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

Count backward from the last spring frost, which falls between March 24 and April 4 in Louisville. That puts onions and leeks in pots from December 9 to December 27. Broccoli, cabbage and head lettuce go in January 6 to January 24, peppers and eggplant January 20 to January 31, and tomatoes February 10 to February 28. Colder parts of Kentucky start later than these dates, and milder parts start earlier.

The date everything counts back from

Seed packets give weeks, and weeks only mean something once you know what they are counted from. For indoor sowing the anchor is the last spring frost, because it is the reference date for moving seedlings outside, with hardy crops going out a little before it and tender ones a little after. NOAA’s 1991-2020 Climate Normals for the Louisville station give a last spring frost between March 24 and April 4, based on the band where the chance of a 32°F freeze falls between 30 and 70 percent. Every sowing window below is that range minus the weeks a crop needs indoors.

Why is the last frost a range and not one day?

A freeze after the average date is normal, not a freak event. The 30-70 percent band means that on March 24 a freeze afterward is still more likely than not in many years, while by April 4 it has become the exception. Gardeners pick a date inside that band according to how much risk they will tolerate and how fast they can cover plants on a cold night.

The range matters for sowing too. Seedlings sown at the early end of a window are ready sooner than the weather allows. They sit in pots getting tall and thin, with roots circling the container and little room left. Sowing at the late end of the window costs you weeks, so the plants are small and green when the garden is ready, and a crop like peppers, which is slow to start, can lose its early harvest. A sensible habit is to choose the middle of the window and shift toward the early end only if you have good light and space for larger plants.

The window for a crop is only as good as your frost date, so check the first and last frost dates in Kentucky for your own county before you write anything on a calendar.

Does the Louisville range apply to every Kentucky garden?

Not quite. The Louisville figures come from a single station in a large city, and urban areas tend to run warmer than the rural land around them, so the last spring frost in rural parts of Jefferson County and beyond often comes later than the figures here. Rural gardens may need to start somewhat later. The colder parts of the state, farther north or at higher elevation, have a later last frost, so spring tasks move later there. The milder parts have an earlier one, so sowing can move earlier.

USDA hardiness zones do not settle this. The USDA Plant Hardiness Zone Map (2023 edition, built on 1991-2020 normals) places Kentucky roughly from zone 6b to 7b, but a zone rests on one number, the average annual extreme minimum temperature. It tells you which perennials survive winter and says nothing about when the last spring frost arrives. For dating a sowing, use the Louisville frost range as the reference and adjust it for your own ground.

The decision to write on the calendar: count weeks back from a last frost date of March 24 to April 4, then shift a week or two later if your garden sits north of Louisville or on higher ground.

The calendar, crop by crop

University of Minnesota Extension publishes a widely used table of how many weeks each crop should stay indoors before planting out. It is the usual horticultural guidance, not a law, and a week either way will not ruin anything. The table below takes those weeks and counts them back from the Louisville last-frost range of March 24 to April 4. The Minnesota calendar months from the original table do not apply here, only the weeks.

Copy the windows as they stand and then adjust for your place. A garden in the colder part of the state sows later than these dates, and one in the milder part can start earlier.

Which crops need the longest head start?

Onions and leeks need the earliest start of anything on the list, up to 14-15 weeks, because their seedlings grow very slowly. A December sowing sounds absurdly early, yet the seedlings are thin as grass blades for most of their time indoors. Celery follows, and then the cabbage family: broccoli, Brussels sprouts, cabbage and cauliflower stay indoors 10-11 weeks, which puts them in pots in January. Those crops tolerate cool spring weather and can go out while nights are still cold, so counting back from the early part of the frost range is reasonable for them.

Peppers, eggplant and okra take 9 weeks, so sow peppers and eggplant January 20 to January 31. These crops hate cold, and they need the full 9 weeks to grow a stocky plant. Tomatoes are quicker, at 5-6 weeks, so they start in February. If you are new to them, the guide to growing tomato seedlings indoors covers pots, feeding and potting up in detail. Sweet potatoes, at 3-4 weeks, are the last to start on the list, and their slips go out only once the soil has warmed.

What about lettuce?

Head lettuce sits in the 10-11 week group, so January is the time to sow it. Many gardeners skip indoor starting for leaf lettuce altogether and sow it in the garden as soon as the soil can be worked, since it germinates in cool conditions and grows quickly. If you do want a head start, see starting lettuce for how to sow it in trays and when to move it outside.

Which crops should not be started indoors?

Root crops such as carrots, beets, radishes, turnips and parsnips go straight into the garden. Their taproots do not like being moved, and a transplanted carrot tends to fork and grow misshapen. Beans, peas, corn, cucumbers, melons and squash grow so fast outdoors that an early start buys little, and their roots resent disturbance too. In most cases it is better to sow them where they will grow once the soil is warm. If you want a short head start on squash or cucumbers, use a biodegradable pot that goes into the ground whole, and plant out young seedlings.

For the outdoor half of the schedule, including when to direct-sow and when to set out transplants, the Kentucky planting calendar picks up where this table stops.

Light, warmth and hardening off

A sunny windowsill looks like the ideal place for seedlings, but in January and February it rarely is. Winter days are short, the sun sits low, and a window delivers light from one side only. Seedlings respond by stretching toward it: stems get long and thin, and plants lean and flop. That leggy growth is a lack of light, and more warmth makes it worse. A fluorescent or LED grow light hung just above the seedlings fixes it. Keep the light a few inches from the leaves, raise it as the plants grow, and run it on a timer so the plants get a long, regular day.

How warm should seed trays be?

Seeds germinate best in steady warmth, and an unheated windowsill or a chilly basement swings too much from day to night. Peppers and eggplant are among the fussiest, tomatoes also like warmth, and cold, wet mix slows them down or rots the seed. A shelf in a warm room works, and a waterproof heat mat under the trays holds the mix at an even temperature until the sprouts appear. Take the mat away once most seeds have emerged, because seedlings that have sprouted prefer slightly cooler air, which builds thicker stems. The seed packet gives the right soil temperature range for each crop.

A small fan on low, set so the seedlings stir gently, toughens the stems and dries the surface of the mix. That helps against the fungal trouble covered in the next section.

How long does hardening off take?

Plants grown indoors have never felt wind, direct sun or a chilly night. Planted straight out, they scorch, wilt or snap. University of Minnesota Extension advises hardening off all plants over the last 1-2 weeks before planting out. In practice: set the trays outside in shade and out of the wind for a couple of hours, then add a little more time and more sun each day. By the end of the period the plants stay out all day and, if the forecast allows, through the night. Begin hardening off 1-2 weeks before your chosen planting-out date, and bring the trays in if frost is forecast.

What goes wrong

Most failures with seed starting trace back to three things: sowing too early, damp conditions that favor fungus, and moving plants out before the garden is ready. Each leaves a recognizable mark.

What does starting seeds too early look like?

Plants sown ahead of the window outgrow their pots before the weather allows planting. They are tall and floppy, the roots wind around the container, and the leaves may turn yellow because the mix has run out of nutrients. Tomatoes and peppers can start to bud while still in a small pot, and early buds drain energy from a plant that is not yet established. If it has already happened, move the plants into larger pots, bury tomato stems deeper, pinch off the early flowers and hold back on fertilizer. Next year, sow nearer the late end of the window.

How do you prevent damping off?

Damping off is a fungal disease that attacks young seedlings at the soil line. The stem turns thin and water-soaked, then collapses, usually after the seedling has come up looking healthy. It spreads fast through trays kept wet and still. To prevent it, use fresh seed-starting mix, wash used containers, water from below, let the surface dry between waterings and keep air moving over the trays. Once a tray is infected, remove the affected seedlings and the mix around them at once.

What happens if you plant out right after the average last frost?

The last frost range is a probability band. Even on April 4, the late end, a freeze is still possible, and the soil may be cold well past the last frost. Tomatoes, peppers and eggplant set into cold soil stop growing, and their leaves often turn purplish. A late freeze can kill them outright. So hold tender crops until after the frost range has passed and nights stay mild, and keep row cover or buckets handy for the weeks that follow. Cabbage-family plants can go out earlier, since they take a light freeze better.

Common questions

Can I start tomatoes in January in Kentucky?

You can, but you should not. Tomatoes need 5-6 weeks indoors before planting out, so the window in Louisville is February 10 to February 28. Sown in January, they would be crowded in their pots long before the soil and nights are warm enough to plant them.

Do I count from the first or the last date in the frost range?

Count from the date that fits your garden. The range of March 24 to April 4 is for Louisville. In a colder part of Kentucky, farther north or higher up, the last frost comes later, so sow later than the dates in the table. In a milder part, you can sow a bit earlier. The county extension office can tell you how your area compares.

Which vegetables should go straight into the garden?

Carrots, beets, radishes, turnips, parsnips, beans, peas, corn, cucumbers, melons and squash do best sown where they will grow. Their roots do not like being moved, and most of them grow fast enough once the soil warms that an indoor start gains little.

Do I need a grow light to start seeds?

A bright south-facing window can carry a few hardy crops, but in winter it rarely gives enough light for peppers, eggplant, onions or tomatoes, and seedlings will stretch and weaken. A grow light kept close above the plants gives a sturdier result, and it matters most for the crops that start earliest in the calendar.

Sources: ncei.noaa.gov | extension.umn.edu