEI data.
Note that this is a probability, not a guarantee. A "50% probability" last frost date means there's a 50% chance of frost after that date in any given year. Some gardeners use the "10% probability" date as a more conservative anchor, meaning frost after that date is unlikely in only 1 year out of 10. Whether you use the 50% or 10% date depends on your appetite for risk and how much you've invested in your transplants.
The crop's number is its weeks-before-last-frost headstart. Each crop has a different developmental timeline. Tomatoes want about 6–8 weeks of indoor growth before they can go outside; lettuce only wants 3–4 weeks; peppers, which are slower, prefer 8–10 weeks. This isn't arbitrary, it reflects how long each crop's seedlings take to grow from germination to a robust transplant size that can handle outdoor conditions without setback.
Put the two numbers together and the math becomes simple. If your last frost is May 15 and tomatoes want 6 weeks of indoor growth, you start tomato seeds around April 3. If your last frost is April 1 and peppers want 10 weeks, you start pepper seeds around January 20. The framework holds across the entire vegetable garden.
The crop-by-crop timing table
The table below summarizes the consensus weeks-before-last-frost timing for the most commonly grown vegetable and herb crops, drawing from the cooperative extension guidance referenced in the methodology block. Where extension services publish different numbers (which happens), the listed timing falls within the range across multiple regional sources.
| Crop | Start indoors | Transplant outside | Notes |
|---|---|---|---|
| Tomatoes | 6–8 weeks before | After last frost | The most common crop. Don't start too early; leggy 10-week tomato seedlings rarely outperform tighter 6-week ones. |
| Peppers (sweet & hot) | 8–10 weeks before | 1–2 weeks after last frost | Slower than tomatoes. Want consistently warm soil (70°F+) to germinate well. |
| Eggplant | 8–10 weeks before | 2 weeks after last frost | Even more heat-loving than peppers. Transplant only when nights are reliably warm. |
| Broccoli, cabbage, cauliflower | 6–8 weeks before | 2–4 weeks before last frost | Cool-season. Can be transplanted into the garden well before last frost, light frost is tolerated. |
| Lettuce | 3–4 weeks before | 2–4 weeks before last frost | Or direct-sow outdoors as soon as the soil can be worked. Doesn't require indoor headstart. |
| Onions (from seed) | 10–12 weeks before | 2–4 weeks before last frost | The longest indoor cycle. Sets and transplants are easier for most home gardeners. |
| Cucumbers | 3–4 weeks before | After last frost | Can also direct-sow outdoors. Don't start too early, cucumbers don't transplant well at a large size. |
| Squash, melons, pumpkins | 2–3 weeks before | After last frost | Similar issue to cucumbers. Better to start late than early. Some gardeners skip indoor start entirely. |
| Basil | 4–6 weeks before | After last frost | Genuinely cold-sensitive. Don't rush this one outside. |
| Parsley, cilantro, dill | 4–6 weeks before | 2 weeks before to after last frost | Cool-tolerant herbs. Cilantro especially bolts in heat, start successive plantings. |
| Beans (bush & pole) | Don't start indoors | 1–2 weeks after last frost | Direct-sow only. Beans transplant poorly and germinate quickly outdoors. |
| Carrots, beets, radishes, turnips | Don't start indoors | 2–4 weeks before last frost | Direct-sow only. Root crops resent transplanting. |
| Peas | Don't start indoors | 4–6 weeks before last frost | Among the first crops planted outdoors each year. Cold-hardy. |
| Corn | Don't start indoors | 2 weeks after last frost | Direct-sow. Wants warm soil (60°F+) for reliable germination. |
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Hardiness zones, and what changed in 2023
The USDA Plant Hardiness Zone Map is the most widely-used reference for connecting a US location to a plant's expected winter survival. The map divides the country into zones based on the average annual minimum temperature, the coldest night of the year, averaged over the past 30 years. Zone 5 averages -20°F to -10°F; Zone 7 averages 0°F to 10°F; Zone 9 averages 20°F to 30°F. Each numbered zone is further split into "a" and "b" half-zones spanning 5°F each.
In November 2023, the USDA released its first major update to the map since 2012. The new map was developed jointly with the PRISM Climate Group at Oregon State University, and is based on 30-year temperature data from 1991 through 2020. It incorporates data from 13,412 weather stations, up from 7,983 in the 2012 map, which produces both a more accurate national picture and meaningfully more granular regional resolution.
The headline change: about half the contiguous United States shifted into a warmer half-zone compared to the 2012 map. Across the lower 48, the average warming between maps is approximately 2.5°F. The central plains and Midwest warmed the most; the southwestern US warmed very little. Notably, the 2023 map for Alaska is "warmer" than the 2012 version, though this is partly explained by better data coverage of mountain regions where warm air overlies the cold air that settles into low-elevation valleys.
The USDA has been careful to note that "temperature updates to plant hardiness zones are not necessarily reflective of global climate change", partly because the underlying statistic (extreme minimum temperature in a given year) is highly variable from year to year, and partly because some of the apparent warming is an artifact of better mapping methods. Chris Daly, the map's lead author and director of PRISM, has stated that climate change is playing some role in the longer-term picture: "Over the long run, we will expect to see a slow shifting northward of zones as climate change takes hold."
The hardiness zone tells you what perennials can survive your winter. It tells you almost nothing directly about when to start seeds, that's a separate calculation based on your last frost date.
This is an important distinction that catches a lot of new gardeners. Your hardiness zone determines what trees, shrubs, and perennial plants will reliably survive in your yard year over year. Your last frost date determines when annual vegetables can go outside. The two are correlated (warmer zones tend to have earlier last frost dates) but they are not the same number, and they are not used for the same purpose. If you're starting tomato seeds, your last frost date matters and your hardiness zone is largely irrelevant; if you're planting fruit trees, your hardiness zone matters and your last frost date is largely irrelevant.
What the zone shift means in practice
For most home gardeners, the 2023 USDA map update is interesting but doesn't change very much in the year-over-year seed-starting calendar. If your area moved from Zone 6b to Zone 7a, that primarily affects what perennials you might plant, it doesn't significantly change when to start tomato seeds, which depends on your last frost date and not directly on your hardiness zone.
That said, the longer-term implication is worth noting: across many regions, the average last frost date has been gradually moving earlier over the past several decades. The University of New Hampshire Cooperative Extension and many other regional services have updated their recommended planting calendars to reflect this. If your seed-starting reference dates are more than a decade old, they may be slightly conservative.
Zone 3 (-40°F to -30°F): Northern Maine, Minnesota, Wisconsin, North Dakota, Montana. Last frost typically late May to early June.
Zone 4 (-30°F to -20°F): Most of upper Midwest, northern New England, mountain West. Last frost mid-May.
Zone 5 (-20°F to -10°F): Central Midwest, much of New England, Pacific Northwest interior. Last frost late April to mid-May.
Zone 6 (-10°F to 0°F): Mid-Atlantic, southern Ohio, much of Pacific Northwest, parts of mountain West. Last frost mid-April to early May.
Zone 7 (0°F to 10°F): Mid-Atlantic coast, parts of South, parts of California coast. Last frost late March to mid-April.
Zone 8 (10°F to 20°F): Coastal South, Pacific Northwest coast, much of Texas. Last frost mid-March.
Zone 9 (20°F to 30°F): Florida (much of), Gulf Coast, much of California, southern Arizona. Last frost late February to mid-March.
Zone 10 (30°F to 40°F): South Florida, southern California coast, parts of Texas Gulf Coast. Effectively year-round growing.
These are the median expectations. Your specific location may differ from the regional zone, a south-facing wall, an urban heat island, a frost pocket at the bottom of a slope, all create microclimates that can shift effective conditions by half a zone or more in either direction. Watch your specific yard for a few seasons and you'll learn your microclimate better than any map can describe.
Practical mistakes that cost the most time
Starting too early
The most common mistake. Excited gardeners look at the calendar in mid-January and start tomatoes "to get a head start," then end up with 12-week-old seedlings that are stretched, root-bound, and producing flower buds indoors. These plants don't outperform 6-week-old seedlings planted at the same outdoor date, they often underperform, because the long indoor stretch sets them back. For most crops, starting later is safer than starting earlier. Tighter, healthier seedlings transplant better than older stressed ones.
Not hardening off
Plants that have spent six weeks indoors under stable temperatures and gentle indoor light are not ready for direct outdoor exposure. The transition needs to be gradual: 1 hour outside on day one in dappled shade, 2 hours on day two with some direct sun, building up over 7–10 days. Plants put directly from indoor seed starting to full outdoor exposure typically show leaf burn within a day or two, and may take weeks to recover (if they recover at all). Per Penn State Extension's guidance, this hardening-off process is one of the most-skipped steps in home seed starting, and one of the highest-impact when done properly.
Inadequate light
Windowsills produce leggy seedlings in nearly all but the sunniest southern-exposure rooms. Plants reaching for inadequate light produce thin, pale stems that flop over. The most cost-effective fix is a basic LED shop light positioned 2–4 inches above the seedlings (raised as the plants grow). Fluorescent T5 fixtures work too. The light should run 14–16 hours per day on a timer. This is not optional equipment for serious seed starting, it's the difference between healthy seedlings and disappointment.
Overwatering
Seedlings die more often from too much water than too little. The cool, wet, low-airflow conditions of overwatered seed trays are ideal for damping-off fungi, which kill seedlings at the soil line. Per University of Wisconsin and University of Maryland Extension guidance, the key is to keep the surface dry between waterings and water from the bottom (sitting trays in shallow water) rather than from above. Sterile potting mix and good airflow also reduce damping-off risk substantially.
The simplest schedule that works
If you've read this far and want a single takeaway: figure out your last frost date, count backwards using the timing table above, and write three or four start dates on your calendar. Most home gardeners only need to remember four indoor start dates per year, the rest can be direct-sown outside at the appropriate time.
- Onions and leeks (if growing from seed): 10–12 weeks before last frost
- Peppers and eggplant: 8–10 weeks before last frost
- Tomatoes, broccoli, cabbage: 6–8 weeks before last frost
- Basil, herbs, cucumbers, squash: 3–4 weeks before last frost
That's the entire indoor seed-starting calendar for most home gardens. Everything else either gets direct-sown outdoors at the right time (lettuce, root crops, beans, peas, corn) or skips the home start entirely (most gardeners are better off buying tomato and pepper transplants if they don't have indoor space and lighting set up).
This is also a meaningful constraint of the broader gardening category: scaled commerce in seed starting has historically been limited by the fact that it's a niche, January-to-March activity, with the majority of US gardening households not engaging with indoor seed-starting at all. That's part of what makes the seed-starter-versus-transplant market a structurally different question from the broader gardening retail category, and why most consumer-facing gardening media focuses on the broader category rather than this specialist subset. We've documented the broader gardening industry sizing in our definitions guide if you want the wider context.
For most readers of this guide, the practical advice is simpler: pick four dates, start the right crops on each, and don't second-guess the calendar once it's set. The plants don't care about your enthusiasm in January; they care about being healthy seedlings on transplant day.