Seed Starting Checklist
Growing your own seedlings is six weeks of small decisions that buy months of harvests. Dates, mix, light and airflow decide almost all of it.
Kit coverage
0%
0 of 28 items handled · tick states are saved on this device only.
Schedule the sowings
Dates come from arithmetic on your frost, not from the catalogue.
Choose seed and mix
Fresh viable seed and a sterile fine mix are the whole foundation.
Set up trays and containers
Clean containers with drainage and a label system beat fancy gear.
Sow at the right depth
Depth and temperature decide germination more than seed quality does.
- CORESow at the packet depth: roughly twice the seed's width as a rule Amazon
- Surface-sown light-lovers like petunia and many herbs pressed in, not buried Amazon
- Moisten the mix before sowing so water does not flush seed deep later Amazon
- COREA humidity dome or cover until germination, removed the day it sprouts Amazon
Manage heat for germination
Warmth under the tray is the upgrade that changes germination speed.
- COREA heat mat set for the crop: around 20 to 25 Celsius for most vegetables Amazon
- COREA thermostat control so the mat cannot cook the tray on a hot day Amazon
- Trays off the mat the moment germination finishes across the tray Amazon
- Cool-tolerant crops like lettuce sown without heat to avoid dormancy Amazon
Run light, water and air
After sprouting, the seedling's fate is set by these three inputs.
- COREGrow lights within a hand's width of the leaves, on a 14 to 16 hour timer Amazon
- COREWater from below by tray flooding, letting the surface stay slightly dry Amazon
- A gentle fan a few hours daily for airflow and stronger stems Amazon
- Thin or prick out early so survivors get light and root room Amazon
Harden off and transplant
The week between indoors and the garden decides the first month after.
Raising your own seedlings opens varieties the nursery never stocks, starts the season weeks earlier, and costs pennies per plant. It also fails in three predictable ways, and every one of them happens before the seedling has its second set of leaves.
The first is dates. Sow too early and you own an overgrown, leggy plant that never recovers after transplanting. Sow too late and the season ends before the fruit ripens. The fix is arithmetic: count back from your last frost date using the days-to-transplant on the packet, and the correct week appears by itself.
The second is water and disease. A wet surface, no airflow and recycled soil grow damping off, the fungal collapse that takes a whole tray overnight. Clean trays, fine fresh mix, watering from below and a small fan make it nearly impossible to catch.
The third is light. True leaves need real brightness, and an ordinary windowsill under-delivers in most climates through winter and early spring. Grow lights or a very bright sill decide whether seedlings stand up.
This checklist runs the seedling season from date to garden. It covers scheduling, seed and mix, trays and sowing, heat for germination, light and water after it, thinning, and hardening off into transplanting. For the season around these weeks see our vegetable garden seasonal checklist. Every buyable line links to current options on Amazon.com, and Spare Theory may earn a commission on those links. We never set or verify Amazon's prices.
At a glance
- Every sowing date is counted backwards from your last frost, not forwards from the seed catalogue's enthusiasm.
- Garden soil in a seed tray is how damping off starts. Use a fresh, fine, low-nutrient seed mix.
- Seedlings stretch toward weak light. A bright window is usually the difference between stems and spaghetti.
- A week of hardening off prevents a day of collapse. Indoor leaves are not built for wind yet.
The full list
The same list the builder above ticks. Use it for printing or when JavaScript is off.
Schedule the sowings
Dates come from arithmetic on your frost, not from the catalogue.
Choose seed and mix
Fresh viable seed and a sterile fine mix are the whole foundation.
Set up trays and containers
Clean containers with drainage and a label system beat fancy gear.
Sow at the right depth
Depth and temperature decide germination more than seed quality does.
- CORESow at the packet depth: roughly twice the seed's width as a rule Amazon
- Surface-sown light-lovers like petunia and many herbs pressed in, not buried Amazon
- Moisten the mix before sowing so water does not flush seed deep later Amazon
- COREA humidity dome or cover until germination, removed the day it sprouts Amazon
Manage heat for germination
Warmth under the tray is the upgrade that changes germination speed.
- COREA heat mat set for the crop: around 20 to 25 Celsius for most vegetables Amazon
- COREA thermostat control so the mat cannot cook the tray on a hot day Amazon
- Trays off the mat the moment germination finishes across the tray Amazon
- Cool-tolerant crops like lettuce sown without heat to avoid dormancy Amazon
Run light, water and air
After sprouting, the seedling's fate is set by these three inputs.
- COREGrow lights within a hand's width of the leaves, on a 14 to 16 hour timer Amazon
- COREWater from below by tray flooding, letting the surface stay slightly dry Amazon
- A gentle fan a few hours daily for airflow and stronger stems Amazon
- Thin or prick out early so survivors get light and root room Amazon
Harden off and transplant
The week between indoors and the garden decides the first month after.
Dates are arithmetic, not optimism
The seed catalogue sells dreams in February. Your last frost date turns them into a schedule. Every crop's sowing week is found by counting backwards from that date by the days-to-a-transplantable-seedling, usually four to eight weeks for vegetables.
Sow six weeks early and you inherit a monster: root-bound, leggy, flowered-out on the windowsill. That plant stays stunted for a month after transplanting no matter how well you treat it. Early seedlings rarely catch up. Timely ones never fall behind.
Sow too late costs less dramatically but still bites. Peppers and tomatoes started three weeks late ripen three weeks later, and a short season can close before the last fruit colours.
Build the list once a year: crop, sow date, expected transplant date, tray number. A seed-starting log turns the whole spring into checked boxes rather than a guessing game played against the weather.
The February rule: Few vegetable crops belong under lights in February. Onions and chillies are the honest exception. Everyone else can wait for March without losing a harvest.
Count backwards from your last frost date by each crop's weeks-to-transplant. A seedling sown three weeks early is a worse seedling than one sown on time.
The mix is not soil and never was
Seedlings need three things from their medium: fine texture, drainage, and cleanliness. Garden soil offers the opposite of all three. It is coarse, it compacts, and it carries fungal spores and weed seeds that make trays a lottery.
A commercial seed-raising mix solves the problem for the cost of a bag. It is sterilised, fine and deliberately low in nutrients, because a germinating seed carries its own lunch and rich mix burns fragile roots.
Potting mix is not the same product. Its coarser, richer build suits established plants and is genuinely too strong and heavy for most seed. Read labels and buy the one named for raising seed.
Pre-moisten the mix in a bucket before filling trays. Dry mix repels water at first, and watering an already-sown tray then flushes seed into a crust. Damp, crumbly, loose and labelled is the standard to hit.
The surface crust trap: If a shiny hard crust forms on trays, your water is too forceful and the mix too fine on top. Switch to bottom watering and a light grit sprinkle; crusts lock seedlings out.
Use fresh, dampened seed-raising mix: fine, drained and sterile. Garden soil carries damping off, and rich potting mix burns the seedling it was not made for.
Heat germinates, but it also rots
Soil warmth is the lever that changes germination from a wait into a schedule. Most vegetable seed sprouts fastest between 20 and 25 degrees at root level. A thermostatically controlled heat mat delivers that floor through a cold March.
The same warmth without control becomes a problem. A mat running hot under an unattended tray cooks the surface. Add a closed dome and it invites exactly the fungal losses the method was meant to avoid. The thermostat is not optional.
Heat belongs only to the germination phase. The day the crop is across, trays come off the mat and the priority flips to light and cooler air. Seedlings grown warm and dim stretch; grown bright and cool they stand stocky.
One exception deserves respect: lettuce and some herbs go dormant in high heat. Their packets quietly say to sow cool, and a warm mat is precisely the wrong answer for them.
The dome's one job: A humidity dome keeps the surface damp until germination and nothing after that. Leave it on past the sprout and you have built a fungal nursery with your own hands.
A thermostatted heat mat buys fast, even germination; taken away at sprouting and replaced by light and cooler air, it buys stocky seedlings too.
Light is the growth habit dial
Leggy seedlings are not a variety trait. They are a light measurement. A stem stretching thin toward a dim window is the plant's honest attempt to find brightness that the site never provided.
Most winter and early-spring windowsills under-deliver even in sun-loving climates. Sky light through glass in those months is a fraction of what a seedling stem needs. The sill's single bright direction then bends everything toward it.
An LED or fluorescent grow light a hand's width above the leaves, on a timer for fourteen to sixteen hours, fixes the problem outright. Raising the lights as seedlings grow keeps the distance honest, and cool LEDs let you keep them close without scorching.
Airflow belongs in the same sentence as light. A gentle fan for part of the day removes the still, humid layer above the mix. It toughens stems and completes the bright-cool combination that produces transplant-ready plants.
The quarter turn: Rotate trays a quarter turn daily under any light, sill or lamp. Seedlings bend toward brightness, and an untwisted tray at transplant time is a tray that was turned.
Fifteen hours under a close, cool light plus daily airflow turns stretched windowsill seedlings into stocky transplants. Leggy is a lighting decision.
Water from below, never the surface
Damping off, the collapse that takes a tray overnight, needs a wet surface and still air. Both are optional, and most growers accidentally provide them with a watering can. The fix is to change where the water enters.
Bottom watering floods the tray for ten or fifteen minutes, lets capillary action lift moisture through the mix, then drains fully. The surface stays slightly dry, exactly the condition fungal spores dislike, and roots grow downward seeking the wet.
A mist bottle belongs to the pre-sowing stage and to surface-tied seed only. Once seedlings exist, forceful droplets damage, and a perpetually sodden top layer selects for loss. Slightly dry on the surface is the correct look.
Thirst has a visible language: a seedling wilting with damp mix is root trouble, while wilting with dry mix is simply thirsty. Reading the difference correctly keeps you from drowning a plant you meant to save.
The one-timing rule: Water seedlings when the mix feels light in your hand, not when the clock says so. Tray weight is the most reliable moisture meter in the nursery, and it costs nothing.
Flood-and-drain bottom watering keeps the surface dry, the air unfungal and the roots reaching down. A dry-looking top is the safe look of a healthy tray.
Thin early, prick out kindly
Generous sowing is natural; leaving every sprout is not. Crowded seedlings shade each other, tangle roots and share a pot of mediocre mix until the whole tray underperforms. The first true leaves are the moment to choose.
In modules, keep the strongest single seedling per cell and snip the rest at the surface with scissors. Pulling rivals disturbs the survivor's roots; snipping leaves it untouched. This is cheaper and safer than re-sowing.
Flat-sown seed gets pricked out into module holes once leaves are handleable. Work by the leaf, never the stem; the stem is the plant's future and the leaf is expendable. A dibber or pricking stick opens holes to depth without disturbing roots.
Thinning teaches a habit the whole garden runs on later. The strongest single plant in a cell becomes the strongest single plant at spacing. Crops that finish the season crowded always started the season crowded.
The scissors habit: Keep small scissors on a string at the tray edge. Growers who snip rivals the day they appear never own a crowded tray; growers who mean to get to it eventually do.
Snip to one seedling per cell at first true leaves and prick out by the leaf, not the stem. Crowding is a decision, not an accident.
Hardening off is the last exam
An indoor seedling is a greenhouse animal. Its leaves were built for still, humid, filtered air, and full sun with wind can strip that plant back in a single bright afternoon. The week between tray and garden is its own stage of growing.
The pattern is a ladder. First days: shade, calm and an hour or two. Middle days: morning sun and longer exposure. Final days: a full day outside, and a mild night if forecast allows. Each step teaches the leaves the next condition.
Transplanting on a dull evening rather than a bright noon does half the work for free. Water in gently and shade the new plant for its first two days if the sun is fierce. Expect a pause rather than a panic in growth.
A garden that goes from indoors to outdoors overnight loses plants the season had already paid for. The week of hardening costs one daily walk to the shed. It buys the whole transplant establishment.
The wilt that returns: A transplant wilted at noon that perks by evening is fine. One still wilted the next morning lacked hardening or water at planting; lift its shade cover and check its root zone.
Build exposure a few hours a day, plant out on a dull evening, and expect a settle week. Hardening off is what transplanting stands on.
Count-back sowing guide for common crops
| Crop | Weeks before last frost to sow | Germination warmth | Transplant notes |
|---|---|---|---|
| Tomato | 6 to 8 | Warm, 22 plus | Plant deep for extra roots |
| Pepper and chilli | 8 to 10 | Warmer still | Loves heat; late as any crop |
| Brassicas | 5 to 7 | Moderate | Tolerates a cool start |
| Lettuce | 4 to 6 | Cool only | Barely cover the seed |
| Squash and cucumber | 3 to 4 | Warm | Distest root disturbance |
| Onion | 8 to 10 | Moderate | Long season under lights |
Weeks are typical counts to a transplantable seedling in mild-climate practice. Match your own last frost date and adjust with the packet's days figure.
Key terms
- Damping off
- A fungal collapse of seedling stems at the mix surface, favoured by wet tops and still air; it takes whole trays within days.
- Pricking out
- Lifting flat-sown seedlings into individual modules at the first true leaf stage, holding them by leaf not stem.
- Hardening off
- The staged week of outdoor exposure that rebuilds an indoor seedling's leaves for sun, wind and dry air.
- Seed dormancy
- A seed refusing to germinate under the wrong condition, including lettuce's habit of ignoring heat above the mid-twenties.
- Cotyledon
- The first rounded seed leaves, not true leaves; the stage that tells you thinning and feeding can now wait a day or two.
- Capillary watering
- Moisture drawn up through the mix from a flooded tray base, leaving the surface dry while roots get their drink.
Tips and common mistakes
Field-tested tips
- Fill and dampen all trays the evening before sow day. Morning-you with a dry mix and eager seed makes hasty, uneven decisions.
- Sow two extra chilli seeds per cell; they are slow and sometimes duds. Snip the weaker seedling the moment both hold a leaf.
- Write the sowing date on the label in permanent marker on the day. Every label dated later is a label dated wrong.
- Stand trays on feet or inverted crates, never on cold floors. Root-zone chill is the quiet variable that stalls a whole shelf.
Common mistakes
- Sowing everything in February because the catalogue arrived, then carrying leggy monsters to the garden.
- Reusing last year's mix or trays, and meeting damping off in a tray that looked perfect on day four.
- Watering the surface of trays daily, keeping the top layer constantly wet for fungal spores to find.
- Skipping the hardening week and transplanting directly from a warm room into bright wind and sun.
Frequently asked questions
When should I start seeds indoors?
Count back from your last spring frost date using each crop's weeks-to-transplant figure. Tomatoes need six to eight weeks of indoor life and brassicas five to seven. Squash want only three or four because they hate root disturbance, and peppers take eight to ten because they are simply slow. Very few vegetable crops genuinely belong under lights before March in most climates.
Do seedlings really need grow lights?
In most climates and seasons, yes, if you want stocky transplants rather than leggy ones. Winter and early-spring window light through glass is a fraction of what growing seedlings need, and every seedling bends toward the one bright direction. A cool LED or fluorescent strip fixes that. Kept a hand's width above the leaves on a fourteen to sixteen hour timer, it turns weakness into straight, thick stems.
Why are my seedlings falling over and dying?
That is almost certainly damping off: a fungal stem collapse at the mix surface. It is driven by a constantly wet top layer, still air and unclean trays or mix. Prevent it with fresh sterile seed mix, cleaned trays and bottom watering that keeps the surface slightly dry. Remove the humidity dome the day of sprouting and run a gentle daily fan. Lost trays are rebuilt cheaper than a guilty conscience.
Can I use garden soil or potting mix to start seeds?
Neither. Garden soil carries fungal spores and weed seed and compacts into a brick around fragile roots. Standard potting mix is too coarse and too rich for seed, whose first leaves run on the seed's own energy. Buy a fine, sterile seed-raising mix, dampen it before filling, and reserve the richer potting mix for established plants and later potting on.
How do I harden off seedlings properly?
Build outdoor exposure a little each day over about a week. Start with one or two hours of shade and calm, then longer, then gentle morning sun. Finish with a full day outside and a mild night if the forecast allows. Then transplant on a dull evening, water in, and shade the plant for its first two days if the sun is fierce. The week exists to rebuild leaves built for indoor air.
Is seed starting worth it for a small garden?
For a handful of tomato, pepper and brassica transplants, buying them is often the saner choice and costs little. Raising your own earns its keep when you want varieties the nursery never carries. It pays when you need many plants of one crop for serious harvests. Sometimes the six weeks under lights is itself part of the enjoyment. The arithmetic rewards the second and third reasons equally.
About this checklist
- Prepared by
- Spare Theory editorial · research-based
- Last checked
- 2026-10-16
How we choose what earns a spotWe build the seedling schedule from frost-date count-back arithmetic and standard germination temperature ranges, following horticultural nursery practice for mix, watering and hardening. Temperature and timing figures are marked indicative against each seed packet. We state no prices, endorse no brand, and defer local frost dates and variety timing to regional seed suppliers and extension services.
Reference basisLand-grant university cooperative extension guidance on home seed startingRoyal Horticultural Society practical guide to raising plants from seed