How to Start Seeds Indoors for a Thriving Garden

It's super rewarding to watch a plant grow from a tiny seed! Each seed can produce so much food for you and your family while providing more seeds for a continuous cycle of bounty!

We want to start vegetable plants early to maximize the harvest, but it can feel daunting if you’re new to starting seeds indoors. In this guide, I’ll take the guesswork out of how to start seeds indoors, which containers work best, what supplies you need, and more.

Why I Start Seeds Indoors

Each year, I start seeds indoors that do not benefit from winter sowing. I’ve had great success using DIY seed starting soil, homemade compost, soil blocks, and even vermicompost to start seeds of all types, then transplanting seedlings into the garden when the weather is right.

By sharing this gardening know-how and other gardening guides, I hope to empower others to grow their own organic garden and establish food security through home gardening.

Especially if you are new to gardening, I advise you to start with a small garden or raised bed garden and thoroughly plan your garden design before planting a seed. That way, your garden will be more organized and productive.

Benefits of Starting Seeds Indoors

Starting your own seeds will not only give you the satisfaction of saying, “I grew that from start to finish," but it will also open up a whole world of options and money-saving benefits.

You can go to your local garden center and buy plants instead of starting seeds, but starting seeds indoors is a great way to save money. Purchasing quality seeds gives you the opportunity for many plants per packet, whereas a starter plant will only give you one plant for that season of harvest.

When you are successful with seed starting but run out of room in the garden, selling leftover seedlings is a great way to earn money back!

Here are more benefits to starting seeds indoors:

Selecting Containers For Seed Starting

If you are anything like me, you probably already have or will have a smorgasbord of seed starting containers. More than personal preference, the correct size container is important to the health of your seedlings!

Choosing a larger than needed starter pot so that you don’t have to transplant as often is a rookie mistake that will cost you later on. When the starting pot is too big, the root development is inferior, the growth is far less vigorous, and seedlings will suffer from things like damping off and root rot more frequently.

The larger the pot, the longer it takes water to evaporate. That will stress seedlings that don’t always like to be wet, specifically peppers and tomatoes.

Varieties like tomatoes, squash, beans, and peppers do need more soil to start properly, while other varieties can succeed in smaller containers. These videos on How to Pick The Right Size Container for Seed Starting and Shallow vs. Deep Seed Starting Trays can help you weigh the pros and cons of different containers for the plants you are starting.

Crops To Start Indoors

The most popular plants to start indoors are tomatoes, peppers, onions, kale, cabbage, Brussels sprouts, celery, eggplant, and herbs like sage and basil. Other varieties, like asparagus, can also be started indoors after cold stratification.

But not all plants will germinate well when started by seed indoors or they just need more space than indoor growing allows. Crops like spinach, beans, sunflowers, squash, lettuce, peas, carrots, radishes, beets, and other root vegetables grow best when directly sown outdoors.

When to Start Seeds Indoors

You can properly time seed starting based on your area's estimated last frost date and natural growing days. These, along with the information on the seed packet, will help you decide how many weeks in advance to start seeds indoors.

Because all types of seeds should not be started simultaneously, this comprehensive post will help you determine  the best time to start your seeds indoors.

I also recommend getting Clyde’s Garden Planner. It is a helpful tool in all climates and zones for gauging the best time for sowing seeds indoors and outdoors, when to expect your first harvest, and more.

Choosing Grow Lights for Indoor Seed Starting

Grow lights can make or break the strength of your plant starts, but grow lights don't need to break the bank! When choosing grow lights, the most important factors to consider are the Kelvin and Lumen capacities and the length and width of lights for your setup.

Kalvin describes the color temperature of a light source. Typically seed starting requires lights that are at minimum 5000 Kalvin. I like mine to be closer to 6000 Kalvin.

Lumen refers to the brightness of the light emitted. For starting seeds, a Lumen capacity of 3000 provides enough light for proper and strong growth.

I love this linkable grow light system! I have had these lights for three years and have seen great success.

How to Start Seeds Indoors

It’s true that seeds can be started in a simple cup of soil placed in a sunny window. But years of experience have taught me that poor seed starting methods lead to poor yields and precious time lost in the growing season.

Using the following seed starting method, I get high germination rates and healthier, more resilient plants for a beautiful, thriving garden. Let’s grow bigger!

Supplies Needed for Seed Starting

If this is your first year starting from seed, you may need to make a few one-time purchases of seed starting materials. Reusable starting containers, grow lights, and shelves may be necessary, depending on the setup you are going for.

Here are the supplies that I am using to start seeds indoors:

• Containers to Plant In - Reusable seedling trays or plug trays are great for at-home seed starting. I will use 6 Cell Plug Tray Inserts and 72-Cell Seed Starting Trays for this guide. You could also use a soil blocker, solo cups, or seed starting pots.

• Propagation Tray - Something sturdy to hold your Cell Plug Tray Inserts is essential. This extra-strength propagation tray is ideal and can hold water for bottom watering. It has been tested with over 50 lbs of weight without breaking!

• Seeds - Sourcing seeds is an important aspect of growing from seed. Make sure you purchase seeds from a reputable source. MIgardener is a great resource for quality heirloom seeds!

• Heat Mats - Heating mats provide ample warmth to your precious seedlings promoting rapid germination and healthy root development. Pro-Tip: A word of caution, don’t leave the heat mat on too long. Once the seedlings begin germinating, remove the heat mat! It is no longer necessary or beneficial to raise the soil temperatures.

• Grow Lights - Natural sunlight from south facing windows is okay, but grow lights will provide the optimal lighting conditions. These linkable grow lights work great!

• Seed Starting Mix - The perfect seed starting medium can be made at home, and it is the most cost-efficient way to start. This ready-to-use seed starting mix is another great option.

• Fertilizer - I like to have fertilizer in the soil so that when the seedlings need it, they have it. Choose a slow-release, non-water-soluble fertilizer like Trifecta+ . (Adding fertilizer is required in some store bought seed starting mixes because the mix does not contain any nutrients.)

• Water - I recommend watering from the bottom. Bottom watering promotes root growth, doesn’t disturb the soil, decreases the chances of fungus growth, and won't damage the delicate seedlings.

• Labels and a Marker - We recommend finding a compostable or reusable label like popsicle sticks or wooden coffee stirrers to mark your seeds. Write on the labels with wax pencils or permanent Garden Markers to combat fading.

Instructions for Seed Starting

When the glorious time finally comes to start your seeds, follow these steps to give your seedlings a strong start.

1. Prepare Your Containers - Gather the number of containers you will need. Make sure they are clean and sterile.

2. Sift the Seed Starting Mix - Use ¼ inch mesh to sift the seed starting mix. Sifting the soil mix makes it finer, gives it more surface area, helps to hold moisture, and removes the big chunks allowing the roots more room to grow. Sifting the seed starting mix has made a dramatic improvement in my seedlings!

3. Add Fertilizer - Mix fertilizer into your seed starting mix. I use Trifecta+ at a ratio of 3 Tbsp of Trifecta per gallon of soil mix plus 2 Tbsp of worm castings per gallon as well. Fertilizing now eliminates the need to fertilize after your seedlings have emerged.

4. Pre-moisten Seed Starting Mix - Dampen the mix to help it retain water over time. It should be damp but not dripping. Misting it with a water bottle works well.

5. Fill Containers With Seed Starting Mix - Fill the cells or pots with the prepared seed starting mix and nest the cell plug tray inserts or starting tray inside the propagation tray. Pro-Tip: Don’t pack the mix into the containers tightly! Fill each one, then gently tap the container on the table to help it settle. You want the mix to be fluffy.

6. Make Indentations - Dibble the soil in each seed tray. To do this, simply use the back of a pencil or marker to create holes ¼ inches deep where the seeds will be planted.

7. Plan - Take a few minutes to look at your containers and make smart decisions about which seeds to plant together. Pro-Tip: Start seedlings that will germinate at the same time and grow at the same rate together in the same trays! Otherwise, your seedlings can shade each other, causing slower growth.

8. Sow Seeds - Read the seed packet for instructions on how to plant each seed. A good rule of thumb is to plant a seed twice as deep as it is wide. For example, if your seed is 1 centimeter wide, then plant it 2 centimeters deep in the soil. Pro-Tip: Start more than one seed per cell to conserve space and maximize efficiency! When planting peppers and tomatoes, I plant one or two seeds per cell. For other seeds, I plant two or three per cell.

9. Cover the Seeds - After planting seeds, cover them with the starting mix to the correct depth.

10. Water - It is important to get watering right to avoid disturbing the soil or damaging the seedlings. To bottom water, lift out a cell plug tray insert and pour water into the propagation tray. Return the cell plug tray insert to its place. The water will be absorbed into the cells from the bottom.

11. Labeling - Label popsicle sticks or wooden coffee stirrers with the type of seeds you planted. Insert the stick into the starting mix at the edge of the tray.

12. Heat - Place the trays on a heat mat and turn the heat mat on.

13. Give Them Light - Place grow lights 2-3 inches above your seedlings to avoid leggy growing. Raise the lights as the plants grow higher. The temperature surrounding your seedlings should be between 68-72°F. Pro-Tip: Seedlings will need 14 to 16 hours of light daily. Too much light can also damage seedlings.

14. Stress - Adding a fan to stimulate air movement in the room (not directly on your plant starts) provides necessary stress to our new starts. This will help strengthen their stems, giving you the strongest plants.

15. Monitor - Watch your seedlings grow. Continue bottom watering your seeds as needed. Remove the heat mat when germination has started and raise the grow lights as the seedlings grow taller and grow their first set of leaves. Seedlings should stay in the small starting cells or pots for 5 to 7 weeks.

Once your plants grow their first true leaves, it is time to transplant seedlings to a bigger pot or into the garden, depending on your growing schedule. Remember to harden off the seedlings before planting them in the garden!

Pro Seed Starting Tips

Let’s discuss some common seed-starting mistakes and my tips for your best crop to date!

The nuances of how to start seeds indoors don’t have to be a mystery. Implementing these seed starting techniques will get you on the way to your most bountiful garden season ever.