How and When to Start Vegetable Seeds Indoors

Sowing seeds indoors is the best way to jumpstart your garden while temperatures remain chilly outside. From peppers to tomatoes to chard, a wide range of vegetables thrive as transplants.

Compared to direct sowing in the garden, seed starting indoors has a range of benefits:

• Faster growth
• Higher germination rates (less wasted seed)
• Protection from pests
• Moderated temperature and environment
• Consistent seedlings
• Easier monitoring

Before you spend a bunch of money and time sowing seeds for the new season, use the following guide to ensure the best germination possible. With these tips, your garden starts will be stronger and healthier than ever before! Let’s dig in!

Tools and Supplies

The most basic seed starting setup requires:

• Seeds
• Containers: Pots, cell trays, or flats
• Seed-starter mix
• Watering can or hose
• A warm environment with plenty of light
• Germination mat (optional)
• Grow lights (optional)

We’ll dive into the details of how you will use these supplies and how to choose the best products below.

Keep in mind that starting vegetable seeds can be as simple or as complicated as you want to make it! Toss some seeds in a container of soil, water them, and they may grow very well.

This guide goes into deeper detail based on the time-tested successes (and inevitable failures) of professional farmers and gardeners who have mastered the art of seed starting.

Starting Seeds Indoors in 10 Steps

As you tuck a tiny seed into the soil, you are also sowing the hopefulness and excitement of a new gardening season. But you want to be sure that your investment is worthwhile.

Think of this as your roadmap to mastering indoor seed starting. While you can’t guarantee every seed will survive, these steps will maximize your chances for strong, high-yielding plants.

Step 1: Planning & Timing

The most successful gardeners aren’t just planters; they’re planners. Before you jump into the seeding process, be sure that you correctly time your planting. Nature does not operate on a reliable clock, but we have several tools to help us guess her next move:

Key Takeaway: Plan before you seed! This will save you a lot of time, effort, and disappointment. No amount of planning will eliminate the risk of crop failures, but it will certainly make you a better gardener for next season.

Step 2: Order 20% More Seed Than You Need

When ordering seeds, consider how many plants you want to grow for a given season. If I want to feed my family one head of broccoli every week from a north Texas garden, I will need at least 40 broccoli seeds to succession plant throughout the season. This means I should order a packet of 50-60 seeds to account for failed germination  or crop loss.

If you are growing heirloom seeds, you may want to order even more extras. Some heirloom or specialty varieties have lower germination rates than hybrid seeds.

Key Takeaway: It’s better to have more plants than not enough. Sow a few extra seeds and if the plants are successful, you can always give them away to your neighbors.

Step 3: Select Your Soil Mix

Seed-starter mix is the next most important step for your seeding success. A baby plant is only as strong as the environment where it begins its life. While advanced gardeners often make their own seedling mix to save money, it’s best for beginners to purchase a store-bought blend.

The best soil mix for seed-starting has a balanced combination of organic matter like compost or peat moss and high-drainage materials like perlite or vermiculite.

High Quality Soil Mix Characteristics

The mix is well-aerated to allow quick water drainage and plenty of airspaces for baby seedling roots to form. Remember that seedling mix is different from potting mix.

A high-quality soil blend is essential for:

• Even germination rates
• Preventing damping off disease
• Proper drainage and irrigation
• Healthy root formation

If you decide to make your own seed starter mix, be sure that it has plenty of high-porosity materials that improve the drainage. When mixing your soil, you can mix in perlite or vermiculite. Coco coir and peat moss can also be used.

Step 4: Choose Your Containers

Seeds can technically be started in any type of container. You could use plastic bowls, empty egg cartons, or even soil-blocks that don’t need a pot. However, the most common types of seed-starting containers are small pots, 6-packs, open flats, or cell trays.

Before seeding, check that your containers have drainage holes. To prevent seedling diseases like damping off, clean and sanitize the pots with a spray bottle of diluted bleach solution. Here’s a breakdown of what types of vegetables should be inside each seed tray, as well as what you should plan for.

3-4″ Pots

The advantage of a round or square pot is that it allows a seedling to grow to a robust size before transplanting. With proper handling, these containers also minimize root disturbance for Cucurbit crops (squash, cucumbers, pumpkins, etc.) that are very sensitive to transplanting.

The downside is that they require more soil and take up more space. To save soil and improve their chances for success, some gardeners start larger quantities of certain crops (like tomatoes or peppers) in 6-packs or cell trays.

Then, they up-pot the most successful ones to a 4” pot so they can mature further before transplanting.

6 Pack Seed Trays

Six-celled trays or “6-packs” are the most common seedling trays found at garden stores because they are easy to handle and transport. However, the closeness of the cells means that seedlings are more prone to overcrowding or becoming “leggy.”

When seeding in 6-packs, be very aware of how fast the plants fill out their cells. When you start to see roots poking out the bottom, it’s time to get those babies in the ground! You don’t want your seedlings to become overgrown and rootbound before transplanting.

Open Flats

Open flats are the only containers that have little to no drainage holes. The flat-bottomed tray is a bit like a bowl of seed starter mix. You make furrows or indented lines in the soil and broadcast or sprinkle the seeds close together.

Allium-family crops like leeks and onions do great with this form of seeding. Because these crops don’t mind root disturbance, you can plant them in clumps of 3-4 or gently separate the roots.

50 or 72 Cell Seed Trays

These multi-celled flats may be too large for small gardeners; however, they are very beneficial for larger homesteads and small farms. Keep in mind that you can save soil mix by starting crops in smaller cells, then later transplanting to a larger pot.

For example, let’s say you have a 50-cell tray. You can sow 25 ‘Sungold’ tomato seeds and 25 ‘Beefsteak’ tomato seeds in the same tray. Of those seeds, let’s assume 40 total plants survive.

After a couple weeks, you see that 35 of the tomato seedlings look robust enough to up-pot. You prepare 4” square pots with a little bit of soil and transplant the tomato cells into the larger container. You can plant it deeper in the new container to encourage stronger roots before transplanting.

Filling Seed Cells

Filling a seed cell or tray is all about striking a balance: You want the container to be full of soil, but you don’t want to compress the seed starting to mix into the cell.

1. Begin with a flat surface and pre-moistened seedling mix.
2. Use a plastic cup or scoop to dump the soil mix over the seeding container.
3. Use your hands to spread it out over the cells, allowing the soil to naturally fall into each slot.
4. Avoid compressing or pressuring the mix into the pot.
5. Instead, lift up the tray and gently tap it on the table to let the mix settle.
6. Dump a bit mix more on top and spread it out with your hands until each cell is full.

Step 5: Sow at the Proper Depth

One of the biggest mistakes you can make while seeding is planting your seeds at an improper depth. If the seed is too deep, it won’t have enough energy to germinate.

If it is too shallow, it will be prematurely exposed to light, pests, or dry conditions. Find the happy medium with this simple trick:

Seed Depth Rule of Thumb: Sow seeds twice as deep as their largest dimension. It really is that simple!

The main reason for different seed depths has to do with a part of the seed called the endosperm. Think of a seed as a baby (embryo) in a package (seed coat) with some food (endosperm).

The baby only has enough food (endosperm, or the starchy part of the seed) to fuel it to germinate and reach the soil surface where it can start making its own food via photosynthesis.

A large seed like a pumpkin has a lot of endosperm, which means it can germinate deeper in the soil and sustain itself for longer in the darkness. But if you plant a tiny seed like basil deep in the soil, it won’t have enough energy to crawl up toward the light.

Key Takeaway: Examine how large each seed is. Plant it at a depth that is about twice its size. No need to pull out the measuring tape— a knowledgeable estimate will do!

Step 6: Water Gently

Never plant a seed without immediately supplying water! Once your trays are filled and your seeds are tucked in at the perfect depth, you need to give the seed a hefty drink to “awaken” it from dormancy. When seed-to-soil contact is made, the plant needs a continuous supply of moisture until germination.

When watering seeds, remember to:

• Make the first soaking count.
• This ensures the seed is completely surrounded by moist soil.
• This initiates the germination process.
• Evenly distribute the water over the trays.
• Never hold the hose in one place.
• Do not allow water to “pool up” or run off the surface.
• Avoid overwatering! The soil should be moist but never soggy.
• Irrigate until water comes out of the drainage holes, then stop.
• Use a fan-spray hose nozzle to mimic a pleasant drizzle of rainfall.
• Do not blast seeds or seedlings with a heavy stream of water.
• This can dislodge the fragile baby roots.

For very tiny seeds, lightly tamper down the soil surface to ensure that the seeds don’t float away when watering.

Step 7: Label Your Trays

Use wooden popsicle sticks and a permanent marker to label every container with the seed variety and date. Some people use tape, but it is not as reliable. A pencil or pen will wash off with watering.

I like to arrange my plant labels so they are all facing the same direction and can be easily referenced when I walk by. You will want to remember which varieties worked best in your garden, so keep your popsicle sticks with the plant even after transplanting into the garden.

Step 8: Use a Germination Heating Mat

A germination heat mat is an electric waterproof mat that is sized to fit under a seedling tray. The mat can warm the soil in the container up to 10-20°F hotter than the ambient temperature.

If you are growing in an unheated greenhouse or near a cold window in your home, a germination mat ensures more consistency for the emerging seeds.

While it is not necessary to use germination mats, most seeds germinate more evenly and quickly with bottom heat. Warm-weather crops like tomatoes, peppers, zucchini, and cucumbers are especially fond of bottom heating that keeps the soil temperature around 70 to 75°F.

Simply plug in the mat and place your seedling containers on top of it for the duration of the germination process.

Some seeds (like brassicas or lettuce) do best when they are removed from the mat immediately after they germinate. Others (like peppers and squash) enjoy the extra warmth for several weeks after germination.

Step 9: Provide Optimal Growing Conditions

Once you’ve mastered the seeding process, you need to pay close attention to the germination environment. Luckily, you only need to create your setup once and you can use it for years to come.

The most popular seed growing options include:

• A mini greenhouse with shelves.
• A bright, south-facing windowsill.
• Seedling germination shelf with lights
• Germination dome with light.
• A sunroom or Florida room with glass windows
• A bright patio or porch.

Depending on your setup, you may need to monitor and adjust the light, humidity, and temperature to optimize your seedling success. You can get seeds started very cheaply and easily, but serious gardeners may want to add monitoring devices over time to streamline the process.

Step 10: Properly Thin Out Seedlings

When your seeds begin emerging, it’s easy to become invested in every little seedling. After all, you worked so hard to sow and nurture them!

However, thinning is an essential part of the seeding process because it ensures that your plants will be successful. You have to kill a few baby plants to optimize the growth of the ones you plan to keep.

When to Start Thinning

The best time to thin seedlings is when they have 1-2 pairs of true leaves or they are 2-3” tall. It’s important that you don’t think too soon (during the cotyledon stage) because it can be difficult to tell which plants will survive.

But you should also beware of letting seedlings get too large and overcrowded before thinning. For most crops, you need to thin to one seedling per cell or pot.

When thinning, remember to:

1. Select the strongest, tallest, healthiest seedling and cull out the rest.
2. First remove seedlings that look deformed, discolored, or stunted.
3. Be careful not to disturb the roots of the seedlings you desire.
4. Cut unwanted seedlings at the base rather than pulling them out.

Use small scissors or needle-nose pruners to cut off unwanted seedlings at their base. Take care not to disturb the roots of the seedlings you intend to keep. Avoid yanking out seedlings.

Bonus Tip: Harden Off Your Plants

When the weather has warmed and your seedlings are ready to transplant, don’t forget to harden them off. While you don’t need to do this if you are winter sowing, it’s essential for vegetables sown indoors.

About a week before you plan to plant seedlings out in the garden, give them a little bit of time to adjust. You can remove them from their baby “nursery” and let them acclimate to the outdoors in a protected area like a patio, porch, or under row-cover near the garden.

At the same time, slightly reduce your watering to strengthen the plant’s roots. The hardening-off process will prevent transplant shock and allow the baby plants to become accustomed to colder nights and reduced water.