How to Read a Seed Packet: 6 Numbers That Decide If Your Seeds Sprout, Rot, or Come Up Weeks Late

Every seed packet has the same shape of information on the back, printed in the same cramped type, and most gardeners skim it once and toss it in a junk drawer. That’s a mistake. The back of a seed packet is a complete, tested growing recipe — condensed by a horticulturist into six numbers because that’s all the room a 3×5-inch envelope has. Miss one, and the failure mode isn’t vague: seeds rot in the ground, sprout three weeks apart from each other, or come up so crowded they smother each other before they fruit.

If you’re still mapping out where everything goes, it’s worth working through our step-by-step guide to starting a garden from scratch first — this article picks up once you’ve got seed packets in hand and need to know what to actually do with them. Here’s what each of the six numbers means, why it’s there, and where the seed companies and extension services themselves don’t fully agree.

The “Packed For” Date Isn’t Marketing — It’s Federal Law

That small “Packed for 2026” line, or a “Sell by” date, or sometimes just a lot number, looks like an afterthought. It’s actually a legal requirement. Under the Federal Seed Act, any seed label that shows a germination percentage has to also show the month and year that germination test was completed — and the seed can’t legally ship more than 5 calendar months after that test date, or 24 months if it’s in a hermetically sealed container [1]. That’s the mechanism: the date on your packet isn’t when the seed was grown or bagged, it’s a legal freshness clock tied to an actual lab test, and it’s the reason two packets of the same variety from different companies can carry different years even when bought off the same rack.

So what happens once that clock runs out? Seed doesn’t go bad on a specific day — it degrades gradually, and how fast depends heavily on the species. A peer-reviewed study on onion seed found that accelerated aging cut germination rates sharply within days of stress exposure, and traced the decline to real physiological damage: seed cell membranes started leaking, and the enzymes that normally protect the seed from oxidative damage dropped off [2]. That’s onion-specific lab data under artificial stress, not a universal timeline every crop follows — but it explains why “expired” seed doesn’t fail all at once. It fails a little more every season it sits.

Three pre-planned garden beds, free

Stop staring at empty beds: printable plans with exact layouts, plant lists and planting calendars — yours free from the Garden Library.

Send me the plans

I agree to receive emails from Blooming Expert and accept the Privacy Policy. Unsubscribe anytime.

In practice, some seed is remarkably durable and some isn’t. Tomatoes, cucumbers, and brassicas routinely stay viable 5 years or more in cool, dry storage, while onions, leeks, parsnips, and spinach are already declining after a year or two [3]. If you’re planting seed that’s past its packed-for date, the fix isn’t to throw it out — sow it a little thicker than the packet recommends to compensate for a lower germination rate, or run a quick viability check first: fold 10 seeds into a damp paper towel, seal it in a bag, keep it warm, and count how many sprout after about a week [3]. If it’s under 50%, replace the seed rather than gamble a whole bed on it.

Days to Germination: The Number That’s Really a Soil Temperature

“Germinates in 7–14 days” looks like a fixed countdown. It isn’t. That range assumes your soil is sitting in that crop’s ideal temperature window — and outside that window, germination doesn’t just slow down, it can stop entirely or fail outright. Cool-season crops like lettuce, peas, and onions will germinate in soil most people would call too cold for gardening, while carrots, corn, cucumbers, peppers, squash, tomatoes, and zucchini need genuinely warm soil to sprout at all [4].

Here’s roughly where that line sits, using minimum, optimal, and maximum soil temperatures for germination pulled from extension research [5]:

The number on the packet is calibrated for the optimal range, not the minimum. Plant a warm-season crop the day soil temperature technically crosses the minimum, and you’re not saving time — you’re extending how long the seed sits vulnerable underground. That matters more than it sounds like it should: the longer a seed sits in cold, wet soil before it emerges, the more its factory-applied seed protectant coating dissipates, and the more time soil pathogens get to attack it before it’s strong enough to fight them off. One extension trial found snap beans took roughly two weeks to emerge in 60°F soil versus a week or less once soil reached the mid-70s — twice the exposure window for the same seed, in the same ground, over one temperature difference [6]. That’s the real reason “plant when the soil is ready, not when the calendar says so” isn’t just folk wisdom — it’s the difference between a seed that sprouts and one that rots.

Days to Maturity: The Same Number Counts From a Different Starting Line

Days to maturity tells you how long from sowing to harvest — except it doesn’t always start counting from sowing. For crops you direct-sow into the garden (beans, carrots, radishes, most root vegetables), the clock starts at germination or first true leaves. For crops you start indoors and transplant out later (tomatoes, peppers, most brassicas), the days-to-maturity number restarts the day you transplant the seedling outside, not the day you first planted the seed indoors [7][8]. Ignore that distinction and your harvest-date math will be off by however many weeks you spent growing seedlings on a windowsill.

The spread between crops is bigger than most beginners expect. Radishes are one of the fastest vegetables you can grow from seed — 25 to 35 days from sowing to harvest [9]. Parsnips, sown at almost the same depth in the same bed, take 120 to 180 days [10] — a four-to-seven-month gap for two root vegetables that look similar on the seed rack. That’s why succession planting works so well with radishes specifically: you can tuck a fast crop between rows of a slow one and harvest it before the slow crop needs the space.

Once you’ve decoded the days-to-maturity number and checked it against your local frost dates, the next question is usually whether to start that crop indoors at all — and if so, exactly when to start it so the seedling is neither leggy nor too small at transplant time. That backward calculation from transplant date to indoor sowing date is its own process, covered in full in our guide to starting seeds indoors.

Plant Spacing: The “Thin To” Number Is a Competition Limit

Seed packets usually give two spacing figures — how far apart to sow the seed, and how far apart to thin the seedlings once they come up. The gap between those two numbers exists on purpose: germination is never 100%, so packets have you oversow and then remove the weaker seedlings once you can see which ones actually took.

Spacing isn’t just about avoiding a crowded-looking bed. Each plant is drawing water and nutrients from a fixed radius of soil, and taller neighbors block light from shorter ones if you don’t respect mature plant height and width when you space [4]. Crowding also recreates the same disease risk described above with cold soil — dense foliage traps humidity at soil level, and that damp, still air is exactly the environment fungal pathogens like the ones behind damping-off need to spread from plant to plant. Under-thinning a bed of carrots or beets doesn’t just mean smaller roots; it means longer leaf-wetness periods and a higher disease load across the whole row.

Planting Depth: Extension Services Don’t Actually Agree on the Rule

You’ve probably heard “plant seeds twice as deep as they are wide.” It’s a reasonable starting point, and some extension guidance backs it directly [8] — but it isn’t universal. Other extension research on small-seeded crops recommends a depth closer to three times the seed’s maximum width, and flags planting too deep as one of the most common causes of a field or bed simply failing to come up at all [11]. Neither figure is wrong, exactly — they’re describing slightly different seed categories, and the honest takeaway is that there’s no single rule precise enough to apply blindly across every crop.

What actually matters mechanically is that a seed only carries enough stored energy to push a sprout a limited distance through soil before it runs out. Bury it too deep and it exhausts that reserve before reaching daylight; too shallow in dry conditions and it dries out before it can root. When in doubt, round down rather than up — a seed planted slightly shallow that gets watered consistently will usually still germinate, while one buried too deep often won’t recover at all.

There’s one category where the “twice as deep” rule doesn’t apply whatsoever: seeds fine enough to look like dust, like lettuce, need light to trigger germination and shouldn’t be buried at all. Press them onto the soil surface and barely cover them, if you cover them at all [8][12].

The Sun Icon: Two Different Light Requirements Wearing One Symbol

The little sun, half-sun, or shade icon on the front of the packet tells you how much direct light the mature plant needs once it’s established — full sun generally means 6 or more hours of direct light a day, part sun runs roughly 4 to 6 hours, part shade drops to 2 to 4, and anything under 2 hours counts as shade [13]. Worth knowing: there’s no single accredited industry standard behind those definitions [13] — different growers and seed companies apply them slightly differently, which is why “part sun” on one packet and “part shade” on another sometimes describe nearly identical light levels.

What trips people up is conflating that icon with a completely different trait: whether the seed itself needs light to germinate. Those are unrelated. The sun icon describes a mature plant’s daily light budget in the garden bed; the germination-light trait — true of lettuce and a handful of other fine seeds — is a seed-coat response that only matters for the few days before sprouting, and has nothing to do with how sunny the spot is once the plant is established [12][8]. I’ve seen beginners bury lettuce seed a half-inch deep because the packet’s sun icon said “full sun,” reasoning the plant “likes sun” — but that icon has no bearing on whether the seed should be covered. The two numbers answer completely different questions and just happen to share a symbol.

Putting All Six Numbers Together

Picture a packet of bush beans with these numbers: germinates in 8–16 days, matures in 50–55 days, sow ½ inch deep, space 3–4 inches apart, full sun. Read correctly, that tells you: don’t sow until soil holds steady above 60°F, because beans rot fast in anything colder; count on harvest roughly 7–8 weeks after sowing since beans are direct-seeded and the clock starts at germination; bury the seed about an inch deeper than a lettuce seed would ever tolerate, because beans carry enough stored energy to push through that much soil; and give each plant a genuinely open 6+ hours of light, because the “full sun” icon here is describing the grown plant, not the germinating seed. Every one of those decisions traces back to one of the six fields on the back of the packet — the packet was never vague, it just assumed you knew what each number was actually measuring.

Frequently Asked Questions

Can I still plant seed after its “packed for” date has passed?
Usually yes — the date marks when the germination test was run, not a hard expiration. Sow it a bit more thickly than the packet suggests to cover for a lower germination rate, or run a damp-paper-towel test first if you’re planting a full bed and want to know the odds before committing [3].

My flower seed packet doesn’t list days to maturity — is that normal?
Yes. Days to maturity is most consistently listed for vegetables, where “harvest” is a clear event. Many flower packets swap it for “days to bloom” or skip a firm number entirely, since bloom timing varies more with weather and deadheading than a vegetable’s harvest date does.

Do I have to hit the depth number exactly?
No — treat it as a target, not a tolerance. Being off by an eighth of an inch on a small seed rarely matters. What matters is the category: don’t bury light-dependent seed like lettuce, and don’t scatter large seed like beans or squash on the surface uncovered.

What if my soil is colder than the packet’s minimum germination temperature?
Wait. Seed planted below its minimum won’t just germinate slowly — it can sit and rot before it ever sprouts, since cold soil extends how long the seed is exposed to fungal pathogens before it’s strong enough to resist them [6]. A soil thermometer is a more reliable planting-date guide than the calendar.

Sources

1. Federal Seed Act, 7 CFR § 201.30a — Date of Test. Cornell Law School Legal Information Institute
2. Physiological and molecular changes of onion (Allium cepa L.) seeds under different aging conditions. PMC / peer-reviewed
3. How Long Do Seeds Last? (Seed Viability Chart). Homestead and Chill
4. 2.5 Reading a Seed Packet. Cornell Garden-Based Learning
5. Soil Temperature Conditions for Vegetable Seed Germination. Alabama Cooperative Extension System
6. Cold, Wet Soils and Vegetable Seed Emergence. Michigan State University Extension
7. Read Your Seed Packet. Penn State Extension, Pike County Master Gardeners
8. Understanding Seed Packet Information. University of Georgia Extension, Urban Ag
9. Growing Radishes in a Home Garden. University of Maryland Extension
10. Carrot, Beet, Radish & Parsnip. Clemson Cooperative Extension Home & Garden Information Center
11. 25 Rules of Thumb for Field Crops. Utah State University Extension
12. Struggling with Lettuce: Germination. Durham County Extension Master Gardener Volunteers
13. Definitions of Light Requirements. UC Master Gardeners of Placer County, UC ANR