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HorticultureLesson 3 of 9

Seed starting

Breakout About 16 min readNew to you

What you’ll learn

  • How to find your frost dates and count back to sowing day
  • A step-by-step indoor setup: mix, trays, heat and lights
  • How to stop damping-off and harden seedlings off
  • When to sow straight in the garden, by soil temperature
  • How long seed keeps, and which grocery-store produce will really grow

The short version

  1. Know your frost date, and use the cautious one. The "average" last frost is the date half of years still freeze after. For tomatoes and peppers, plan around the 10% date (a freeze after it only 1 year in 10) from NOAA's 1991–2020 normals.

  2. Count back from the day you'll plant out. Roughly: onions 10–12 weeks, peppers and eggplant 8–10, tomatoes 6–8, cabbage family 4–6, lettuce 3–6, squash and cucumbers 2–4. Sowing too early gives tall, root-bound plants that do worse.

  3. Fresh seed-starting mix, clean trays with drain holes, warm water. Garden soil and dirty trays bring damping-off, the disease that topples seedlings, and it can't be cured once it starts.

  4. Warm to sprout, cool and bright to grow. About 65–75°F (75–85°F for tomatoes and peppers) until they come up, then about 60–70°F under lights 14–16 hours a day. A windowsill alone makes leggy seedlings.

  5. Harden off for 1–2 weeks before planting out, starting in shade for an hour or two a day.

  6. Sow outdoors by soil temperature, checked 4 in deep at 8 a.m.: peas and carrots from about 40°F, corn from 50°F, beans and squash from 60°F. Beans, corn, squash, carrots and peas are usually best sown straight in the ground.

  7. Keep seed dry, sealed and cool. Temperature (°F) plus humidity (%) under 100. Test anything older than a year on a damp paper towel.

  8. Grocery store: scallions, celery bases and sweet potato slips work. Don't plant store garlic (usually sprout-treated, poorly adapted, possible disease carrier), and expect store zucchini and cucumber seeds not to grow.

When to start: frost dates and the countdown

What a frost date really means

NOAA's National Centers for Environmental Information (NCEI) publish the official U.S. frost dates as part of the 1991–2020 Climate Normals. For each weather station they give the last spring and first fall 32°F date at probability levels from 10% to 90%.

  • 50% date (the "average" last frost): half of years still have a freeze after it.

  • 10% date (the cautious one): only 1 year in 10 has a freeze after it. Use this for tender plants.

Example stationLast spring freeze, 50%Last spring freeze, 10%First fall freeze, 50%
Boston Logan Airport, MAApr 4Apr 17Nov 9
Boulder, COMay 6after May 18—
Modesto, CA—about Mar 3—

How to find yours: search NCEI's Annual/Seasonal Normals tool for your nearest station and pick "10% probability date of last 32F occurrence or later," or ask your county extension office. In the West, elevation and the coast change dates sharply over short distances, so use the nearest station. Hardiness zones describe winter lows, not frost timing.

The countdown

Pick your planting-out date, then count back:

CropWeeks indoors before planting outPlant out
Onions, leeks10–12 (some say 14–15)3–4 weeks before last frost
Peppers, eggplant8–10About 2 weeks after last frost, warm soil
Tomatoes6–8 (5–6 for stocky plants; 7–9 in Alaska)About 2 weeks after last frost
Broccoli, cabbage, cauliflower, kale4–62–4 weeks before last frost
Lettuce3–62–4 weeks before last frost
Cucumbers, squash, melons2–4 (if at all)About 1 week after last frost (melons and winter squash 2+)

Sources disagree most on brassicas and onions. When in doubt, choose the shorter time: an over-age transplant does worse than a young one.

Example: your 10% last-frost date is April 17.

  • Tomatoes go out about May 1, so sow them around March 6–20.

  • Peppers go out about May 1, so sow them around Feb 20–Mar 6.

  • Broccoli goes out about March 27, so sow it around Feb 13–27.

Diagram coming soonCountdown calendar
Figure 1. Count back from the day you plant out.
Sources: NOAA National Centers for Environmental Information, NOAA Climate.gov and 7 more

Starting seeds indoors

What to start indoors and what to sow outside

Start indoorsEither waySow directly outside
Tomatoes, peppers, eggplant, cabbage family, lettuce, onions, leeks, most herbsSquash, cucumbers, melons, sweet corn, beans: only as 2–4-week transplants in big cellsCarrots, beets, radishes, peas, beans, corn, squash

Squash-family roots are "extremely intolerant of being disturbed". Carrots should "never" be started in pots.

Supplies

ItemWhat to look for
Seed-starting mixFresh soilless mix (peat or coir with perlite or vermiculite). Not garden soil, which is about 75 lb per cubic foot against about 10 for mix and brings disease.
ContainersDrainage holes, at least 2 in deep. Cell size: greens 3–6 sq in; squash and cabbage family 4–9; tomato, pepper, eggplant 9–16
Clear domeHolds moisture until seeds come up
Heat matWarms mix to 70–75°F; speeds sprouting, cuts damping-off
Lights + timerLED or fluorescent shop lights, 14–16 hours a day
Small fanGentle air movement for sturdier stems
Water-soluble fertilizerUsed at ¼–½ strength
VermiculiteLight cover for small or light-needing seeds

Costs: none of our sources gave prices, so none are listed yet.

Soil blocks are compressed cubes of mix with no pot. Roots stop at the block's edge instead of circling, so plants suffer less transplant shock. Eliot Coleman uses ¾-in blocks to germinate seeds and 2-in blocks to grow them on.

Step by step

  1. Clean reused trays. Scrub them, soak 30 minutes in 1 part household bleach to 9 parts water, then rinse well. Bleach can make old plastic brittle.

  2. Wet the mix first. Stir water in until a squeezed handful holds together without dripping. Dry peat repels water.

  3. Fill and sow about 2–3 times as deep as the seed is wide. Don't sow thickly, because crowding invites disease. For big seeds (squash, beans, corn), sow 2–3 per cell and keep the strongest.

  4. Light-needing seeds (lettuce, celery): press them onto the surface or dust with vermiculite.

  5. Cover and warm. Put on the dome and set the tray on a heat mat at about 70–75°F, or 75–85°F for tomatoes, peppers and eggplant. Lettuce germinates poorly above about 75°F.

  6. Watch daily. Remove the dome as soon as seedlings appear. Take the tray off the heat mat.

  7. Lights on right away, 14–16 hours a day with a dark period, and keep them close (see Light and heat).

  8. Grow them cooler: about 60–70°F, nights 60–65°F, and cool-season crops near 55°F at night.

  9. Water with room-temperature water (68–77°F) so the mix stays moist but not soggy. Cold water slows growth and invites damping-off.

  10. Feed once true leaves show: ¼-strength fertilizer weekly (USU uses ½ strength). If the mix already has fertilizer, wait several weeks.

  11. Run a fan 5–10 minutes or more once to several times a day, or brush your hand over tomato tops.

  12. Pot up into a bigger container when the first true leaves are out or roots fill the cell. Set tomatoes deeper, because they root along the buried stem.

  13. Harden off for 1–2 weeks (see Hardening off), then plant out.

Diagram coming soonIndoor setup cutaway
Figure 2. An indoor setup: heat mat, dome, lights on a timer and a small fan.
Sources: University of Maryland Extension, Johnny's Selected Seeds and 8 more

Light and heat

Light

A sunny window "seldom" gives enough light. Seedlings stretch, lean and go pale.

FixtureDistance above leavesHours
Fluorescent tubes or LED fixtures (most extension guides)2–6 in; raise as plants grow14–16
T5 high-output fluorescent shop light9 in14
4-ft LED shop light, 3,000+ lumens, "daylight"18 in14

Target strength is about 200–400 µmol/m²/s (PPFD), if you have a meter. Light distances differ because fixtures differ:

  • Stretching: lower the light.

  • Bleached or pale leaves at the top: raise the light.

Heat

StageTemperature
Germinating most crops65–75°F mix
Germinating tomato, pepper, eggplant75–85°F
Germinating lettuce60–68°F (poor above 75°F)
Growing on60–70°F; nights 60–65°F
Diagram coming soonLeggy vs stocky seedling
Figure 3. Too little light makes tall, pale seedlings; good light makes short, sturdy ones.
Sources: University of Maryland Extension, Johnny's Selected Seeds and 4 more

Damping-off (the seedling killer)

What it looks like:

  • Seedlings pinch at the soil line, fall over and die.

  • Or seeds rot before they come up.

It is caused by soil fungi and water molds (Pythium, Rhizoctonia, Fusarium, Phytophthora).

What favors it: cool, wet mix, overwatering, low light, crowding, cold water and too much fertilizer.

Prevention is the only cure. Once a tray has it, throw out the seedlings and the mix.

  • Use new mix, and scrub and bleach reused trays.

  • Use clean tools, and keep hose ends off the floor.

  • Heat the mix to 70–75°F, and water with warm water.

  • Sow thinly, use good drainage, and give strong light and moving air.

Sources: University of Minnesota Extension, UConn Extension and 4 more

Hardening off and transplanting

Hardening off (7–14 days)

  1. Day 1: put plants outdoors in shade, out of the wind, for about an hour. SDSU uses 3–4 hours in the warmest part of the day.

  2. Each day: add an hour or two outside and a little more sun.

  3. Water a bit less, but don't let them wilt. Skip fertilizer.

  4. Bring them in at night:

    • cool-season crops if frost is coming
    • warm-season crops if it will drop below 50°F
    • any tender seedling on days below 45°F or with strong wind or heavy rain

  5. Last 1–2 days: leave them out overnight if the night looks mild.

Points to watch:

  • Cabbage, broccoli and cauliflower going out before the last frost need hardening most.

  • Warm crops planted after frost into soil above 60°F need it less.

  • Don't overdo it. Cauliflower stressed to a stop can make "thumb-sized heads".

  • Squash-family plants shouldn't be hardened with temperatures below 55°F.

Planting out

  1. Pick a cloudy, calm day or early morning or late afternoon.

  2. Water the plants a few hours before.

  3. Peat or coir pots: tear off any rim above the soil, which wicks water out of the root zone, and slit or remove the bottom.

  4. Leggy tomatoes: lay them in a trench with only 2–3 sets of leaves above ground.

  5. Water in well, if you like with a weak starter fertilizer. Alaska uses ½–1 cup per plant of 1 Tbsp high-phosphorus soluble fertilizer per gallon.

  6. Water every morning until roots are established.

What a good transplant looks like:

  • stocky, with white, fibrous roots

  • not woody or root-bound

  • ideally without open flowers. Alaska prefers tomatoes "near bloom" for its very short season.

Fall transplants

  • North: start broccoli and cabbage in June–July. CSU direct-seeds them in early July.

  • Georgia: set them out Aug 15–Oct 1, starting trays 5–6 weeks earlier.

  • Texas: set them out from mid-July (north) to Nov 1 (far south).

  • Mild-winter areas: brassica transplants go out September to February.

  • In late-summer heat, give young transplants shade after 3 p.m. and daily water for about 2 weeks.

Diagram coming soonHardening-off week
Figure 4. Hardening off over one to two weeks.
Sources: High Mowing Organic Seeds, University of Maryland Extension and 7 more

Sowing directly in the garden

Sow by soil temperature

Push a soil thermometer 4 in deep at 8 a.m. (6 in for beans).

CropMinimum soil °FDays to come up at 50 / 59 / 68 / 77°FDepth
Lettuce357 / 4 / 3 / 2¼ in
Spinach3512 / 7 / 6 / 5½ in
Onion3513 / 7 / 5 / 4¼ in
Parsnip3527 / 19 / 14 / 15½ in
Pea4013 / 9 / 7 / 61 in
Carrot4017 / 10 / 7 / 6¼ in
Beet4017 / 10 / 6 / 5¾–1 in
Radish4011 / 6 / 4 / 3½ in
Sweet corn5022 / 12 / 7 / 41–1½ in
Snap bean60 (CSU: 55)none / 16 / 11 / 81–1½ in
Cucumber60none / 13 / 6 / 41 in
Squash, melons60—1–1½ in

Days-to-emerge figures are from Harrington's UC Davis table.

Published copies of the optimum-temperature table differ, so treat them as a guide. Hot soil also fails: Harrington found little or no lettuce or spinach germination at 95°F.

Planting windows (CSU, keyed to your average last frost)

GroupCropsWhen
HardyBroccoli, cabbage, kohlrabi, onions, lettuce, peas, radish, spinach, turnips2–4 weeks before
Semi-hardyBeets, carrots, cauliflower, parsley, parsnips, potatoes, chard0–2 weeks before
TenderBeans, corn, cucumbers, summer squashAround the date
Very tenderMelons, eggplant, pepper, pumpkin, winter squash, tomato2+ weeks after

A week of days below 55°F may stunt very tender crops.

Getting small seeds up

  • Carrots and other fine seed need a surface that stays moist and never crusts. They can take up to 3 weeks to come up.

  • The board-and-burlap trick:

    • Sow on the surface.
    • Cover with moist vermiculite.
    • Lay damp burlap, newspaper or a light board over the row.
    • Lift it the moment seedlings show.

  • Sow again for a steady supply: for example carrots 2–3 times, 3 weeks apart.

  • For fall: sow storage carrots about 100 days before the first fall frost. Sow cool-season crops again in midsummer.

Sources: Colorado State University Extension, Oregon State University Extension and 4 more

Keeping and testing seed

How long seed lasts (stored well)

YearsCrops
1Onion, parsnip (UGA: onion only 8–10 months)
2Sweet corn, pepper
3Beans, peas, carrots
4Beets, tomatoes, squash
4–5Radish
5Cucumber
1–6 (sources disagree)Lettuce: treat as 1–3 and test

Pelleted (clay-coated) seed stores poorly: Johnny's says use it within 1 year, and USU says test it after 2. Primed seed should be used the same year.

Storing it

  • Dry is what matters most. Aim for temperature (°F) plus relative humidity (%) under 100, with humidity under 35%.

  • Seal it in a jar and keep it dark and cool. USU's ideal is 35–40°F at under 40% humidity. Avoid anything above 70°F or 60% humidity, and never over 100°F.

  • Dry it first: seal seed with an equal amount of silica gel for 1–2 weeks. No dehydrators or microwaves.

  • Fridge: only for dry seed in a sealed jar, because fridges are humid inside. Let the jar warm to room temperature before opening.

The paper-towel germination test

  1. About a month before planting, roll 10–20 seeds (more is better) in a damp paper towel.

  2. Put the roll in a plastic bag at 70–80°F.

  3. Check from day 2–3 for about a week and count the sprouts.

Reading the result (Illinois, 10 seeds):

  • 10 sprout: sow as normal.

  • 7–9 sprout: sow more thickly.

  • 6 or fewer sprout: buy new seed.

Hybrid, open-pollinated, heirloom

  • F1 hybrids are crosses of two parent lines made by ordinary breeding. They are not GMOs.

  • Seed saved from F1 plants gives plants that vary, "typically for the worse." Buy new F1 seed each year.

  • Open-pollinated and heirloom varieties come true from saved seed if kept from crossing.

Diagram coming soonPaper-towel test
Figure 5. The paper-towel germination test.
Sources: Colorado State University Extension, University of Illinois Extension and 7 more

Growing from the grocery store

ItemVerdictHow
Green onions✅ WorksStand the root end (lower third) in water and change it every few days. One gardener got 4–6 regrowths
Celery, romaine, bok choy bases✅ Greens onlySet about 2 in of base in about 1 in of water and change it often. Leaves start in days; move to soil after 2–3 weeks. Results "may not match" store produce
Sweet potato slips⚠️ Works, with caveatsHalf-bury a root in moist mix at 75–85°F. Sprouts in 2–3 weeks, slips in 6–8 (8–10 if cooler), about 7–10 per root. Cut slips ½–1 in above the soil. Plant 3–4 weeks after frost-free date, 12–18 in apart. Water-jar slips have weaker roots
Ginger⚠️ Pick carefullyOften treated with a sprout inhibitor, so choose plump pieces with swelling or green buds (organic sprouts more often). Grows only in soil above 68°F; in-ground only in zones 8–12. Baby ginger in about 4 months, mature in 8–10
Herb cuttings (mint, oregano, sage, basil)✅ WorksStrip the lower leaves, root in water for a few weeks, then pot up
Dried beans⚠️ Try itDo a germination test first. Gardeners report pinto, kidney and great northern growing, one at about 60%. Canned beans won't sprout
Tomato, pepper, melon seeds⚠️ Grows, but not trueMany are hybrids, so the plants won't match the fruit
Zucchini, cucumber seeds❌ Usually noThe fruit is picked before the seeds are mature
Pineapple top🪴 HouseplantDry the crown several days, plant about 1 in deep. Roots in 6–8 weeks; fruit maybe in 2–3 years; protect from frost
Avocado pit🪴 HouseplantDoesn't come true; about 5–13 years to fruit, usually smaller fruit. Bring in when cold
Garlic❌ Don'tSee the safety box below

Myths:

  • "Planted beet or turnip tops grow new roots": they only grow greens and seed. Only UF says otherwise.

  • "Cutting garlic scapes splits the bulb into more cloves": extension says scapes are removed to put energy into the bulb.

Sources: University of Florida IFAS Extension, Permies forum and 12 more

What you'll need

Indoor setup:

  • seed-starting mix

  • trays or cells with drainage holes, plus a clear dome

  • heat mat

  • shop lights or grow lights on a timer

  • small fan

  • water-soluble fertilizer

  • vermiculite

  • household bleach and a scrub brush

  • labels

  • soil thermometer

  • optional: a soil blocker

Seed storage:

  • glass jars with tight lids

  • silica gel packets

  • envelopes and labels

  • paper towels and zip bags for testing

Costs: none of our sources gave prices yet. We'll add a dated price survey separately.

Sources: University of Minnesota Extension, University of Maryland Extension and 3 more

Common mistakes

  1. Starting too early, which gives tall, woody, root-bound plants.

  2. A windowsill instead of lights, which gives leggy, leaning seedlings.

  3. Lights too far away, which also makes seedlings stretch.

  4. Garden soil or reused mix in trays, which brings damping-off.

  5. Overwatering, or cold tap water, which also brings damping-off.

  6. Leaving the dome on after sprouting.

  7. Keeping seedlings too warm after they sprout, which makes them leggy.

  8. Skipping hardening off.

  9. Leaving a peat-pot rim above the soil, which wicks water away from roots.

  10. Sowing beans, squash or corn in cold soil, where they rot or are slow.

  11. Letting carrot beds dry or crust.

  12. Planting store-bought garlic.

  13. Storing seed somewhere warm or humid, or in a fridge without a sealed jar.

Sources: University of Maryland Extension, Rutgers Cooperative Extension and 8 more

Troubleshooting

ProblemLikely causeFix
Seeds didn't come upToo cold, too dry, too deep, old seedHeat mat at 70–75°F; check depth; do a paper-towel test
Seedlings collapse at the soil lineDamping-offDiscard the tray and mix; clean everything; warm, drier, brighter next time
Tall, pale, leaningToo little light, too warmLower the lights, 14–16 hours; cooler room; fan
Lettuce won't sproutToo warm (above ~75°F)Germinate at 60–68°F; shade or cool the tray in summer
Purple leavesCan mean low phosphorus, often from cold, wet mixKeep the mix warmer and less soggy; feed at ¼–½ strength once true leaves show
Roots circling the cellLeft too long in a small cellPot up sooner; use bigger cells or soil blocks
Pale or damaged leaves after going outsideToo much sun or wind too fastHarden off more slowly, starting in shade out of the wind
Transplants wilt and stallRoot disturbance, hot sunny dayPlant on a cloudy day and water each morning; for the squash family, use plants under 3 weeks old or sow directly
Carrot row came up patchyCrust or dry surfaceBoard, burlap or vermiculite cover; sprinkle gently
Old seed sprouted poorlyAge, humiditySow more thickly (7–9 of 10) or buy new (6 or fewer)
Sources: University of Minnesota Extension, Iowa State University Extension and 9 more

Resources

By zone

Frost dates matter more than your zone number. Use your nearest NOAA station's dates and slide these by region.

3–4
Short season, so lead times run long: Alaska starts tomatoes 7–9 weeks before last frost, and Minnesota starts onions in early February. Start fall brassicas in June–July. Ginger and sweet potatoes do best in containers or with slips started early.
5–6
Standard countdowns (tomato 6–8, pepper 8–10 weeks). Example: Boston's 10% last-frost date is Apr 17. Direct-sow beans and squash once soil hits 60°F.
7–8
Earlier frost dates and long falls. Georgia sets out broccoli and cabbage Aug 15–Oct 1. Shade cool-crop trays in summer heat.
9–10
Spring tomatoes may start in December–January, counting back from a very early frost date (Modesto: about March 3). Brassica transplants go out September–February. Short-day onions are sown mid-Sept to mid-Oct. Ginger grows in the ground. Hot soil stops lettuce.
Northeast
late, uneven springs. Count back from the 10% date and harden off carefully.
Southeast
fall is a second main season for transplants. Cool seed trays in summer heat.
Midwest
cold, wet spring soil brings damping-off outdoors too, so wait for soil temperature.
Southwest
heat and dryness. Give late-summer transplants shade after 3 p.m. and daily water. Frost dates change with elevation.
Pacific Northwest
cool summers. Use heat mats for warm crops. White rot garlic quarantines apply in ID, OR and WA.
Mountain West
short, dry season, so use your nearest station (Boulder's 10% date is after May 18). Peat pots rot slowly in dry soil.

Where this comes from

Every fact in this lesson was checked against 66 published sources: 53 from extension services, the USDA and research journals, 11 from experienced growers and practitioners and 2 from grower forums, where several agreed. Each section lists its own at the bottom.

See all 66 sources
National Pesticide Information Center
Pacific Northwest Pest Management Handbooks
Rutgers Cooperative Extension
University of Alaska Fairbanks Extension
University of California, Riverside
University of Saskatchewan
University of Wisconsin Extension
Washington State University Extension

Summary

The whole lesson on one page. Print it or come back to it.

Key information

  • Plan tender crops around your 10% last-frost date, not the average.
  • Weeks indoors: onions 10–12, peppers and eggplant 8–10, tomatoes 6–8, cabbage family 4–6, squash 2–4.
  • Germinate warm (65–75°F; 75–85°F for tomatoes and peppers), then grow cooler (60–70°F) under lights 14–16 hours a day.
  • Damping-off is prevented, not cured: fresh mix, clean trays, warm water, good light.
  • Harden off 7–14 days. Sow outdoors by soil temperature (peas 40°F, corn 50°F, beans and squash 60°F).
  • Keep seed dry and sealed (°F + % humidity under 100). Don't plant grocery-store garlic.

Step by step

  1. 1.Clean reused trays. Scrub them, soak 30 minutes in 1 part household bleach to 9 parts water, then rinse well. Bleach can make old plastic brittle.
  2. 2.Wet the mix first. Stir water in until a squeezed handful holds together without dripping. Dry peat repels water.
  3. 3.Fill and sow about 2–3 times as deep as the seed is wide. Don't sow thickly, because crowding invites disease. For big seeds (squash, beans, corn), sow 2–3 per cell and keep the strongest.
  4. 4.Light-needing seeds (lettuce, celery): press them onto the surface or dust with vermiculite.
  5. 5.Cover and warm. Put on the dome and set the tray on a heat mat at about 70–75°F, or 75–85°F for tomatoes, peppers and eggplant. Lettuce germinates poorly above about 75°F.
  6. 6.Watch daily. Remove the dome as soon as seedlings appear. Take the tray off the heat mat.
  7. 7.Lights on right away, 14–16 hours a day with a dark period, and keep them close (see Light and heat).
  8. 8.Grow them cooler: about 60–70°F, nights 60–65°F, and cool-season crops near 55°F at night.
  9. 9.Water with room-temperature water (68–77°F) so the mix stays moist but not soggy. Cold water slows growth and invites damping-off.
  10. 10.Feed once true leaves show: ¼-strength fertilizer weekly (USU uses ½ strength). If the mix already has fertilizer, wait several weeks.
  11. 11.Run a fan 5–10 minutes or more once to several times a day, or brush your hand over tomato tops.
  12. 12.Pot up into a bigger container when the first true leaves are out or roots fill the cell. Set tomatoes deeper, because they root along the buried stem.
  13. 13.Harden off for 1–2 weeks (see Hardening off), then plant out.

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