19 Greatest Seeds To Begin Indoors in Winter

Starting seeds indoors is the best way to get a headstart on your gardening season and ensure robust, healthy transplants. Instead of relying on expensive nursery seedlings or imported plants at garden stores, you can easily start your seeds indoors in the winter so they are ready to plant in the spring.

But before you get too eager, be realistic about what crops benefit from indoor sowing. Seeding indoor tomatoes in December may be reasonable for gardeners in the subtropics and tropics, but it doesn’t make sense for northern growers. Unless you have a sophisticated greenhouse or grow light setup, waiting to sow warm-weather crops is best until late winter. 

Don’t worry. We will clear up all the details of indoor seeding times in this comprehensive guide to indoor winter planting!

Our Favorites for Indoor Winter Planting:

Romaine Lettuce

Rouge d’Hiver Romaine Lettuce Seeds


Sandwich Mix Sprouts Seeds



Mild Mix Microgreens Seeds


Sweet Peppers

Sweet Bell Blend Sweet Pepper Seeds

What Month Do You Start Seeding Indoors?

A close-up of small, delicate sprouts pushing through dark soil in a white pot on a windowsill. Surrounding pots showcase various thriving plants, all bathed in sunlight against the backdrop of a bright window.In winter, indoor seed sowing for early crops usually starts in late January or February.

Indoor winter seed sowing is a delicate dance between giving plants a headstart and not planting too soon. For most temperate zones, late January or February is a good time to start indoor seeding the earliest crops like leeks, onions, and cold-hardy greens.

February and March are best for starting long-season warm-weather crops like tomatoes and peppers indoors. However, the seasonality of winter seeding varies dramatically across landscapes. 

Defining Winter

Two brown pots rest on a clean white windowsill, each cradling a verdant plant. Beyond the window, delicate snowflakes gracefully descend, painting a wintry scene against the glass, contrasting with the warmth indoors.
Winter spans from the Winter Solstice to the Spring Equinox.

Most of us consider the winter months December through March-ish. The Winter Solstice (December 21) technically marks the Northern Hemisphere’s descent into winter due to the Earth’s tilt away from the sun. The Spring Equinox (March 19) is the technical first day of spring. 

For most climates, frost dates are more reliable data for marking the beginning and end of winter. After all, November is still a colorful autumn in Texas and Oregon, but high elevations and northern zones often start frosting around Halloween. Similarly, Montana and New Hampshire are often covered in snow in March, while Arizona and Georgia are frost-free and warm. 

If you don’t know your zone or frost dates, use a frost date calculator to understand your winter timing better.

What Plants Can I Start Indoors In Winter?

A diverse collection of herbs thrives in brown and blue pots. Each pot cradles a different herb, creating a vibrant and aromatic display. The sunlight enhances the lush greenery, promoting healthy growth.
Optimal indoor winter planting includes long-season alliums, cool-season brassicas, herbs, and warm-weather crops.

To prepare for early spring transplanting, the best plants to start inside during winter months, in order of planting, are:

  • Long-season alliums (onions, leeks, shallots, chives)
  • Cool-season brassicas (Brussels sprouts, kale, cauliflower, broccoli)
  • Herbs (parsley, cilantro, basil)
  • Warm-weather crops (tomatoes, peppers)

You can also grow a few plants indoors for winter harvests while the garden is dormant. Here are 19 plants to seed inside!

Late Fall/Early Winter (Right After First Frost)

A close-up of delicate seedlings nestled in individual clear plastic cups, each harboring a tiny promise of growth. The green leaves catch the sunlight, creating a mesmerizing play of light and shadow, illuminating their tender, hopeful existence.
For temperate gardens in November and December, focus on harvesting existing crops.

Unless you live in a subtropical southern zone, November and December are typically the months that temperate gardens are slowing down. You may continue harvesting established frost-hardy crops, but there isn’t much you can seed or transplant outdoors now. 

In the weeks after your first hard fall frosts, these veggies can be seeded indoors to grow in containers and enjoy modest harvests through the winter. It’s usually best to avoid seeding plants for transplanting out in the garden because it’s still too early, and they may become rootbound. 

1. Lettuce

Rows of lettuce thrive indoors within a specialized lettuce table setup. The leaves soak in ample sunlight, showcasing rich shades of green. The controlled environment ensures optimal conditions for their growth and health.
These are harvested as baby greens in a “cut and come again” style.

Salad greens like head lettuce and baby greens mix are perfect for seeding indoors once the ground has frozen overnight. These quick-germinating seeds grow easily in shallow containers with well-drained potting soil and a moderately sunlit window. 

While you can space them 6” apart for head lettuce, I much prefer to broadcast (scatter) the lettuce seeds and harvest them as baby salad mix. This means you can wait until they are 6-8” tall, then grab a bundle and cut at the base, leaving about 1” of growth above the soil. This “cut and come again” method ensures the growing tips stay intact so you can enjoy another harvest in a few weeks.

2. Sprouts

A clear plastic tray holds broccoli sprouts, their tiny green leaves unfurling under sunlight. The tray rests neatly on a white table, accentuating the lush, budding greens. The sprouts eagerly reach for the light, thriving in their nurturing environment.
Indoor sprouting is easy and quick and requires minimal care and low light.

Sprouts are the quickest, easiest green you can seed indoors. They don’t need as much light as most vegetables and barely require any tending. The key is to keep them moist and warm. This Botanical Interests Seed Sprouter is the perfect container kit to get started.

These seeds are sowed in dense trays and harvested just after they sprout from the soil. They provide quick nutrition and remain very tender with white stems, perfect for winter sandwiches and salads. Some popular options include broccoli sprouts, alfalfa sprouts, and ‘Sandwich Mix’ sprouts.

3. Microgreens

Vibrant microgreens flourish in green pots. The leaves boast an assortment of rich colors and textures, ranging from delicate emerald hues to robust, serrated edges, displaying a spectrum of tastes and nutrients within their tiny forms.
These reach 3-5 inches in height with slightly larger true leaves.

The difference between sprouts and microgreens is that sprouts are the youngest seedlings, with only cotyledons (the earliest sprouted leaves). Microgreens are more mature, around 3-5 inches tall, and have slightly larger true leaves.

You can harvest microgreens with clean shears to cut them off right at their base. For the cleanest greens possible, use a bottom watering tray so you don’t splash soil onto the microgreen foliage.

4. Indoor Herbs

White pots filled with a variety of herbs sit in white metal mesh baskets next to a kitchen window. The herbs flourish in separate pots, each displaying healthy foliage against the window's natural light.
Provide careful watering and a warm, well-lit environment for indoor herbs.

Mint, chives, lemon balm, and scallions are just a few of the herbs you can grow indoors almost year-round. If you have a warm south-facing windowsill or some grow lights, you can easily start these plants from seed or cutting during the winter. 

Always seed indoor herbs in containers without drainage holes and catchment trays. Terracotta is a great option for ensuring breathability in the root zone and longevity of your herb harvest.

The keys to successful indoor herbs are well-drained soil mix and modest watering. A potting blend with plenty of compost, peat moss, and vermiculite is ideal. Keep seeds moist without overwatering.

Always check the soil with your finger before adding any moisture. If the soil sticks heavily to your skin, it is sufficiently wet. If your skin comes out dry, it’s time to irrigate until water flows from the bottom into the catchment tray. When the water flows out of the drainage hole, you can stop watering because you know it has saturated the lower layers of soil.

Mid-Winter (8-12 Weeks Before Last Frost)

Green vegetable seedlings rest in clear and black trays, their tender leaves unfurling, hinting at future harvests. Placed on a windowsill, they soak in the warm, golden sunlight, fostering growth and vitality.
Winter marks the start of seeding long-season crops indoors before the last frost for spring transplantation.

In the frigid middle of winter, propagation season technically begins for most farmers. This is the time to start seeding long-season crops in seedling trays so they are ready to transplant when the ground thaws. Many alliums, brassicas, and perennials can be sown 8-12 weeks before your expected last frost date in the spring.

For example, if you’re in zone 7 and your last frost date is in early April, you would start these seeds indoors in late January or February. In warmer zones 8 and 9, you can seed these crops indoors as soon as December or early January to prepare for early spring transplanting.

As we’ll explore below, seeding too soon is the most important thing to avoid. You don’t want your seedlings to get overgrown, leggy, or rootbound before it’s time to transplant. When in doubt, wait until closer to March.

5. Leeks

Long, slender leeks with delicate blue buds peeking from their tips grow abundantly within a sturdy brown pot. The intricate blossoms atop the leeks add a touch of ethereal beauty to the verdant scene against the crisp white backdrop.
They start indoors as seeds in late winter due to their slow growth and long maturation.

We often consider leeks an autumn or winter crop because the plants can tolerate frost and wait in the cold garden to harvest. However, this versatile allium starts in the late months of winter.

Long-season giant leek varieties like ‘Bandit,’ ‘Giant Musselburgh,’ or ‘Electra’ take up to 100 or 150 days to mature! These onion relatives grow quite slowly and benefit from indoor sowing in mid-winter.

Leeks are best grown in open flats, making them easy to cut back in the spring. Prepare a shallow open tray with a well-drained seed starting mix.

Broadcast the leek seeds over the surface about ¼” apart and ¼” deep. Keep in bright sunlight and supplement with a grow light if needed. When they are about 6” tall, you can give them a “haircut” to encourage stronger root formation and thicker stalks. They should be pencil-thick and ready to transplant when your outdoor garden soil is workable.

6. Onions

Fresh green onions sprout from rich, brown soil nestled in a rectangular, green pot. The slender, elongated leaves stand tall, capturing the sunlight that streams through a nearby window, embracing the warmth for nourishment and growth.
Starting onions indoors 8-10 weeks before the last frost date boosts growth for shorter seasons.

Many storage onions and sweet onions take over 100 days to mature, which means indoor sowing is the perfect head start for areas with short growing seasons. You can start onions 8-10 weeks before the last frost date.

Broadcast in open flats or sow into cell trays at about ¼” deep. When the onion tops reach about 5” tall, snip off a few inches of the slender leaves to encourage stronger bulb development. The clippings can be used like chives!

Onions form bulbs based on the day-length. Be sure to choose the right type of onion for your region! In the US, it’s simple to break down which is best for you:

  • Short-day onions are best for the southernmost latitudes
  • Intermediate-day onions are best in latitudes 32-42
  • Long-day onions are best for the northernmost latitudes

As a general rule, most parts of the US are able to grow two day-length types of onions, location-depending, but those furthest north or south points may be limited to just one day- length.

For the largest onions possible, thin to 1 seed per cell if they stay in the tray for a while. You can grow smaller onions in bundles of 2 to 3 per cell or separate a densely-seeded tray to transplant later individually.

7. Celeriac 

Verdant, vibrant celeriac leaves stretch upward, textured with serrated edges and a lush green hue. Behind the celeriac, a robust plant stands, boasting broad, glossy leaves that unfurl elegantly, capturing the essence of vitality.
These plants require consistent moisture and warmth until seedlings emerge.

Also known as celery root, this unique alien-looking bulbous celery cousin has a very long growing season. This vegetable takes up to 120 days to mature, but the potatoey texture and cozy homestyle flavor are worthwhile for incredible fall soups, stews, and roasts.

It’s best to seed celeriac inside about 10-12 weeks before you plan to transplant outdoors. For most zones, this falls in mid-winter, around January through March. Celeriac seeds are very small and should only be sown about ⅛” deep. 

They require consistent moisture and soil temperatures around 70-75°F until the seedlings emerge. Be patient! This can take 2-3 weeks. Once germinated, the temperature can be reduced to 60-70°F. Grow indoors until outdoor weather is consistently above 55°F. If you transplant too soon, celeriac may bolt in cool weather.

8. Celery

Green celery leaves form a lush and dense carpet under the golden rays of the sun. Each leaf glistens with dew, creating a mosaic of shadows and highlights that dance across the foliage.
Like carrots, celery requires careful attention to moisture and temperature fluctuations.

This more well-known parsley family member is one of the most finicky crops I’ve grown. Celery is not necessarily hard to cultivate, but it is sensitive to large fluctuations in moisture or temperature, sort of like carrots. Don’t let this deter you from growing celery! Just be sure you carefully follow seed instructions and keep the seedlings protected through the early winter and spring.

Seed celery indoors in late February or early March, approximately 10-12 weeks before your transplant date. I usually push the transplant date at least one week after the frost date because the seedlings are so prone to bolting in cool weather. 

You never want young celery plants to be exposed to temperatures below 55°F. Don’t lower the temperature or put them out in the cold when hardening them off. Reduce the water for 7-10 days and plan to transplant under protection, like a row cover or a low tunnel.

9. Stevia 

A close-up of green Stevia leaves, showcasing serrated edges and delicate veins, emanating a fresh, herbal aroma. The blurred background gracefully showcases additional stevia leaves, their muted hues forming a natural tapestry.
Grow stevia indoors by sowing seeds shallowly 6-10 weeks before your area’s estimated frost date.

You can grow this popular sugar substitute right in your garden! Stevia seeds are best sown indoors about 6-10 weeks before your estimated frost date. The huge range depends on your climate because stevia needs to stay indoors until temperatures are reliably above 45°F.

If you want to keep it as a container herb, you can sow it almost any time in the winter and keep it by a south or west-facing window. It tolerates partial shade in the South but enjoys more light in windowsill settings.

Sow the tiny seeds very shallowly, and be careful to avoid overwatering. To encourage bushiness, pinch back the top growing tips every few weeks. Scientifically, this is called removing the apical meristem (top growing point). Doing so signals the plant to grow out rather than up. Bushier plants are more attractive and yield more usable leaves for your teas or recipes.

10. Rosemary

A rosemary nestled in a clean white container. Positioned near a window, the herb bathes in sunlight. Its needle-like leaves radiate a deep green hue, bringing a touch of nature's elegance to the space.
Growing rosemary from seed requires patience, taking 2-4 weeks indoors for germination.

Rosemary is primarily propagated by cutting, but growing from seed is certainly a fun project that unlocks more diverse varietal options. However, this process requires extreme patience. After sowing the seeds indoors in February or March, you must wait 2-4 weeks for germination. This slow-growing woody plant takes up to a year before it will start producing harvestable springs. 

Before planting, thoroughly moisten your seed starting mix so it feels like a wrung-out sponge. Watering rosemary requires a balance between consistent moisture and never creating soggy conditions that may cause rotting. Rosemary seeds require light for germination, so only press them into the soil surface and do not cover them. A heating mat and grow lights can speed up the process. Don’t up-pot or transplant until the plants are at least 3-5” tall and develop a robust root system.

If you’re impatient, I recommend growing rosemary from a cutting. You can root the cuttings indoors during mid-winter and have transplantable seedlings by mid-spring. Remember that rosemary is a Mediterranean plant that loves drainage, so be sure the soil is gravelly and very porous.

11. Lavender

A close-up of lavender blooms and green foliage flourishing in a terracotta pot. The delicate purple petals contrast beautifully with the lush, aromatic leaves. Bathed in sunlight, the intricate textures and hues create a serene natural display.A popular Mediterranean herb, lavender requires patience, taking up to 2 years to bloom.

Another popular Mediterranean perennial herb, lavender, is rarely propagated from seed, but I think the project is worthwhile for genetic diversity and affordability if you’re doing a large planting. Seeds are significantly cheaper than established plants. While most lavender plants are grown from nursery starts or cuttings, you could source a unique lavender cultivar like Portuguese lavender (Lavandula latifolia) or ‘Blue Cushion’ English lavender. Sometimes, these varieties aren’t available at nurseries, making seed-sowing the only option.

Before getting too excited, note that lavender seeds take even longer than rosemary to get established. This is not for the impatient gardener! It can take up to 2 years for the plants to bloom from the sowing date, so buckle up and sow many seeds!

Lavender seeds should be started indoors 10-12 weeks before the last frost date. But first, they require cold exposure to germinate. It’s best to scatter the seeds on a moist paper towel, place them in a plastic bag, and refrigerate them for 30-40 days in mid-winter.

Ensure they stay moist! In mid-February or March, pull the seeds from the refrigerator and place them in a window with direct sunlight. Wait 7-14 days for germination, then use a tiny spoon or tool to transplant the baby seedlings into the soil mix.

12. Strawberries

Fresh strawberries growing in black pots within a greenhouse. Juicy, red fruits alongside unripe ones hang from green plants, showcasing a mix of ripeness and growth stages in this thriving strawberry patch.
Indoor planting of bare-root strawberry crowns in winter provides a head start for transplanting.

While strawberries are not typically grown from seed, bare-root strawberry crowns benefit from indoor establishment in mid-winter. I like to order the bare root crowns in early winter as soon as they’re available. Most seed companies will ship the crowns at the perfect time for your zone. 

It helps to establish the plants in containers so they have a head start. Plant the crowns in a seed-starting mix in 4” pots. Be very careful not to bury the crown too deep or too shallow. The roots should be submerged, and the upper twigs should be exposed above the soil. Once the plants begin growing leaves and anchoring their roots in the pot, you can transplant around your last frost date, just like you would plant a standard vegetable seedling.

Strawberries are a unique crop because there are so many varieties and methods for growing. I prefer day-neutral varieties like ‘Seascape’ and ‘Albion’ because they fruit in the same year they are planted. Sometimes, I start the plants indoors, transplant them out in late spring, and eat fresh berries by June! 

If you don’t want to establish a perennial bed, day neutrals have the added advantage that they can be grown like an annual. This makes them easier to rotate with your annual vegetable crops. It also makes weed maintenance far easier than the “matted row” system of perennial strawberry production.

13. Bell Peppers

A close-up of two red bell peppers with green stems and leaves. The peppers glisten in the sunlight, showcasing their glossy surface. Their rich color and textured skin create a visually appealing composition.
These require indoor starting for ample ripening time in colder climates.

Peppers are notoriously slow-growing and can be started indoors several weeks before tomatoes. In cold climates, bell peppers especially need indoor growth to have enough time to ripen and turn colorful in the late summer. Otherwise, you end up with a bunch of green bells! 

Start bell pepper seeds 8-10 weeks before transplanting. The transplant date should be 2-4 weeks after the average last frost when temperatures are consistently above 55°F. Peppers adore warm weather and have very little tolerance for cold. The young seedlings are particularly vulnerable to chilly weather and need soil temperatures around 70-90°F to germinate properly.

Sow ¼” deep in cell trays and wait 10-25 days to emerge. Thin the plants to 1 pepper plant per cell. Maintain consistent moisture. 

14. Hot Peppers

A cluster of hot peppers, showcasing purple, round-shaped pods amid lush green leaves and delicate white flowers. The plant bursts with life, a mix of fiery colors and lush greenery.
Transplant hot peppers after 8-10 weeks when the weather is warm.

If you can handle the spice, hot peppers are a joy to grow, harvest, and make into hot sauces! However, the seeds can be very spicy on their own. When you seed the peppers, it is essential to wear gloves and avoid touching your eyes or face after handling the seeds! Trust me, this common mistake is not fun to endure.

Sow hot peppers indoors 8-10 weeks before transplanting, at the same time as bell peppers. They require the same warm conditions and shouldn’t be moved outside until the weather is thoroughly settled and the seedlings are 4-6” tall. Pinching the tops encourages bushier growth.

Late Winter/Early Spring (6-8 Weeks Before Last Frost)

A close-up of homegrown kale microgreens flourishing in a clear tray, each leaf showcasing intricate patterns. Sunlight cascades over the miniature leaves, highlighting their lush green hues and delicate textures.
This marks a crowded period in indoor seeding for warm-weather crops like tomatoes and peppers.

Late winter is when the real garden magic starts happening! Once you reach 6-8 weeks before your last frost date, your greenhouse or indoor seeding area can get quite crowded! Most of our favorite warm-weather crops, like tomatoes and peppers, should be sown now.

The time window for seeding brassicas, nightshades, and other warm-weather annuals could be anywhere from February to April or even May in very cold climates. For example, zone 8 growers can sow these seeds in February, but zone 5 gardeners should probably wait until March.

15. Kale

Sunlight illuminates a lush ensemble of kale leaves, highlighting their rich, verdant tones and crinkled textures. Each leaf showcases a blend of green shades, with delicate frilly edges that catch the light.
They thrive from early spring planting and yield harvests through late fall.

On most commercial organic farms, kale was among our top crops (alongside tomatoes and melons). The cool thing was we usually only planted kale once in the early spring and harvested the leaves until late fall. You can enjoy the same prolonged harvest window in your garden by establishing robust kale seedlings indoors and transplanting them out around the first frost date.

Sow kale seeds in the late winter, 4-6 weeks before your last frost date. The little round seeds should be planted about ¼” deep in cell plugs or open flats. Cover with a well-drained seed starting mix and maintain continuous moisture. Kale seedlings prefer a cozy room temperature during germination but enjoy cooler weather once established. 

You can transplant slightly before the last frost date when they are 5-6” tall with several sets of true leaves. I always cover kale with row fabric to protect it from early-emerging flea beetles and aphids. These practices apply to almost any variety of kale. ‘Black Magic’ and ‘Red Russian’ are two of my favorites!

16. Cauliflower

Abundant cauliflower leaves basking in sunlight, showcasing green hues and intricate, frilled edges. The textured surface glistens with dew, emphasizing the veined patterns that sprawl across each leaf, a testament to their robust health and vitality.
Begin cauliflower seeds indoors about 4-6 weeks before your last frost.

An early spring cauliflower crop is quite a treat and not hard to accomplish! About 4-6 weeks before your last frost, sow cauliflower seeds ½” deep in cell plugs filled with seed starter mix. Keep the soil around 70°F until germination and 60°F after. If you don’t have a soil thermometer, keep the trays in your home windowsill at room temperature.

Cauliflower mustn’t get too large while still in containers. Transplant outdoors when seedlings are 4-5 weeks old. Putting them outside 1-2 weeks before your last frost is okay.

Be sure to harden off the seedlings by gradually increasing cool weather on a patio or porch before putting them in the ground. Again, I like to cover them with row fabric or a low tunnel to prevent temperature stress and keep early-emerging pests from damaging the young leaves.

17. Broccoli

A close-up of two fresh broccoli heads with thick, green stems and tightly clustered florets. The stalks appear vibrant and firm, showcasing their rich green hues against a soft, blurred background.
Brassicas, including broccoli, are uniformly treated for seeding and transplanting.

By now, you can see the theme: Brassicas (cole-family crops) are all seeded and transplanted in the exact same way. I treat my broccoli, cauliflower, kale, and cabbage pretty much the same as seedlings. They benefit from the same conditions, watering, sunlight, and timing. 

Broccoli is another crop that benefits from indoor sowing in late winter, about 4-6 weeks before the last spring frost. Sow seeds ¼” deep and follow the same procedures as cauliflower. Choose a spring or summer variety like ‘Belstar’ for bolt resistance in the heat.

18. Tomatoes

Three green pots aligned in a row next to a yellow square pot positioned near a white window. Lush tomato plants thrive within each pot, showcasing healthy foliage and growing stems against the bright window backdrop.
Starting tomatoes indoors too early can lead to leggy plants.

This is usually the summer crop gardeners are most excited to start indoors, but starting too early can hinder your production. Leggy, wimpy tomato plants usually result from seeding too soon and keeping the plants in containers for too long. By March or April, you are usually in the clear! Avoid seeding more than 4-6 weeks before your frost date. Larger containers can prolong healthy indoor development.

Most varieties of tomatoes can be seeded ¼” deep in a well-drained seed starting mix rich in compost. I like to use 4-cell plugs or 4” pots, but you can also start in smaller cells and up-pot as needed. 

Tomatoes germinate most successfully in extra-warm conditions like a heated seed starting mat. The soil should be 70-90°F, and ample light is essential. If you don’t have a bright south-facing window, I highly recommend supplementing with grow lights. The sun-loving tomato seedlings will reach up toward the light and develop weakened, leggy stems if they don’t have enough sunshine in the early stages.

19. Basil

A close-up of a white pot containing flourishing basil. The sun's rays illuminate the verdant foliage, highlighting its texture and shades of green. The contrast between the bright leaves and the pot's neutral color is visually strikinRegularly pinching basil encourages a bushy shape and continuous leaf production for a consistent supply.

No summer garden is complete without basil! This warm-weather essential appreciates an indoor start in the late winter. Seed ¼” deep in cell plugs about 4-6 weeks before transplanting. I usually wait to transplant basil until several weeks after the last frost date because even the slightest chill under 40°F can damage or stunt these cold-sensitive plants.

Basil benefits from pinching to stimulate bushy growth with lots of lateral shoots and a profusion of flavorful leaves. I like to sow several successions of basil every three weeks for a continuous supply of pesto ingredients!

Mistakes to Avoid

The biggest mistake gardeners make is starting seeds indoors too early. As a rule of thumb, the winter solstice (December 21) marks the first day of winter and the earliest possible indoor sowing. Since this time of year is usually pretty busy, I’d recommend starting your earliest seeds in January if you are in zone 6 or warmer. In zone 5 and colder, sowing isn’t typically sensible until March. 

How to Seed Indoors at the Right Time

A close-up of a black seedling tray showcasing young hot chili pepper plants, their verdant leaves bursting with life. Each delicate seedling stands erect, promising a future harvest of fiery peppers.
Start indoor seeds by calculating maturity days before transplanting, accounting for the last frost date.

Timing is everything with seed-starting. To get your timing right, you’ll need to determine three dates:

  1. Last Frost Date
  2. Seeding Date
  3. Transplanting Date

Your last frost date and the days to maturity listed on your seed packets serve as your best “insurance policy” against planting too early. Use these to calculate your seeding date.

If you are unsure about when to start something indoors, here’s a quick rundown:

  1. First, check the seed packet recommendation. It should say how many weeks to start indoors before transplanting. This is your seeding date.
  2. Find your last frost date online.
  3. Use your last frost date to determine an ideal transplanting date.
  4. For tomatoes, peppers, and squash, wait until 1-2 weeks after the frost date.
  5. For onions, leeks, and cold-weather greens, consider transplanting 1-2 weeks before.
  6. Count backward from your transplant date to determine your sowing date.

Don’t worry; this isn’t complex math! You can easily Google “X days before X date” to get an estimation. There is no need to be perfectly precise, as the weather is always changing. Here are some examples to clear things up.

Tomato Example

Tomato seedlings thriving in a black tray and four dark green pots. These containers are placed neatly on a white windowsill, receiving ample sunlight. Pink and green label sticks in the pots add a touch of organization to the budding garden.Consider starting ‘Glacier Bush’ tomatoes indoors about 4-6 weeks before the last frost date.

This ‘Glacier Bush’ tomato from Botanical Interests recommends starting indoors 4 to 6 weeks before transplanting. If I were gardening in Santa Fe, New Mexico (USDA growing zone 6b), my estimated last frost date would be around May 20. 

To be extra safe, I may want to wait to transplant my tomatoes until May 27 because I know the spring mountain weather can be finicky. Six weeks before May 27 is around April 15. I could get away with seeding tomatoes indoors in larger cell containers around April 1. If I sow them too soon, they may be leggy or rootbound when it’s warm enough to put them outside.

Pepper Example

A close-up of red sweet bell peppers showcasing glossy, smooth skin. Adjacent, verdant stems and leaves complement the peppers' vivid color, creating a vivid display of freshness and ripeness in this close-up shot.
Start peppers indoors earlier due to their slow growth compared to tomatoes.

Peppers are a long-season crop that typically grows much slower than tomatoes, so they must be started indoors earlier. However, they are extra cold-sensitive and often need a heating mat to germinate evenly.

This ‘Sweet Bell Blend’ recommends starting seeds indoors 8 to 10 weeks before transplanting. You should wait to transplant pepper seedlings outside 2 to 4 weeks after the last frost date. Let’s do that quick calculation:

  1. If I’m gardening near Nashville, Tennessee (USDA zone 7a), my last frost date is April 7.
  2. Since last spring was pretty cold, I will wait until three weeks after that date to transplant peppers.
  3. Three weeks after April 7 is April 28. This is my transplant date.
  4. Ten weeks before April 28 is February 17.
  5. I should seed these peppers indoors around the second or third week of February.

Crops to Avoid Starting Indoors in Winter

Not all crops are suited to indoor winter sowing. Wait until spring to avoid seeding these plants too early!

Cucurbit Family

A close-up of green zucchinis showcasing glossy skin and long, fresh stems. Delicate yellow flowers crown the tips of the zucchinis, adding a bright touch to the scene. The verdant hues and textures reveal the youthful vitality of these zucchinis.
Begin cucurbit-family crops indoors 1-2 weeks before the last spring frost.

Avoid starting cucurbit-family crops (squash, cucumbers, zucchini) indoors too early. These plants grow rapidly and are sensitive to transplanting.

If you start them too soon, they may become rootbound in their containers and have difficulty adjusting to transplanting in your garden. Most squash, cucumber, and zucchini varieties should be started 1-2 weeks before your last spring frost.

Root Crops

Carrots growing in a brown pot, showcasing small roots and leafy tops. Despite efforts, these indoor-grown carrots struggled to reach their full size. The confined space limited their growth, resulting in stunted development.
Sowing root crops directly in the garden is ideal when the soil is ready.

Avoid starting root crops indoors. Carrots, parsnips, turnips, and radishes are best direct-sown in the garden unless you have biodegradable pots that you can easily transplant without disturbing the fragile taproots.

Wait until the soil is workable, then seed these cool-weather roots outside. Protecting with a cold frame, low tunnel, or floating row fabric is best.

Final Thoughts

The key to success with indoor winter sowing is timing! Be sure to calculate and write out your last frost date, transplant date, and sowing date so you can properly time your plants. Starting too early is often a bigger problem than starting too late. You may be eager to get in the garden, but your plants mustn’t sit in containers for too long.

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