When to Start Seeds Indoors: Four to Twelve Weeks

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 Mississippi that day is anchored to a last frost around Late March (north) to late February (coast). 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 7b–9a.

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 Mississippi — a last frost around Late March (north) to late February (coast) — 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 Extension Lawn and Garden at Mississippi State University Extension Service publishes for Mississippi 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.

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.

And 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.

Skip 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.

How many cells you actually need is a season-length question. Mississippi runs from a last frost around Late March (north) to late February (coast) to a first frost around Early November (north Mississippi) to early December (Gulf Coast), and the shorter that span, the more of the garden has to start indoors rather than being sown in place — which is the difference between one tray and a shelf of them. Count the crops that cannot be direct-sown in your season before buying cells for the ones that can.

The 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.

That 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 Mississippi's last frost — Late March (north) to late February (coast) — because that is when the tray's occupants can go outside. Garlic counts forwards to the first frost — Early November (north Mississippi) to early December (Gulf Coast) — because it needs to root before the ground closes. One date opens your season and the other shuts it, and in Mississippi they are far enough apart that a single "planting time" is not a useful idea.

Is an indoor start even worth it in Mississippi?

Worth asking honestly, because the answer is not always yes. Mississippi runs from a last frost around Late March (north) to late February (coast) to a first frost around Early November (north Mississippi) to early December (Gulf Coast). 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 Mississippi'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:

• Pre-emergent late February on the coast, mid-March up north
• Lime the piney-woods sand per soil test; Delta ground rarely asks for it
• Hold off nitrogen on centipede — feeding it like bermuda is how it dies

The fortnight outdoors, in Mississippi

Hardening off happens outdoors, so it happens in your weather rather than in a controlled tray. The pattern here: Very wet (~55–60 in/year), spring-heavy with tropical systems in late summer and a short dry stretch most Augusts

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 The highest sustained heat-and-humidity load of any state we cover — disease is not seasonal here so much as continuous from May to October

In humid air, damping-off is the indoor risk that carries outdoors with the tray. Airflow is the answer rather than less water — UNH's phrase for the post-germination location is light and AIRY, and the second word is doing as much work as the first.

And the bed it is heading for: Piney-woods sand drains fast and leaches nutrients through. Free-draining ground warms with the air, so the tray and the bed tend to be ready at about the same time.

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.