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The More One SowsSmall-space food growing, by the numbers

GuideEntry 41Reviewed 2026-10-16

Seed starting indoors: timing, lights and hardening off

Starting seed indoors gives slow crops such as onions, peppers and tomatoes a head start before the last frost. This guide sets out what UNH, Minnesota, Colorado State, UMass and Penn State Extension publish on timing, containers and mix, warmth, light, feeding, hardening off and damping-off, and shows where their numbers differ.

When to start: count back from the last frost

Find your average last spring frost date (see frost dates explained), decide when each crop goes outside, then count back the weeks it needs indoors (UNH). UNH puts seed-to-transplant time at 4 to 12 weeks, depending on how fast a crop germinates and grows; Colorado State says most seeds are started 4 to 8 weeks before the last killing frost. Seeds started too early give poor-quality plants by planting time (UNH). Carrots and parsnips are direct-seeded, not transplanted (UNH), and UMass sows peas, beans, corn, mesclun, root crops and spinach straight into the garden.

CropWeeks indoors (UNH)Plant out vs last frost (UNH)Sow indoors, weeks before last frost*Weeks before planting out (UMass)Weeks indoors (UMN, Minnesota)
Onion, leek8–104 wk before12–146–8 (onion)14–15
Parsley9–102–3 wk before11–13——
Celery10–12After last frost10–12 before the planting-out date—12–13
Cabbage4–64 wk before8–104–710–11
Kale4–64 wk before8–10——
Broccoli4–62–4 wk before6–104–710–11
Cauliflower4–62 wk before6–84–710–11
Lettuce4–53–4 wk before7–93–510–11 (head lettuce)
Spinach4–63–6 wk before7–12Direct sow—
Pepper8–102 wk after6–86–89
Eggplant8–102–3 wk after5–87–99
Tomato6–81–2 wk after4–76–85–6
Basil6–82 wk after4–6——
Cucumber3–41–2 wk after1–32–3—
Summer squash, melon3–42 wk after1–22–3—

*Calculated here from UNH's two columns: planting-out offset minus weeks indoors. Example: tomatoes go out 1–2 weeks after the frost after 6–8 weeks indoors, so sowing falls 4–7 weeks before it. UMN's figures are weeks indoors in a Minnesota schedule that ends with hardening off all plants in early to mid May. UNH's own worked example: for a May 27 transplant date, 6–8 weeks indoors means sowing between April 1 and April 15. The planting calendar does this count for 25 crops.

Containers and mix

Step by step

  1. Fill and moisten. Water the mix before sowing (UMN). UNH fills containers to within ¾ inch of the top.
  2. Sow at about twice the seed's diameter (UNH, UMN). Leave light-requiring seed such as lettuce, begonia and petunia uncovered or under a thin layer of fine vermiculite; cover dark germinators such as calendula, annual phlox and vinca (UNH). With old, low-germination seed, sow two or more per cell (UMN). Label every container.
  3. Cover and keep warm. A clear dome or plastic bag holds moisture (UNH, Colorado State, UMN); keep covered trays out of direct sun (UNH, Colorado State). Most seed germinates at 65–75°F (UNH) or 70–75°F (Colorado State, UMass). Tomato's optimum is near 80°F (UNH), as is the nightshade family's in general (Colorado State). Potting mix can run up to 5°F cooler than the air, so a heat mat helps; do not plug it into the light timer (UMN).
  4. Uncover at emergence: once the first seedling leaves appear (UNH) or seedlings emerge (Colorado State); UMN removes the dome once seedlings touch it.
  5. Give strong light at once (table below). Insufficient light is the main cause of tall, thin stems (UMN).
  6. Water evenly. Mist the surface or let water wick up from a tray, then drain the excess; keep the mix moist, never dry or soggy (UMN). UMN waters seedlings with clean water at 68–77°F; 50°F water slows growth and raises infection risk.
  7. Thin after true leaves form: snip extra seedlings at soil level with scissors rather than pulling them (UMN).
  8. Feed lightly. Seedlings live on the seed's reserves at first; UMN starts ¼-strength soluble fertilizer weekly once several true leaves have formed. Other sources start sooner (see the table at the end).
  9. Pot up seedlings that outgrow their cells into containers 1–2 inches wider (UMN). Lift them by the root ball or a leaf, never the stem: a lost leaf is recoverable, a lost growing tip can be fatal (UMN).
  10. Grow cool and bright. Once established, UNH moves seedlings to 55–60°F nights and 65–70°F days to prevent leggy growth.
  11. Harden off for one to two weeks (next section), then plant out on a cloudy day or in late afternoon (UMN).

Light and temperature by source

SettingUNHUMNColorado State
Germination warmth65–75°F for most; tomato near 80°FAir above 60°F; heat mat warms mix to 70–75°F70–75°F for most; nightshade family about 80°F
Hours of light18 h a day for germination; about 16 h for seedlings12–16 h with a dark period each night12–16 h
Lamp height6–12 in above seeds; seedlings 6 in from the tubesAbout 2 in above the tops, no more than 4 in3–4 in above the seedlings
LampsTwo 40-watt cool white or full-spectrum fluorescent tubesFluorescent or LED; natural light alone is not enoughOne cool white and one warm white tube, or a bright window

UMN advises against windowsills, which turn cold at night and hot by day; a south window may do for a few plants. Colorado State keeps the light as low as possible to prevent stretching. Hang the fixture on chains so it can be raised as plants grow (UMN).

Indoor seed-starting setupA fluorescent or LED fixture hangs on two chains a few inches above a divided tray of seedlings. A timer on the left switches the lamp. The tray sits in a shallow water tray on a heat mat that has its own plug. A clear dome that has been lifted off rests on the right. A double arrow marks the gap between the lamp and the seedling tops. Numbered markers match the list below the drawing. 12–16 h own outlet heat mat: mix at 70–75°F 2–4 in 1 2 3 4 5 6 7
Drawing by The More One Sows, based on the cited sources. Not to scale; distances, hours and temperatures follow the UNH, UMN and Colorado State figures in the text.
  • 1. Fixture hung on chains so it can be raised as seedlings grow (UMN)
  • 2. Gap between lamp and seedling tops: about 2 in and no more than 4 in (UMN); 3–4 in (Colorado State); 6 in from fluorescent tubes (UNH)
  • 3. Timer: 12–16 h of light with a dark period each night (UMN, Colorado State); about 16 h for seedlings (UNH)
  • 4. Clear dome holds moisture; off at emergence (UNH, Colorado State) or when seedlings touch it (UMN)
  • 5. Divided tray of new soilless mix, one seedling per cell, with drainage holes (UMN)
  • 6. Water from below, then drain the excess; moist, not soggy (UMN)
  • 7. Heat mat warms the mix to 70–75°F; plugged in separately, not into the light timer (UMN)

Hardening off

Hardening off gradually gets indoor plants used to sun, wind and temperature swings, which prevents leaf scorch and wilting (UMN). Start about two weeks before planting out (UMN, Colorado State) or one to two weeks before (UNH).

  1. Day 1: a few afternoon hours outside in shade, out of the wind (UMN). Colorado State starts with a couple of hours of gradual direct sun.
  2. Each day after: lengthen the time outside and the hours of direct sun (UMN); add a few hours of wind exposure daily for a week (Colorado State).
  3. Nights: bring plants in before night temperatures drop (UMN), and whenever it nears freezing (Colorado State). UNH names 45–50°F as the ideal hardening temperature and keeps tender seedlings in on windy days or below 45°F.
  4. Water less to slow growth, without letting plants wilt (UNH).
  5. After about two weeks plants can stay in a sunny spot until transplanting, unless a freeze is forecast (UMN). A cold frame vented a little more each day also works (UMN, UNH).
  6. Plant out on a cloudy day or late afternoon; some wilting usually recovers within a day. Trim the rims of peat or paper pots to soil level and tear the bottoms so roots can spread (UMN).

Crop pages give each crop's outdoor timing, for example tomatoes, peppers and parsley.

Damping-off

Damping-off is the rotting of seeds in the mix and the collapse of newly emerged seedlings (Penn State). It often kills a large part of a tray or all of it, and infected seedlings rarely grow into vigorous plants (UMN). The usual causes are Rhizoctonia, Fusarium and the water mold Pythium (UMN); Penn State adds Phytophthora.

StageWhat to look for
Before emergenceSeedlings never come up (UMN); the young root and stem are destroyed below the surface (Penn State)
After emergenceWater-soaked, mushy, gray to brown stems and seed leaves; stems shrink to a thread and the seedling falls over (UMN); stem rot at or just below the soil line (Penn State)
In humid airFluffy white, cobweb-like growth (UMN); white to tan-brown (Rhizoctonia), white to orange-pink (Fusarium) or white cottony growth (Pythium, Phytophthora) (Penn State)

Conditions that slow growth favor it: low light, overwatering, salt buildup from heavy feeding and cool soil (UMN). Penn State names excess moisture, high humidity, poor drainage and overcrowded seedlings. UMN's page focuses on prevention; Penn State removes and discards plants with symptoms.

Damping-off prevention checklist

Shallow trays sown densely for microgreens face the same warm, humid conditions.

Where sources disagree

QuestionPositions
When to start feeding¼ strength weekly once several true leaves form (UMN); ¼ to ½ strength at true leaves (Colorado State 1034); ½ strength one week after germination, then full strength every two weeks (Colorado State 1840); houseplant fertilizer at label dilution weekly after germination (UNH fact sheet)
Hours of light12–16 h with a dark period (UMN, Colorado State) vs about 16 h for seedlings and 18 h for germination (UNH)
Lamp height2 in, at most 4 in (UMN); 3–4 in (Colorado State); 6–12 in, given for 40-watt fluorescent tubes (UNH)
Tomato start6–8 weeks before the last frost (UNH Master Gardener answer) vs 6–8 weeks indoors, planted out 1–2 weeks after it (UNH fact sheet); 6–8 weeks before planting out (UMass); 5–6 weeks indoors (UMN)
Pepper startAt least 10 weeks (UNH Master Gardener answer); 8–10 (UNH fact sheet); 9 (UMN); 6–8 (UMass)
SpinachTransplants after 4–6 weeks indoors (UNH) vs direct sowing (UMass)
Hardening offAbout two weeks (UMN, Colorado State) vs one to two weeks (UNH)
Damping-off weatherCool, wet conditions (UMN) vs excess moisture, high temperatures and poor light (Colorado State 1034)

Sources

  1. University of New Hampshire Extension — Growing Vegetables: When to Plant Your Vegetable Garden (fact sheet)
  2. University of New Hampshire Extension — Starting Plants Indoors from Seed (fact sheet, B. Sideman, rev. April 2017, PDF)
  3. University of New Hampshire Extension — How do I know when to start seedlings indoors? (S. Walker, March 2018)
  4. University of New Hampshire Extension — Seed Starting Basics (M. Winchell, March 2018)
  5. University of Minnesota Extension — Starting seeds indoors
  6. University of Minnesota Extension — How to prevent seedling damping off (M. Grabowski, reviewed 2024)
  7. Colorado State University Extension, PlantTalk Colorado — 1034 Starting Seeds Indoors
  8. Colorado State University Extension, PlantTalk Colorado — 1840 Starting Vegetable Seeds Indoors
  9. UMass Amherst Extension (CAFE) — Starting Seeds Indoors (fact sheet, updated March 2018)
  10. Penn State Extension — Safeguard Your Seedlings from Damping-Off (J. Diehl Mazzone, updated January 2026)

All sources were opened and checked in October 2026. Rules and figures change; confirm with the original before acting.

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