When Should You Start Seeds Indoors?
Start most seeds indoors 4 to 10 weeks before your average last frost date. Slow onions, celery and petunias need 10 to 12 weeks, peppers 8 to 10, tomatoes and broccoli 6 to 8, and fast cucumbers and squash only 3 to 4.
The lead time is the number of weeks a seedling needs to reach planting size, typically four to six true leaves and a root system that holds the root ball together. Crops that tolerate frost, such as broccoli, lettuce and onions, are planted out before the last frost, so their indoor start comes earlier in the calendar than the weeks alone suggest. Crops with long taproots or fast growth, such as carrots, beans and corn, are sown directly in the garden instead. The calculator is one of PlantoScan's free garden planning calculators.
| Weeks before last frost | Crops started indoors |
|---|---|
| 10–16 weeks | hot peppers, onions (from seed), leeks, celery, rosemary, petunias, impatiens, wax begonias, pansies & violas |
| 8–10 weeks | sweet peppers, eggplant, parsley, chives, oregano, thyme, snapdragons |
| 6–8 weeks | tomatoes, broccoli, cabbage, cauliflower, brussels sprouts, kale, basil, sage, marigolds, sweet alyssum |
| 4–6 weeks | lettuce |
| 3–4 weeks | cucumbers, zucchini & summer squash, watermelon, cantaloupe & melons |
| Direct sow outdoors | spinach, swiss chard, peas, green beans, sweet corn, winter squash & pumpkins, carrots, beets, radishes, okra, cilantro, dill, chamomile (german), zinnias, cosmos, sunflowers, nasturtiums, calendula, sweet peas |
Grouped by the earliest recommended indoor start. Many direct-sown crops can also be started indoors in short seasons.
How Do You Calculate Your Seed Starting Date?
Find your average last frost date, then subtract the weeks the seed packet or chart lists for starting indoors. With a March 30 last frost, 8 weeks earlier is February 2 and 6 weeks earlier is February 16: the tomato sowing window.
A worked example for New York City (average last frost March 30, safer date April 9): peppers need 8 to 10 weeks, so they are sown January 19 to February 2 and planted out 2 to 3 weeks after the frost date, April 13 to 20. Broccoli is sown February 2 to 16 and, being frost-hardy, transplanted March 2 to 16, four to two weeks before the last frost. Planning from the safer April 9 date moves every date 10 days later. The last frost date calculator by ZIP code shows both dates and the length of your growing season.
Count in whole weeks and round to a convenient sowing day; a few days either way makes little difference. The calculator does the arithmetic in calendar days, so daylight saving time never shifts a date.
What Soil Temperature Do Seeds Need to Germinate?
Most vegetable seeds germinate best in soil or mix at 65 to 85 °F (18–29 °C). Cool-season crops such as lettuce, spinach and peas sprout from about 40 °F (4 °C), while peppers, eggplant and melons stall below about 65 °F (18 °C).
A seedling heat mat raises the temperature of the mix roughly 10 to 20 °F above room temperature, which is often the difference between peppers sprouting in one week or three. A mat with a thermostat probe in the mix prevents overheating. Move trays off the mat once most seeds have sprouted, since warm, humid conditions after germination encourage damping-off and stretched stems. Lettuce and spinach need no heat at all: lettuce seed can go dormant above about 80 °F (27 °C).
How Much Light Do Seedlings Need?
Seedlings need 14 to 16 hours of strong light a day, about 6 to 12 mol/m² of light per day. A grow light a few inches above the leaves provides it; a windowsill in late winter usually does not, and seedlings turn leggy.
Light matters from the moment the first seed leaves unfold, so have lights running before germination. The daily light integral calculator for grow lights converts a light's PPFD and hours on into a DLI. Germination itself is different: most seeds sprout in the dark under a thin layer of mix, but a few, including lettuce, celery, petunias, snapdragons and begonias, need light and are pressed onto the surface uncovered. Calendula, pansies and nasturtiums germinate best fully covered.
When Should You Pot Up Seedlings?
Pot up seedlings when they have one or two sets of true leaves or when roots start circling the cell, typically 3 to 5 weeks after sowing. Tomatoes, peppers, eggplant and slow flowers benefit; cucumbers and squash should be sown in their final pot.
Move seedlings from small cells into 3- to 4-inch (8–10 cm) pots filled with potting mix, handling them by a leaf rather than the stem. Tomatoes can be buried up to their first leaves, and the buried stem grows new roots; the tomato plant care guide covers light and watering once they are in the garden. Sow seeds in a fine, sterile seed-starting mix rather than garden soil, which crusts and carries disease. The potting soil calculator for pots and trays works out how much mix a batch of cells and pots needs.
How Do You Harden Off Seedlings?
Harden off seedlings over 7 to 10 days by setting them outdoors in shade for 1 to 2 hours, then adding time and sun each day until they stay out overnight. Skipping it can leave leaves scorched or plants stunted.
Start hardening off 7 to 10 days before your transplant date, on days above about 50 °F (10 °C) for tender crops, and bring trays in when frost or strong wind is forecast. Water less often, but never let seedlings wilt. Warm-season crops such as peppers are especially sensitive to cold nights; the pepper plant care guide lists the temperature range they grow best in. Transplant on a cloudy day or in the evening and water in well.
Which Seeds Should You Direct Sow Instead of Starting Indoors?
Direct sow root crops such as carrots, beets and radishes, large fast seeds such as beans, peas, corn and sunflowers, and taprooted herbs like dill and cilantro. They grow quickly outdoors and often suffer from transplanting.
Direct sowing depends on soil temperature more than the calendar: peas and radishes sprout from about 40 °F (4 °C), while beans and corn rot in soil below about 60 °F (16 °C). Squash, cucumbers, melons and zinnias work either way; starting them indoors 3 to 4 weeks early gains a few weeks of harvest in short-season areas. Set the calculator to “Direct sow” to plan every crop that allows it without indoor starts.
When Should You Start Seeds for a Fall Garden?
Count back from the average first fall frost instead of the last spring frost. Fall broccoli is typically sown indoors 14 to 17 weeks before the first frost and transplanted 10 to 12 weeks before; lettuce is direct sown 6 to 10 weeks before.
Frost-hardy crops such as kale, cabbage, spinach and Brussels sprouts keep growing after light frosts, and many taste sweeter for it. Fall schedules allow extra time because growth slows as days shorten. Late-summer soil is often too warm for lettuce and spinach to germinate outdoors, so sow them in a shaded spot or start them indoors. The fall tab in the calculator uses your first frost date and lists every selected crop with a fall window.
Seed Starting Chart: When to Start 50 Vegetables, Herbs and Flowers
The seed starting chart below lists, for each crop, the weeks before or after the last frost to start seeds indoors and plant outdoors, with germination temperature, days to sprout, sowing depth and light needs.
| Crop | Start indoors | Plant outdoors | Soil °F (min / best) | Days to sprout | Depth | Light | Frost tolerance |
|---|---|---|---|---|---|---|---|
| Tomatoes | 6–8 wk before | Transplant 1–2 wk after | 50 / 70–85 | 5–10 | ¼ in | Cover to depth | Tender |
| Sweet peppers | 8–10 wk before | Transplant 2–3 wk after | 60 / 75–85 | 7–14 | ¼ in | Cover to depth | Tender |
| Hot peppers | 8–12 wk before | Transplant 2–3 wk after | 65 / 80–90 | 10–21 | ¼ in | Cover to depth | Tender |
| Eggplant | 8–10 wk before | Transplant 2–3 wk after | 65 / 75–90 | 7–14 | ¼ in | Cover to depth | Tender |
| Broccoli | 6–8 wk before | Transplant 2–4 wk before | 40 / 65–80 | 4–10 | ¼ in | Cover to depth | Hardy |
| Cabbage | 6–8 wk before | Transplant 2–4 wk before | 40 / 65–80 | 4–10 | ¼ in | Cover to depth | Hardy |
| Cauliflower | 6–8 wk before | Transplant 0–2 wk before | 45 / 65–80 | 5–10 | ¼ in | Cover to depth | Half-hardy |
| Brussels sprouts | 6–8 wk before | Transplant 2–4 wk before | 45 / 65–80 | 5–10 | ¼ in | Cover to depth | Hardy |
| Kale | 6–8 wk before | Transplant 2–4 wk before | 40 / 60–80 | 5–8 | ¼ in | Cover to depth | Hardy |
| Lettuce | 4–6 wk before | Transplant 1–3 wk before | 35 / 60–70 | 2–10 | ⅛ in | Needs light: press onto surface | Hardy |
| Spinach | Direct sow | Sow 3–6 wk before | 35 / 50–70 | 7–14 | ½ in | Cover to depth | Hardy |
| Swiss chard | Direct sow | Sow 0–2 wk before | 45 / 60–85 | 5–14 | ½ in | Cover to depth | Half-hardy |
| Onions (from seed) | 10–12 wk before | Transplant 2–4 wk before | 45 / 65–80 | 7–14 | ¼ in | Cover to depth | Hardy |
| Leeks | 10–12 wk before | Transplant 0–2 wk before | 45 / 65–80 | 7–14 | ¼ in | Cover to depth | Hardy |
| Celery | 10–12 wk before | Transplant 1–2 wk after | 50 / 60–70 | 14–21 | Surface | Needs light: press onto surface | Half-hardy |
| Peas | Direct sow | Sow 4–6 wk before | 40 / 60–75 | 7–14 | 1 in | Cover to depth | Hardy |
| Green beans | Direct sow | Sow 1–3 wk after | 60 / 70–85 | 6–10 | 1 in | Cover to depth | Tender |
| Sweet corn | Direct sow | Sow 1–3 wk after | 60 / 65–85 | 7–10 | 1 in | Cover to depth | Tender |
| Cucumbers | 3–4 wk before | Transplant 1–3 wk after | 60 / 70–90 | 3–10 | ½ in | Cover to depth | Tender |
| Zucchini & summer squash | 3–4 wk before | Transplant 1–3 wk after | 60 / 70–90 | 5–10 | 1 in | Cover to depth | Tender |
| Winter squash & pumpkins | Direct sow | Sow 1–3 wk after | 60 / 70–90 | 5–10 | 1 in | Cover to depth | Tender |
| Watermelon | 3–4 wk before | Transplant 2–3 wk after | 65 / 75–90 | 4–10 | ¾ in | Cover to depth | Tender |
| Cantaloupe & melons | 3–4 wk before | Transplant 2–3 wk after | 65 / 75–90 | 4–10 | ½ in | Cover to depth | Tender |
| Carrots | Direct sow | Sow 2–3 wk before | 45 / 65–80 | 10–21 | ¼ in | Cover to depth | Half-hardy |
| Beets | Direct sow | Sow 2–4 wk before | 45 / 65–85 | 5–14 | ½ in | Cover to depth | Half-hardy |
| Radishes | Direct sow | Sow 3–5 wk before | 40 / 65–85 | 3–7 | ½ in | Cover to depth | Hardy |
| Okra | Direct sow | Sow 2–4 wk after | 65 / 75–90 | 7–14 | ¾ in | Cover to depth | Tender |
| Basil | 6–8 wk before | Transplant 1–3 wk after | 60 / 70–85 | 5–10 | ⅛ in | Cover to depth | Tender |
| Parsley | 8–10 wk before | Transplant 1–3 wk before | 45 / 65–75 | 14–28 | ¼ in | Cover to depth | Hardy |
| Cilantro | Direct sow | Sow 0–3 wk before | 45 / 55–70 | 7–14 | ¼ in | Darkness helps: cover fully | Half-hardy |
| Dill | Direct sow | Sow 0–2 wk before | 50 / 60–70 | 7–21 | ¼ in | Cover to depth | Half-hardy |
| Chives | 8–10 wk before | Transplant 2–4 wk before | 45 / 60–70 | 7–14 | ¼ in | Cover to depth | Hardy |
| Oregano | 8–10 wk before | Transplant 0–2 wk after | 60 / 65–75 | 7–14 | Surface | Needs light: press onto surface | Half-hardy |
| Thyme | 8–10 wk before | Transplant 0–2 wk after | 60 / 65–75 | 7–21 | Surface | Needs light: press onto surface | Half-hardy |
| Sage | 6–8 wk before | Transplant 0–2 wk after | 60 / 65–75 | 10–21 | ⅛ in | Cover to depth | Half-hardy |
| Rosemary | 10–12 wk before | Transplant 1–3 wk after | 65 / 70–80 | 14–28 | ⅛ in | Needs light: press onto surface | Half-hardy |
| Chamomile (German) | Direct sow | Sow 2–4 wk before | 55 / 60–70 | 7–14 | Surface | Needs light: press onto surface | Hardy |
| Marigolds | 6–8 wk before | Transplant 1–2 wk after | 60 / 70–80 | 4–10 | ¼ in | Cover to depth | Tender |
| Zinnias | Direct sow | Sow 1–2 wk after | 60 / 70–85 | 4–7 | ¼ in | Cover to depth | Tender |
| Cosmos | Direct sow | Sow 0–2 wk after | 60 / 70–80 | 5–10 | ⅛ in | Cover to depth | Tender |
| Sunflowers | Direct sow | Sow 0–2 wk after | 50 / 70–85 | 5–10 | 1 in | Cover to depth | Half-hardy |
| Petunias | 10–12 wk before | Transplant 1–2 wk after | 70 / 75–80 | 7–14 | Surface | Needs light: press onto surface | Tender |
| Snapdragons | 8–10 wk before | Transplant 0–2 wk before | 60 / 65–75 | 7–14 | Surface | Needs light: press onto surface | Half-hardy |
| Impatiens | 10–12 wk before | Transplant 1–3 wk after | 70 / 70–80 | 7–21 | Surface | Needs light: press onto surface | Tender |
| Wax begonias | 12–16 wk before | Transplant 1–3 wk after | 70 / 72–80 | 14–21 | Surface | Needs light: press onto surface | Tender |
| Sweet alyssum | 6–8 wk before | Transplant 1 wk before – 1 wk after | 55 / 60–75 | 5–14 | Surface | Needs light: press onto surface | Half-hardy |
| Nasturtiums | Direct sow | Sow 1–2 wk after | 55 / 60–70 | 7–12 | ½ in | Darkness helps: cover fully | Tender |
| Calendula | Direct sow | Sow 0–2 wk before | 50 / 60–70 | 5–14 | ¼ in | Darkness helps: cover fully | Half-hardy |
| Pansies & violas | 10–12 wk before | Transplant 2–4 wk before | 55 / 65–70 | 7–21 | ⅛ in | Darkness helps: cover fully | Hardy |
| Sweet peas | Direct sow | Sow 4–6 wk before | 45 / 55–65 | 7–21 | 1 in | Darkness helps: cover fully | Hardy |
Weeks are relative to the average last spring frost. Typical ranges from PlantoScan's own planting data; seed packet instructions for a specific variety take priority.
Frequently Asked Questions
- Is it too late to start seeds indoors?
- It is too late when fewer weeks remain before the planting-out date than the crop needs indoors. Fast crops like cucumbers, squash and zinnias need only 3 to 4 weeks, so they can still be started close to the last frost; late peppers are better bought as transplants.
- What happens if you start seeds indoors too early?
- Seedlings started too early outgrow their pots before it is warm enough to plant out. They become leggy and root-bound, and tomatoes and brassicas that sit too long in small pots often flower early or produce smaller harvests after transplanting.
- Do seeds need light to germinate?
- Most vegetable seeds germinate in darkness under a thin layer of mix. A few, including lettuce, celery, petunias, snapdragons, impatiens and begonias, germinate better with light and should be pressed onto the surface instead of buried.
- Can I start seeds on a windowsill instead of under grow lights?
- A bright south-facing window can raise sturdy seedlings for fast crops, but late-winter days are short and light comes from one side, so many seedlings stretch and lean. A grow light kept a few inches above the plants for 14 to 16 hours a day gives more compact starts.
- Should I use the average or the safer last frost date?
- Use the average date for frost-tolerant crops such as broccoli, lettuce and peas. For tender crops such as tomatoes, peppers and basil, the safer 10% date lowers the risk of losing transplants to a late frost, at the cost of a slightly later harvest.
Frost dates: NOAA National Centers for Environmental Information, 1991–2020 U.S. Climate Normals (32 °F freeze dates by probability); ZIP code locations: U.S. Census Bureau ZCTA Gazetteer. Seed timing, germination temperatures and depths are PlantoScan's own rules of thumb; local weather and soil temperature always come first.