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Beginner’s Guide: How to Start Seeds Indoors

Learning how to start seeds indoors is a skill worth developing. Growing crops from seeds is a cost-effective way to expand your garden. Starting seeds indoors offers more variety than store-bought transplants and allows you to grow plants at the perfect time for your climate. Here’s a simple guide to help you get started, covering everything from setup to transplanting.

Article Outline:

1. Choose the Right Location
2. Gather Your Supplies
3. Set Up Your Indoor Seed Starting Space
4. Prepare the Soil
5. Plant Your Seeds
6. Care for Growing Seedlings
7. Water Wisely
8. Thin and Fertilize Indoor Grown Seedlings
9. Transplant to Larger Containers
10. Harden Off Before Planting Outdoors

How to Start Seeds Indoors: 10 Steps for Success

1. Choose the Right Location

The ideal temperature for starting seeds is between 70-80°F. If outside or in your garage isn’t the right temperature, you can get creative and find a spot in your home that is. I use a wire shelving unit that I move into my office or laundry room when it’s time to start seeds indoors. It holds my seed-starting supplies in the garage for the rest of the year.

Use this seed germination temperature guide to double-check the correct temperature for the seeds you’re starting.

2. Gather Your Supplies

It might seem like a lot, but you don’t need everything. Even a single seedling tray with a grow light on your counter is a great start.

Essential Supplies for Starting Seeds Indoors:

• Seedling Pots and Tray: Use what you have on hand. I love these trays from Burpee and these from Bootstrap Farmer. 
• Seed Starting Mix: Use a sterile, bagged mix — avoid regular potting soil. I like this one.
• Light Source: LED or T5 fluorescent lights work well. Choose one that fits your space and budget. The light from a window is insufficient to grow vigorous, healthy seedlings indoors.

Nice-To-Have Indoor Seed Starting Supplies:

• Humidity Domes: Good-quality domes will last for many seasons. You can also cover seedlings with plastic wrap or burlap to retain moisture.
• Vermiculite: It allows light to penetrate the surface of sown seeds while keeping the soil hydrated. It also helps prevent algae from forming and can help prevent fungus gnats.
• Heat Mat: Keeps the soil at the optimal temperature of 70-80°F for germination.
• Labels: These should be small enough to fit inside the humidity dome. Get creative and use what you have on hand. Wood labels may bleed. 
• Fan: This is the one I use. It ensures air circulation, which helps seedlings grow strong and prevents disease.
• Power Strip and Timer: Use one with a timer for your lights and another for the heat mats and fans.
• Watering Can: A long spout makes indoor watering easier.
• Wire Shelving: Ensure the dimensions fit your trays and lights.

Starting a garden can feel overwhelming. Read this guide to help you avoid new gardeners’ most common mistakes.

3. Set Up Your Indoor Seed Starting Space

Before planting, make sure your indoor growing area is ready to go. Set up your lights, plug in the heat mat, and ensure everything fits and works properly. Clean and sanitize your seed pots and trays. Organize your seeds, plan how many to plant, and make your labels before getting your hands dirty.

4. Prepare the Soil

Moisten your seed-starting mix until it’s damp but not soaking wet. Fill your containers, making sure there are no large air pockets. This step ensures that the seeds will have the right environment for germination.

5. Plant Your Seeds

Follow the directions on the seed packets for planting depth and spacing. Plant 2-3 seeds per pot to increase the chances of successful germination. Press the seeds gently into the soil to ensure good contact. Sprinkle with vermiculite (if using). Add labels as you go, cover the trays, and place them under lights on the heat mats. Use a timer to keep the lights on for 12-16 hours a day.

Aim to plant seeds with similar germination times in each tray. This will make it easier to remove the humidity dome or seed cover when it’s time.

6. Care for Growing Seedlings

Remove the cover or humidity dome once a few more than half of the seedlings have sprouted.

Keep the lights on for 12-16 hours a day. Keep the light within an inch or two of the seedlings to prevent them from becoming leggy. Some LED lights may be kept further away; follow the instructions that came with your lights. If the seedlings looked scorched, move them further away. If the seedlings are leggy or elongated, put the lights closer.

Use a fan to provide airflow, which helps strengthen the plants and prevent disease.

7. Water Wisely

Keep an eye on your seedlings’ moisture levels. Water from below using a watering can, ensuring not to let the seedlings dry out or become waterlogged. This also helps prevent fungus gnats.

If water remains in the tray a few hours after watering, remove it with a syringe or carefully dump it. (I like to pour it into my houseplant pots.)

8. Thin and Fertilize Indoor Grown Seedlings

When seedlings develop true leaves, thin them to leave the strongest plant in each pot. The earlier. you thin to one plant per cell, the quicker they will grow. You can gently tease up the extra seedlings with a pencil and replant them in other pots if desired.

Once true leaves appear, it is also time to start fertilizing. Begin by adding a half-dose of seaweed fertilizer to your water. One week later, feed with 1/2 dose of fish fertilizer every other time you water. 

9. Transplant to Larger Containers

Knowing exactly when to start seeds indoors can be tricky sometimes. If your timing is off and your seedlings outgrow their containers or roots start coming through the drainage holes, you need to pot them up into larger containers. This step may not always be necessary, depending on the size of your seedlings and their original containers. Learn more about potting up in this blog post.

10. Harden Off Indoor Grown Seedlings Before Planting Outdoors

Once the seedlings have several sets of true leaves and the outdoor soil is at the correct temperature, you can plant them outdoors. Hardening your seedlings off before moving them outside is crucial to avoid transplant shock. About a week before transplanting, start by turning off the heat mats during the day and keep them on only at night. Gradually expose your seedlings to outdoor conditions by placing them in a shady, protected spot for an hour or two each day, increasing the time and sun exposure daily. Learn more about the process of hardening off in this blog post.

For spring planting, transplant in the morning so plants can adjust before nightfall. For fall or monsoon planting, transplant in the early evening so plants can adapt to cooler night temperatures.

With these steps, you’ll be well on your way to successfully starting seeds indoors and growing healthy, vigorous plants ready for the garden.

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