When to Start Seeds Indoors in South Carolina: A Crop-by-Crop Calendar

In coastal South Carolina, start onions and leeks indoors from November 15 to December 9. Broccoli, cabbage, cauliflower and head lettuce follow from December 13 to January 6. Peppers and eggplant go in from December 27 to January 13, and tomatoes from January 17 to February 10. Each window counts back from the last spring freeze at Charleston, which falls between February 28 and March 17. Inland and upstate gardens sow later than these dates.

The date everything counts back from

Every indoor sowing date starts with the last spring freeze. NOAA’s NCEI 1991-2020 Climate Normals put the last 32°F freeze at the Charleston airport station (USW00013880) between February 28 and March 17, using the 30-70% probability band. University of Minnesota Extension publishes a table of how many weeks each crop needs indoors before it goes out. Other extension services give shorter lead times for some crops, so treat those weeks as a starting point. A sowing window is simply the frost range minus those weeks, worked from both ends.

Peppers show how it works. The extension table gives them 9 weeks indoors. Nine weeks before February 28 is December 27, and nine weeks before March 17 is January 13. The same subtraction gives every row in the calendar below.

Why use a range instead of one frost date?

Because the last freeze moves from year to year. Sow on the early end of a window and a cold March keeps the frost range open to its late end. The seedlings then wait in their pots, outgrow the cells, stretch and tangle their roots. Sow on the late end and an early spring leaves you with plants that are too small to set out when the weather turns. You lose weeks of growth you could have had.

The sensible move is to split the difference. Sow a first tray at the early end of the window and a second tray a week or two later. Whichever spring arrives, one of them will be the right size. Subtract each crop’s weeks from both ends of February 28 to March 17, write down the two dates, and plan around that pair.

Does this range fit all of South Carolina?

It fits Charleston, and only roughly even there. Peninsular Charleston is milder than the airport station, so the peninsula can run ahead of these dates. Upstate South Carolina is colder. Its last spring frost comes several weeks later, so every sowing date on this calendar moves later there, and the milder coast can sow a bit earlier. Your local frost dates are laid out in First and Last Frost Dates in South Carolina, and the rest of the state’s gardening material sits in the South Carolina guides.

The 2023 USDA Plant Hardiness Zone Map puts the state in zones 7 through 9, from the northwestern mountains to the coast. That system rests on one variable, the average annual extreme minimum temperature. It tells you which perennials survive winter, and it says little about when a seed tray should go under the lights. For sowing, use frost dates.

The calendar, crop by crop

The weeks in the table come from University of Minnesota Extension’s guide to starting seeds indoors. They are one extension service’s guidance for weeks indoors before planting out. Other sources give shorter lead times for some crops (often 8 to 10 weeks for peppers and onions), so use them as a starting point, not a law. The sowing windows were calculated by counting those weeks back from Charleston’s February 28 to March 17 frost range. Gardeners in colder parts of the state should sow later than these windows.

The order is the point. The slowest crops come first, and onions and leeks go in from mid-November to early December. Celery follows a couple of weeks behind them. Broccoli, cabbage and head lettuce share one window from mid-December to early January, and the same window covers cauliflower and Brussels sprouts. Peppers, eggplant and okra start around the new year.

Tomatoes surprise people. They are a crop many gardeners want to start first, and they have one of the shortest runs indoors. Tomatoes wait until January 17 to February 10, long after the onions are up. Sown in November or early December, they would be tall, floppy plants by the time the garden is ready for them. Growing Tomato Seedlings Indoors covers the day-to-day care in that window.

Head lettuce shares a row group with cabbage in this table, with a long run indoors. If you want a closer look at how lettuce starts, read Starting Lettuce. Sweet potatoes come last, and the dates in the table apply to starting slips (sprouts grown from a tuber, not from seed). Slip timing varies by method, so treat that row as approximate. Plant the slips out only after the soil has warmed and nights stay mild.

Which crops should not be started indoors?

Root crops resent being moved. Carrots, beets, radishes and parsnips form their roots where the seed lands, and transplanting damages the taproot, so they go straight into the garden. Beans do the same, because they sprout quickly outdoors and gain little from a head start. Squash, pumpkins, melons and cucumbers can be started indoors, but in most cases direct sowing works better. They grow fast, and their seedlings sulk when their roots are disturbed. If you do start them indoors, use large pots and plant out gently.

Light, warmth and hardening off

A sunny window looks bright to us, but to a seedling it is weak. Winter days are short and the sun sits low. A seedling that doesn’t get enough light stretches toward the glass, putting its energy into a tall, pale stem instead of leaves and roots. The stem flops, and the plant is slow to recover once it goes outside. Leggy seedlings are a common result of indoor starting, and a windowsill alone rarely prevents them.

How do you keep seedlings from getting leggy?

Put a grow light close above the leaves and raise it as the seedlings grow. Run it on a timer so the plants get a regular day and a regular night. If you only have a window, turn the trays every day and move them to the sunniest spot you have. A seedling that has already stretched can sometimes be buried deeper at transplant time. That works well for tomatoes, which root along buried stem, and poorly for most other crops.

What temperature do seeds need to germinate?

Steady warmth matters more than any one reading. Seed-starting mix is often cooler than the air above it, especially in a garage or a spare room, and cool mix slows germination and encourages rot. Peppers, eggplant and okra are among the most demanding, and a heat mat under the tray helps them sprout evenly. Take the tray off the mat once the seedlings are up, and keep them in a room that stays comfortable for people. Onions, leeks and cabbage-family seeds sprout in cooler conditions than the peppers do.

How do you harden off seedlings?

Hardening off means getting a plant used to wind, sun and temperature swings before it moves outside for good. University of Minnesota Extension says to start moving seedlings outdoors about two weeks before planting out. Start with a few hours in a shady, sheltered spot, then lengthen the time outdoors each day and add a little more sun. Bring the trays in on any cold night. Water a little less in the final days, so the plants toughen up.

For a tomato tray sown in the January 17 to February 10 window, hardening off falls in the last week or two before it goes out. Skipping the step is a common cause of scorched leaves and stalled growth, even in a mild climate.

What goes wrong

Three common mistakes cause many failed spring starts. They are sowing too early, losing seedlings to damping off, and planting tender crops into cold soil. Each one is easy to spot and easy to prevent.

What happens when you start seeds too early?

The plants outgrow their space before the garden is ready for them. Peppers sown in November are well ahead of their December 27 to January 13 window. By spring they are tall and spindly, with roots circling the cell and lower leaves turning yellow. Plants that stay in a small container too long often sulk after transplanting, even once the weather is right. Follow the sowing windows above, and if a tray is running ahead of the weather, move seedlings into larger pots rather than rushing them outside.

How do you prevent damping off?

Damping off is a disease caused by soil fungi and water molds. A seedling that looked healthy yesterday has a thin, dark, pinched stem at the soil line and falls over. It usually follows wet mix, stagnant air, or dirty containers. Use fresh seed-starting mix and clean trays. Water from below and let the surface dry between waterings. Keep a gentle fan running nearby for air movement. A tray that is crowded, soggy and cool is where it spreads fastest.

Is it safe to plant out the week after the average last frost?

Not always. The February 28 to March 17 range comes from the 30-70% probability band, which means a freeze can still arrive after the middle of the range. Soil also stays cold longer than the air. Tender crops such as peppers, eggplant, okra and sweet potatoes will stop growing in cold ground, even if no frost arrives. Hold tender transplants until the forecast shows no freeze and nights stay mild, and keep row cover or a few spare plants on hand for a late cold snap. Cool-season crops such as onions, cabbage and lettuce can go out earlier. The South Carolina Planting Calendar lays out when each crop goes into the garden.

Common questions

When should I start tomato seeds indoors in South Carolina?

At Charleston, the window runs from January 17 to February 10. That counts back 5-6 weeks from the February 28 to March 17 frost range, using University of Minnesota Extension’s usual guidance. In the Upstate, where the last frost comes several weeks later, sow later than these dates.

Which seeds should I start first?

Onions and leeks, from November 15 to December 9. They need the longest time indoors, 14-15 weeks. Celery comes next, from November 29 to December 23. Cabbage-family crops and head lettuce follow from December 13 to January 6.

Do I need a grow light to start seeds indoors?

For most crops, yes. Window light in winter is weak, and seedlings stretch toward it. A grow light placed close above the plants keeps them short and sturdy. If you only have a window, choose the brightest one and turn the trays daily.

How long should I harden off seedlings?

University of Minnesota Extension says to start moving seedlings outdoors about two weeks before planting out. Begin with a few hours in shade and build up to a full day in sun. Bring the trays in on cold nights.