How to Start Leeks from Seed Indoors

Leeks are part of the Amaryllidaceae or onion family of plants. They’re a mainstay for fine dining restaurants. You’ll see them used in everything from Chinese stir-fries to grilled tapas in Spain. Once you try them, you’ll realize why so many foodies and chefs gush over them. 

These flavorful vegetables have shallow root systems and clusters of green leaves growing from a bulbous white stalk. There are tons of varieties, including early-season varieties and late-season varieties. 

Like other members of the onion family, leeks are easy to grow. You can buy starters at nurseries or even grow them by seed. Growing them by seed lets you plant a larger variety. You can also start them indoors ahead of the growing season. 

Below are the easy steps to starting your leeks indoors by seed. Follow these easy steps to prepare these delectable small onions for your garden. 

King Richard Leek Seeds

This onion relative has compact stems that thicken but do not form bulbs like onions. ‘King Richard’ produces extra long white stems early. Enjoy fresh, grilled, sautéed, and added to soups and stews. Cold tolerant to 20°F (-7°C) and can be left in the garden for an extended harvest.

Buy at Epic Gardening

Step 1: Understand Your Climate and Planting Window

Leeks are relatively hardy crops, but they’ll still succumb to freezing temperatures and late-season frosts. That’s why you need to know when your last frost usually occurs and your USDA hardiness zone. 

Check the USDA website and find your location and its zone. Next, look up your zip code on the Old Farmer’s Almanac site to find your expected last frost date. 

Use the recommended planting dates on the back of your seed packs to figure out the best time to start. The best time to start seeds indoors is usually 8 to 10 weeks before the last frost date. 

Step 2: Choose Your Varieties

Select the varieties you want to grow based on your climate and the flavors you want. There are early-season varieties and late-season varieties. Early season leaks grow quickly and mature between 75 and 100 days. Late-season varieties take longer and can handle cold weather. 

Each variety has a slightly different flavor profile. Some are mild, and others are spicy. If you’re not sure, buy several seed packs for different varieties. 

Generally, early-season varieties have milder flavor and are more compact. Late-season varieties, such as ‘American Flag’ and ‘Blue Solaise,’ are larger with more intense flavors. 

Step 3: Buy Your Seeds and Trays

Start your leek seeds up to 10 weeks before the last frost date in your area. Find a warm space inside your home or greenhouse to start your seeds. 

Select high-quality seeds from a reputable source. Look for organic varieties that are less than a year old. If you buy your packs from a nursery, make sure they’re in good condition. 

The next step is gathering your starting trays. You can use standard 6-cell seed starting trays or open flats. 

Step 4: Add Your Soil

After getting your seeds and trays, prepare your soil. Use organic potting mix amended with compost. It should be well-drained and rich in nitrogen. Try a preformulated seed starting mix for a more sterile option.

Carefully fill your trays up to the rim and avoid compressing. If you’re using open flats, make several shallow lines down the center of the flat. 

Step 4: Sow Your Seeds

Starting leeks from seed requires special attention to depth and spacing. Sow 3 to 4 seed clusters down the rows and cover them with a thin layer of soil. If your seeds are too deep, they won’t germinate. Too shallow, and they’ll be exposed to the elements and probably die before germinating. 

Leeks need about 3/8 inch to ¼ inch of soil above them. Sprinkle a thin layer above the seed clusters. Make sure all of them are covered. 

Step 5: Water Your Trays

Once you sow your seeds in their trays, gently spray them with water. Use a spray bottle to moisten the soil evenly. Be extra careful not to overwater and waterlog the trays. 

Once all the soil surrounding your seeds is moist, it will encourage germination. Continue watering them to prevent the soil from drying out. 

It’s important to be as gentle as possible. Stop watering if you see pools of water form on the surface. Wait until it drains and dries, then water again with a spray bottle or mister. 

Step 6: Put the Trays Over a Heating Mat

Germination heating mats are great for keeping the soil warm while starting leek seeds indoors. These mats work by heating the soil to 10°F warmer than the ambient air. Simply plug them in, put your trays over them, and wait until your seeds germinate. 

You can also add a germination dome on top of the trays. This will trap heat and moisture. This is especially helpful if you live in a cold space near drafts or large windows in winter. 

Your soil should stay between 70 and 80°F (21-27°C). A soil thermometer is a great tool to measure the temperature and keep it within its ideal range. 

Step 7: Label Your Trays

After getting your trays set up and your seeds planted, don’t forget to label them! It’s easy to mix up our seedlings, and labeling prevents any mixup or confusion. 

Paper labels and tape are popular, but wooden stakes or popsicle sticks work the best. Label each tray in case you misplace them. 

Wooden sticks are handy because you can use them in your garden after you transplant your seedlings. Simply stick them in the ground or garden bed soil around your plants, and they’ll last an entire season or more. 

Step 8: Maintain Your Seedlings

Keep your trays near a bright window with plenty of direct light. If your space doesn’t get enough light, try growing them under grow lights. Grow lights mimic the sun’s rays, so you can grow your plants in basements, closets, or any room in your home. 

Once they’re growing in a space with enough light, check on them once or twice a day. Make sure they have enough moisture. The soil should stay moist but not waterlogged. 

Your soil temperature should be between 70 and 80°F (21-27°C). Make sure the leek seedlings never experience freezing temperatures. If you maintain them in their right growing conditions, you’ll have seedlings emerge in one or two weeks. 

Step 9: Thin Out Your Seedlings

Once your seedlings emerge, check to see if they’re growing in clusters. If you see them bunched together, remove the less vigorous growth. 

Wait until they’re a couple of inches tall to start thinning. They’ll look like blades of grass growing in clusters. If there are only two plants next to each other, leave them be. Try to leave one or two seedlings every 6 inches. 

Preserve the tallest and strongest-looking plants. Avoid pulling them out too hard or disrupting the soil. Leeks have strong roots, but you still want to be delicate. 

Step 10: Acclimate Your Seeds to the Outdoors 

Before transplanting them outdoors, get them used to the elements by hardening them off. Indoor seedlings are accustomed to a stable environment. Once you move them outdoors, they’ll experience the intense rays of direct sunlight, wind, and changing temperatures. 

Acclimate your seedlings by moving the trays outside in the shade after the threat of frost has passed. This is usually a week before you intend to transplant them. 

Move them indoors at night for the first 2 days. After the second day, slowly place your trays in the direct sun for an hour. Move them back to the shade and store them on a patio or porch overnight. After about a week, they’ll be hardened off and ready for planting outdoors. 

Step 11: Transplant Them Outdoors

After hardening your seeds off, choose a garden bed or larger container to plant them. They should be about the size of a pencil.

Choose a space with partial shade and well-drained soil. Make a row of holes about 6 inches deep. Space them around 6 to 8 inches from each other. Rows should be spaced about 12 inches from each other. 

Place each leek seedling in one hole and cover its crown with soil. Give the soil a good watering until it’s moist. If you expect a late-season freeze, use a cold frame until the air warms up.