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The seed packets give you a range that looks like advice: sow six to eight weeks before the last frost. It is advice the way a nutritional label is advice — technically true, practically useless, because the number that matters has been averaged away.
Two things decide whether a seedling is ready to go out. How long the plant takes to grow, and how long the season you can hand it actually is. Neither one is a fixed number of weeks, and the gap between them is where most home seed starting goes wrong.
The short answer
- Count back from your target transplant date, not from a week count. Two countdowns, and the earlier one wins.
- A seedling's pace is set by temperature and light together. Warm room, dark corner — the dark corner wins.
- If you must start early, you must extend the light as well as the heat. Heat alone makes a stretched, weaker plant, not an earlier one.
- Aim for a daily light integral of 10–15 across the seedling phase. Anything under about 8 and you are growing etiolation on the calendar.
Why the six-week rule breaks
“Six weeks before last frost” quietly assumes two things: that your season gives the plant six weeks to grow, and that your growing environment lets it do so in six weeks. In northern climates with a short summer, the plant often needs more time than the season has. In a cool basement in spring, the seedling may take twice as long as the packet implies.
Both halves are knowable. The useful calculation is a countdown in two stages:
| Stage | What you need from the seed packet | What you do with it |
|---|---|---|
| Sowing → transplantable | Days to emergence + days to a moveable size (usually on the packet, or from a variety catalogue) | Count back from the date you want the plant in the ground |
| Transplant → harvest | Days to harvest in the open ground | Count back from your first expected frost date |
| Decision | — | Sow on the earlier of the two dates |
The second countdown is the one people forget, and it is the reason the six-week rule survives. If your variety needs 70 days to harvest and your frost date leaves you 65 days, you have already lost — sowing six weeks early just moves the failure earlier. Short-season varieties exist precisely to close this gap.
The one-line version. If the two countdowns disagree, the packet's week range was wrong for your season, not your technique.
What actually sets the pace: two inputs, not one
Seedlings do not run on calendar time. They run on accumulated warmth and photons, and they run at whichever of the two is scarcer. This is the part the week-count rule hides completely.
In practical terms: a seed that wants 70°F to germinate will sit in 65°F soil for several days longer than it would in 75°F, and it will not compensate later. Once it is up, the rate of leaf area increase tracks available light far more closely than it tracks temperature. A tray on a warm shelf under a full canopy in early May is slower than a tray on a cooler shelf under a grow light.
Full-spectrum LED bar on a stand, 2 ft
This is the piece that makes an early sowing possible at all. A seedling under a lamp that only produces enough photons to survive will stretch, and stretching is not merely ugly — it produces a plant with weak stem tissue that has to be hardened off anyway. The stand matters more than people expect: light falls off with the square of distance, so a lamp two inches above a tray delivers several times the dose of the same lamp eight inches up.
For what the daily light integral should be at each stage, and how to measure what your setup is actually delivering rather than guessing, see how much light houseplants need.
What we liked
- Bar form gives even coverage across a shallow tray, with no hot spot in the middle
- Full-spectrum mix keeps stems compact instead of stretching for the red end
- Cheap enough to leave running on a timer for a full seedling season
What we did not
- Bare fixture needs a stand or hang kit, which is an extra purchase
- Bar output drops off fast above about a foot — height is not adjustable on the budget models
- No dimmer, so you cannot dial it back for the first week after emergence
Why a window is not a light source for seedlings
Windows lose light in three separate ways, and the third is the one that surprises people. Glass reflects a little at each surface. The frame and sill block the light that arrives at an angle rather than straight down. And on a windowsill, the plant sits centred in a patch of light that is bright at noon and useless at eight in the morning.
The deeper problem is that seedlings need hours, and a windowsill cannot supply hours. A plant receiving 200 μmol/m²/s for eight hours accumulates a daily light integral of about 5.8 — well under what a compact seedling needs. On a bright day it may double that. That day-to-day swing is what produces the “some of the tray is fine and half of it is leggy” pattern, because the plants facing the glass get 14 usable hours and the ones at the back of the same tray get five.
| Location | Typical PPFD, overcast day | DLI if you left it 12 hours | Verdict for seedlings |
|---|---|---|---|
| North window | 20–60 | 0.9–2.6 | Far too low — etiolation within a week |
| East window, clear | 100–250 | 4–11 | Marginal; only works in shallow trays |
| South window, clear | 250–600 | 11–26 | Workable, but the swing between days is the problem |
| Under a 2 ft LED bar, 6–10 in | 150–350 at leaf level | 7–15 over 12 hours | Reliable — this is the target band |
The failure to recognise. Leggy seedlings are nearly always a light-hours problem wearing a light-intensity costume. Moving the tray half an inch closer rarely fixes it, because the difference between a stretch and a stop is usually a factor of two or three in the daily total, not in the instantaneous reading.
How to work out your own sowing date
- Write down your last expected frost date. Not the average — pick a date you are willing to be wrong about late, usually two weeks after the long-run average.
- Count back the days to harvest for the variety. This is the blocker in short seasons.
- Count back the days to transplant from your target transplant date, which is frost date minus whatever you intend to leave the plant in its pot.
- Sow on the earlier of the two dates, less two or three days for the soak and germination period if you are starting indoors.
- Adjust for your light. If seedlings are stretching where you have them, add a lamp and put the lamp on a timer rather than moving the tray closer.
Frequently asked questions
How do I know when to start seeds indoors?
Count backwards from the date you want the plant in the ground. Take the variety's days-to-transplantable from your target transplant date, and its days-to-harvest from your first expected frost. Sow on whichever of the two is earlier. A generic week count only holds when your season happens to match the one the rule was written for.
Can I start seeds earlier by using a heat mat?
A heat mat warms the soil, which speeds germination, so earlier sowing does become possible. But warming is only half of it. Without added light earlier sowing produces a stretched plant, not an earlier one, and stretching costs you more time in hardening off than the earlier sowing gained.
What does a seedling actually need from a light source?
Both intensity and hours, because what the plant integrates is the product of the two. A daily light integral of 10 to 15 is a reasonable target through the seedling phase, and anything below roughly 8 will show as stretched stems before it shows as pale leaves.
Why are my seedlings stretching even though they are not far from the window?
Light falls off with the square of the distance, and the signal that stops elongation is red light absorbed by phytochrome rather than brightness as such. A tray receives enough photons to photosynthesise and not enough to be told to stop, so it reaches upward. The real fix is hours from a close lamp on a timer, not moving the tray half an inch.
Related reading
- How much light houseplants need — PPFD, DLI and how to read the numbers
- Best light meters for plants — what to measure and where
- Best grow lights for indoor herbs — for a kitchen bench directly under a bar
The bottom line
Stop counting weeks from the last frost and start counting days back from your own two deadlines. Then look at what the seedlings are actually receiving for a full day rather than at what they look like at noon, because the failure mode of too little light is not pale leaves — it is a stem that grows before the plant is ready to stand up.
If your season is short and the countdown says you have already missed it, that is a variety problem, not a technique problem. Buy a short-season line and start again rather than forcing an early sowing into a dark room.