How to Read a Seed Packet: Sow Dates, Depth and Spacing
A seed packet is a condensed instruction manual. Once you know what each line means — and which ones actually matter — you can turn any packet into a sowing plan in under a minute.
Read six fields: variety, days to germination, days to maturity, sow depth, spacing, and the sowing window. Sow at 2–3 times the seed’s diameter deep; space to the packet’s “thin to” distance; and translate “sow after last frost” using your zone’s frost date. “Days to maturity” usually counts from transplanting, not from sowing.
The six fields that matter
Packets vary in layout, but the useful ones all carry the same six facts:
| Field | What it tells you | Example |
|---|---|---|
| Variety / cultivar | The exact plant; days and habits refer to this variety | ‘Celebrity’ tomato |
| Days to germination | How long until seedlings emerge (at the stated soil temperature) | 7–14 days |
| Days to maturity | Harvest timing — usually from transplanting, not sowing | 70 days |
| Sow depth | How deep to cover the seed | 1/4 in |
| Spacing | Distance between plants (and rows); often “sow X, thin to Y” | Thin to 12 in |
| Sowing window | When to sow, relative to frost or in weeks indoors | Start indoors 6–8 weeks before last frost |
“Days to maturity” — what it really means
This is the most misunderstood number on the packet. For crops started indoors (tomatoes, peppers, brassicas), “70 days” almost always counts from transplanting, not from the day you sow. Add your indoor weeks on top: a 70-day tomato started 7 weeks early actually needs about 120 days from sowing to first ripe fruit.
For direct-sown crops (beans, carrots, corn), days to maturity counts from sowing — there is no transplant to count from. When a packet is ambiguous, assume transplant-based for anything the packet also says to start indoors.
Sow depth: the 2–3x rule
The rule of thumb used across extension guidance: cover a seed with soil 2–3 times its own diameter. Tiny seeds (lettuce, carrots) barely want covering; big seeds (beans, corn, peas) go deeper. The table gives the standard depths:
| Crop | Sow depth |
|---|---|
| Lettuce, carrots, celery | 1/8 – 1/4 in (lightly covered) |
| Tomatoes, peppers, brassicas | 1/4 in |
| Cucumbers, squash, melons | 1/2 – 1 in |
| Beans, peas, corn | 1 – 1.5 in |
| Onions (seed) | 1/4 – 1/2 in |
| Beets, chard | 1/2 in |
Sowing too deep is a common cause of poor germination — the seedling runs out of energy before reaching light. When in doubt, go shallower and keep the surface moist.
Spacing and thinning
Packets usually give two numbers: how far apart to sow and how far apart to thin. Sow thickly for insurance, then thin ruthlessly to the final spacing — crowded seedlings compete and none thrive. Standard final spacings:
| Crop | Final spacing | Row spacing |
|---|---|---|
| Tomatoes | 18–24 in | 30–36 in |
| Peppers | 12–18 in | 24–30 in |
| Lettuce (head) | 8–12 in | 12–18 in |
| Carrots | 2–3 in | 12 in |
| Beans (bush) | 3–4 in | 18–24 in |
| Corn | 9–12 in | 30 in |
| Broccoli / cabbage | 12–18 in | 24–30 in |
Decoding sow-date phrases
| Packet says | It means |
|---|---|
| “Start indoors 6–8 weeks before last frost” | Sow under cover; count back from your frost date |
| “Sow after all danger of frost has passed” | Direct sow at or just after your last frost date |
| “Sow 2–4 weeks before last frost” | Direct sow into cool soil before the frost date |
| “As soon as the soil can be worked” | Very hardy crops (peas, spinach); sow when soil is not frozen or waterlogged |
| “After the soil has warmed” | Warm-season crops; wait until soil is at least 15°C / 60°F |
Turn the packet into calendar dates
Our Seed Starting Calendar Generator applies these rules to your zone automatically — pick a zone, get sowing dates for 20 crops.
Direct sow vs transplant: how to tell
The packet usually decides for you. If it gives “weeks before last frost” for indoor sowing, start indoors. If it only gives an outdoor sowing window, direct sow. Crops with taproots (carrots, parsnips, dill) resent transplanting — always direct sow those. Warm-season fruiting crops (tomatoes, peppers, eggplant) repay indoor starting in zones 3–7 with a much earlier harvest.
Frequently asked questions
What does “germination 7–14 days” assume?
Soil at the packet’s stated temperature — usually 20–25°C (68–77°F) for warm-season crops. Cold soil stretches germination out or rots the seed; a heat mat for peppers and tomatoes is the single best germination upgrade.
Should I follow the packet or my extension service when they disagree?
Packets are written for a whole continent; your extension service knows your county. Use the packet for the crop’s biology (depth, spacing, maturity) and your extension for the calendar (frost dates, local timing).
What is “thin to” and why does it matter?
It is the final distance between plants after you pull (thin) the extras. Seedlings sown thickly germinate better, but left crowded they stay small. Thin on time — when seedlings have their first true leaves.
Related guides
How Much Soil Do I Need for a Raised Bed?
Volume, bag counts and the 60/30/10 mix for your beds.
Seed Starting Calendar Generator
Your personalised sowing dates, computed from your zone.
Sources: US cooperative extension seed-starting publications (including Cornell and Clemson); standard seed-catalogue guidance. Depths and spacings are typical values — always check your specific variety.
Pingback: When to Start Seeds Indoors: A Zone-by-Zone Guide