Garden Guides · Michigan
When to Start Seeds Indoors: Four to Twelve Weeks
The indoor window is four to twelve weeks depending on the crop — and the cost of starting early is a leggy seedling, not an earlier harvest. Guide for Michigan, where the dominant grass is Kentucky Bluegrass.
In this guide7 sections
The question is usually asked as though there were one answer, and there is not. UNH Extension’s range is four to 12 weeks before transplanting, depending upon the speed of germination — a span of two months, because the number belongs to the seed rather than to the season.
What that range hides is the direction of the error. Starting late costs you a few weeks of harvest. Starting early costs you the plant.
The offset belongs to the seed
UNH’s phrasing is worth reading closely: the period ranges from four to 12 weeks depending upon the speed of germination. Not depending on the crop’s value, its hardiness, or how impatient the gardener is. On how long the seed takes.
That is why the seed packet is the authority and a generic chart is not. A crop that germinates in four days and grows fast needs four weeks of indoor time to reach transplant size. One that takes two weeks to come up and then grows slowly needs ten or twelve. Both are being transplanted on the same day.
In Michigan that day is anchored to a last frost around Late May (Upper Peninsula), early May (Metro Detroit/Grand Rapids). Work backwards from it: twelve weeks before it is the earliest anything should be sown indoors, and four weeks before it is the latest for the fastest crops. Anything sown earlier than the twelve-week mark is being asked to hold in a tray for longer than a tray can hold it. Your USDA range is 4b–6b.Where to get the date this all hangs on
Every number on this page is measured from one date, and the state-wide range we carry for Michigan — a last frost around Late May (Upper Peninsula), early May (Metro Detroit/Grand Rapids) — is a range because a state is not a point. Where it spans several weeks, sowing on the state figure means half the state sows at the wrong end of it.
MSU Turf at Michigan State University Extension publishes for Michigan at a finer grain than we can, and a county-level frost date is the one piece of information worth looking up once and writing on the shelf where the trays live. It does not change from year to year, and everything above is an offset from it.
Three temperatures, not one
This is the part that most seed-starting advice compresses into one number, and the compression is what produces the classic failure.
| Stage | Temperature | Source |
|---|---|---|
| Germinating, most crops | 65–75°F soil | Utah State |
| Germinating, cool-season crops | 55–65°F soil | Utah State |
| Growing on, day | 65–70°F | UNH |
| Growing on, night | 55–60°F | UNH |
Read the third and fourth rows against the first. The tray is supposed to get cooler after germination, not stay where it was. UNH’s instruction is to move the flats to a light, airy, cooler location once the seedlings are up.
The cool-season exception in row two is the one that catches people running a single heat mat for a whole shelf: broccoli, cabbage and spinach prefer 55–65°F and are being pushed by the 75°F that suits a pepper.
UNH also gives one crop’s full tolerance, which is useful for calibrating how much of this is fussiness: tomato germinates anywhere from 50°F to 95°F, with 80°F the optimum. The seed will come up over a 45-degree span. It will just take much longer at the ends of it.
For the germination number, and only that
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The thermostat is the product. Utah State gives two different germination temperatures for two groups of crops, and a mat without a setpoint can only produce one.
Best for: Holding one tray at a set germination temperature — 80°F for tomato, 55–65°F for the cool-season crops — rather than at whatever the room gives
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View on AmazonAnd then take the tray OFF it. The mat is for germination; UNH’s instruction after emergence is a cooler location, and a tray left on heat is half of why the next section exists.
Why seedlings get leggy
A tall, pale, floppy seedling is not a variety problem or a fertilizer problem. It is the tray being kept at germination temperature under insufficient light, and both halves have published numbers.
The light half. UNH: position the plants six inches from the tubes and keep the lights on about 16 hours each day, using two 40-watt cool white or full-spectrum fluorescent tubes. Distance is the variable people get wrong — light intensity falls off sharply with it, so a fixture left at its original height while the seedlings grow is delivering less and less to a plant that is stretching to reach it.
The temperature half. Leaving the tray on the heat that germinated it. The plant grows fast in warm air and cannot build stem in proportion, which is exactly the stretched seedling. Dropping the night temperature to 55–60°F slows the top growth relative to the stem and produces the stocky plant you actually want.
The indoor period, start to finish
Count back from your own last frost, per cropFour to twelve weeks
UNH's range, set by germination speed. Read the packet rather than a generic chart — the same transplant date is reached from four different sowing dates on a shelf with four crops on it.
Sow at two to three times the seed's diameterSowing day
Utah State's rule, which scales without a table. The exception: some seeds need light to germinate — snapdragons, petunias and basil among them — and those go on the surface, not under it.
Germinate warm: 65–75°F, or 55–65°F for cool-season cropsUntil the seedlings are up
Utah State's two ranges. Broccoli, cabbage and spinach are the ones that want the cooler figure, which matters if a single heat mat is running a whole shelf.
Then move the tray COOLER and under lightAs soon as they emerge
UNH: a light, airy, cooler location at 65–70°F day and 55–60°F night. This is the step that separates a stocky seedling from a leggy one, and it is the step most often skipped.
Lights six inches away, about 16 hours a dayEvery day of the indoor period
UNH's figures, on two 40-watt tubes. Raise the fixture as the plants grow so the distance stays six inches rather than becoming twelve.
Water from below, evenly moist and not waterloggedThroughout
Utah State: bottom watering or a spray bottle. A watering can moves small seed and compacts a fine mix; bottom watering also pulls roots downward, which is what makes a plug hold together when you lift it.
Harden off one to two weeks before the bedThe fortnight before planting
UNH's range. Wind matters more than sun — it is the stimulus a plant raised under a fixed light has never had.
The fixture shape UNH describes
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The modern equivalent of the two 40-watt tubes UNH specifies — and the format matters more than the wattage, because the whole job is hanging it six inches away and raising it as the seedlings grow.
Best for: Hanging two four-foot strips six inches over a tray, and raising them as the seedlings grow — the adjustment that stops the stretching
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View on AmazonSkip the 200-watt panels with 660 nm red and infrared that dominate this category. That spectrum is aimed at fruiting, and a tray of six-week-old seedlings is not fruiting. The specification to match is UNH’s: linear, close, and sixteen hours.
Sowing depth and water
Utah State’s depth rule is the useful kind, because it needs no chart: plant seeds at a depth of 2 to 3 times the seed’s diameter. A tomato seed goes a fraction of an inch down; a squash seed goes an inch. The same sentence covers both.
The exception is a category rather than a list: some seeds require light to germinate, including many small-seeded plants like snapdragons, petunias and basil. Those are pressed onto the surface. Burying them is a silent failure — the seed is viable, the conditions are right, and nothing comes up.
On water, Utah State asks for the mix evenly moist but not waterlogged, with bottom watering or a spray bottle. Both methods exist because a watering can does two things to a seed tray that nothing else does: it moves the seed out of position, and it packs the surface of a fine starting mix into a crust.
What bottom watering actually needs
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Best for: Cells that drain sitting in a tray that does not — and domes that come OFF at emergence, not after
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View on AmazonThe end of the indoor period
The indoor phase ends on a condition rather than a date, and the condition is outside rather than in the tray. A hardened-off seedling still needs somewhere to go — and for a warm-season crop that means soil that has actually warmed, not merely a frost date that has passed.
What decides when the tray is finished
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Best for: Checking bed temperature at planting depth before the hardened-off seedlings come out of the tray
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View on AmazonThat second gate is a whole subject of its own, and it is where most of the seed-starting effort is either banked or lost — when to plant tomatoes covers the 60°F soil threshold and why the frost date on its own is not enough.
The other useful thing to know about this list is what is not on it. When to plant garlic is the clearest counter-example: a clove needs weeks of cold in the ground to form a bulb, so starting it in a tray does not merely waste the tray, it costs you the crop. Sowing indoors is a technique for crops whose season is too short outdoors, not a default that improves everything.
The two crops also run off opposite ends of the same calendar, which is the clearest way to see why one wants a tray and the other wants the ground. This page counts backwards from Michigan's last frost — Late May (Upper Peninsula), early May (Metro Detroit/Grand Rapids) — because that is when the tray's occupants can go outside. Garlic counts forwards to the first frost — Mid-September (Upper Peninsula), early October (Grand Rapids), late October (Metro Detroit) — because it needs to root before the ground closes. One date opens your season and the other shuts it, and in Michigan they are far enough apart that a single "planting time" is not a useful idea.
Is an indoor start even worth it in Michigan?
Worth asking honestly, because the answer is not always yes. Michigan runs from a last frost around Late May (Upper Peninsula), early May (Metro Detroit/Grand Rapids) to a first frost around Mid-September (Upper Peninsula), early October (Grand Rapids), late October (Metro Detroit). Everything between the two is the outdoor season, and the point of an indoor start is to borrow weeks the season does not have.
Where that span is long, several crops can simply be direct-sown and the tray is a convenience rather than a necessity. Where it is short, the indoor period is not optional — it is the only way a long-season crop finishes at all, and the twelve-week end of UNH's range is doing real work rather than being a maximum nobody uses.
And these are Michigan's own spring tasks from our state research, because the seed trays come out of the basement in exactly the fortnight the rest of the yard starts asking for attention:
- Wait for active growth before the first feeding — MSU weights the year to autumn
- Pre-emergent late April (south) to mid-May (north) on soil temperature
- Curative grub treatment is possible in March or April here, because European chafer resumes feeding early
The fortnight outdoors, in Michigan
Hardening off happens outdoors, so it happens in your weather rather than in a controlled tray. The pattern here: Even (~32–36 in/year) with heavy lake-effect snow on the west side and prolonged winter snow cover inland
That decides what the seedlings are actually being hardened against. A wet spring hardens plants against damp and low light rather than against sun, and a tray left out through it can rot at the soil line before it ever meets a dry day. A dry, windy one hardens them fast and can also flatten them, because a seedling raised under a fixed lamp has no stem tissue built for wind at all.
Local humidity is Moderate — humid July and August, with lake-effect cloud and snow on the western side
Drier air is easier on a seed tray indoors and harder on it outdoors: a hardened-off seedling in dry wind loses water faster than its unestablished roots can replace, so the fortnight outdoors here needs watching more closely than the six weeks under the lamp did.
And the bed it is heading for: Fast on the sands, which is why Michigan's fertilizer guidance is written around leaching. Slow-draining ground stays cold after the frost date has passed, which is the most common reason a perfectly grown tray of seedlings sits still for a fortnight after planting.
The honest summary
Four to twelve weeks before transplanting, chosen by how fast the seed germinates rather than by a chart. Germinate at 65–75°F, or 55–65°F for the cool-season crops, then move the tray somewhere cooler — 65–70°F by day, 55–60°F at night — and under light six inches away for about sixteen hours. Sow at two to three times the seed’s diameter, leave the light-germinating seed on the surface, water from below, and harden off across one to two weeks. The failure mode this prevents is not a dead seedling; it is a tall pale one that never recovers the head start it was supposed to have.
Where the tray is heading
The indoor period only pays off if there is somewhere ready to receive it, and that is a construction question as much as a calendar one. Raised bed soil mix covers the bed that reaches transplanting temperature earliest — which is the only way to spend the weeks the tray bought you rather than lose them waiting for cold ground.
Common questions
Frequently Asked Questions
How many weeks before planting should I start seeds indoors?
UNH Extension gives the honest range rather than a single number: the period may range from four to 12 weeks before transplanting, depending upon the speed of germination. That is a wide span because it is really a property of the seed rather than of the calendar — a fast, vigorous crop needs four weeks of indoor time and a slow one needs twelve. The date you count back from is your own last frost, which is why a national seed-starting calendar is close to useless.
What temperature do seeds need to germinate?
Utah State's general figure is that most plants will readily germinate at soil temperatures between 65°F and 75°F, with an important exception: cool-season crops such as broccoli, cabbage and spinach prefer soil temperatures from 55°F to 65°F. UNH adds the single-crop detail that shows how wide a seed's tolerance can be — tomato germinates anywhere from 50°F to 95°F, with 80°F the optimum. Warm is for germinating; it is not for growing.
Why are my seedlings tall and pale?
Almost always too warm and too little light, and the temperature half is the one people miss. UNH's instruction after germination is to move the flats to a light, airy, cooler location — 55°F to 60°F at night and 65°F to 70°F in the daytime. Leaving a tray at germination heat produces exactly the stretched, pale seedling everyone recognises. The light half is a distance problem: UNH positions the plants six inches from the tubes and keeps the lights on about 16 hours each day.
How far should grow lights be from seedlings?
Six inches, on UNH's figure, using two 40-watt cool white or full-spectrum fluorescent tubes, with the lights on about 16 hours a day. Distance matters more than wattage here — light falls off sharply with distance, so a fixture eighteen inches above a tray is delivering a fraction of what the same fixture delivers at six, and the seedlings stretch toward it. Raise the fixture as they grow rather than leaving it fixed.
How deep do I sow seeds?
Utah State gives a rule that works for anything: plant seeds at a depth of 2 to 3 times the seed's diameter. That scales correctly across a tomato seed and a squash seed without a chart. The exception is worth knowing — some seeds require light to germinate, including many small-seeded plants like snapdragons, petunias and basil, and those are pressed onto the surface rather than buried.
How do I water seed trays without washing the seed out?
Utah State's instruction is to keep the seed-starting mix evenly moist but not waterlogged, using bottom watering or a spray bottle. Both exist for the same reason: a watering can moves small seed out of position and compacts the surface of a fine mix. Bottom watering also draws roots downward rather than keeping them at the surface, which is the difference between a plug that holds together at transplant and one that falls apart.
How long do I harden seedlings off?
UNH says to start hardening plants one to two weeks before planting in the garden — a slightly longer window than the one week Penn State and Clemson give for tomatoes specifically. Take the wider range as the safer plan: the cost of an extra few days outdoors is nothing, and the cost of skipping it is a seedling that spends its first week in the ground repairing rather than growing.
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Verify our work
📚 Sources & Further Reading
This guide is written against published land-grant university extension research and USDA data. Verify local details with your own state's extension office — and always follow the product label.
- [1]Starting Plants From Seed [fact sheet] — University of New Hampshire Extension(extension.unh.edu)
- [2]Starting Vegetable Seeds Indoors: III. Germination — Utah State University Extension(extension.usu.edu)
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