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Starting Seeds Indoors: The Indoor Gardener’s Guide

Perhaps you watched your grandparents sow seeds into the garden for decades. Two to three months later, their countertops would be brimming over with tomatoes, squash, and cucumbers.

But, did you know that you can grow almost anything when you learn how to start seeds indoors?

From fragrant herbs to bell peppers and lettuce, you can open up a whole new world in gardening.

In this post, I’ll go over all the essential things you need to know for starting seeds indoors. This includes what you need, when to start and how to do it. Let’s dive in!

A starter tray of broccoli seedlings.

Why Learn How to Start Seeds Indoors?

One of the best reasons for starting seeds indoors is to get a jumpstart on your garden.

Do you live in an area with a short growing season? You can have your vegetables ready to transplant after the last frost. This is especially helpful for warm-season vegetables such as tomatoes and squash.

For easy-germinating vegetables, you can begin the first succession indoors. Then, you can direct sow another round of vegetables a month later. This will give you tasty vegetables all season long!

A starter tray of bean seedlings. Starting seeds indoors opens up all sorts of gardening possibilities.

You can also try your hand at growing unique vegetables, instead of buying transplants from the nursery.

Slow-growing vegetables such as artichokes, cauliflower, and peppers need extra time to form roots. By starting seeds indoors, you’ll also be able to control their growing conditions, giving yourself a better chance of germination.

Don’t be scared to experiment with unfamiliar seeds indoors. After all, you’re not taking up any valuable space in your garden yet.

What You’ll Need


Packets of various seeds sold by Hoss Tools.

The first thing you’ll need for starting seeds indoors is, of course, seeds! Come late winter, online seed distributors will begin releasing their seeds for the upcoming season. That’s the best time to shop and purchase your seeds for the year.

Browse through the vegetables and flowers to see what will grow best in your zone. Some websites even have a ‘wish list’ feature to keep up with which seeds you like.

When picking out seeds, look for the vegetables your family enjoys eating and cooking with. This will make growing them an exciting process. And, you won’t potentially waste whatever you’re growing.

Try out online seed distributors such as Hoss ToolsTrue Leaf Market, and Botanical Interests. Through these websites, you’ll be sure to find a wide variety of seeds.

You can also find seeds at your local nursery and garden center in the spring. Just be sure to check that it’s not too late to start a certain seed when shopping in late spring.

Seed Starting Mix

A bag of Sungro seedling mix.

Next, you’ll need seed starting mix to germinate your seeds in. It can be confusing, but seed starting mix is different than potting soil.

The ideal seed starting mix will contain light ingredients that aren’t packed with fertilizer and heavy soil. Look for mixes containing peat moss, coconut coir, vermiculite, or perlite.

Young seedlings need little to no added nutrients. This will only overwhelm your seedlings and could stunt their growth completely.

Nutrients and fertilizers are best when added after seedlings are established.

We made a list of the best seed starting mixes to help you out. We even have a post on how to create a DIY seed starting mix.

Seedling Trays

Seed starter trays and humidity domes.

Seedling trays are essential for starting seeds indoors. They give each individual seed a place to germinate and form roots.

There are several kinds of seedling trays. Most are made from plastic, but you can also find trays made from biodegradable materials.

You can purchase trays with four cells, or you can find trays with over 300 cells. It all depends on how many seeds you plan on starting. Or, you may prefer for every tray to contain a different type of seed.

If you need help with germination, look for trays with domes, which promote humidity.

For more information, read about the Best Seedling Trays we found.


Newly formed sprouts tend to resemble one another. This makes it easy to lose track of what you’ve planted.

Using labels will help you keep your seeds organized.

Whether you use popsicle sticks or plastic labels, write the name of the seed and the date you planted it. This will indicate the best time to transplant the seedling.

Heat Mats

A heating mat keeping seedlings warm.

A good heat mat is the key to germination when starting seeds indoors. Many warm-weather vegetables will only germinate if the soil is at a certain temperature.

Heat mats sit below the seed tray and gently warm the soil to optimal temperature. Heat mats are safe, and you don’t have to worry about them overheating.

Look for a heat mat that is a similar size to your seed trays. They come in multiple sizes, and you can even find packs of two or more.

Visit our post on the Best Heat Mats for more information.

Grow Lights

Various seedlings under a grow light.

If you’re new to starting seeds indoors, I recommend making grow lights a priority. I’ve used both shop lights and grow lights. And, I can confirm that grow lights truly make a difference.

Grow lights are specifically made to give seedlings the benefits of being outdoors. That’s due to the full-spectrum bulbs that emit more than just white light.

It can be overwhelming to sift through the many grow lights on the market. A helpful way to narrow down your search is by determining what you need.

Measure your space, see if you need free-standing or hanging lights, and determine if you want LED or fluorescent bulbs.

Visit our list of Best Grow Lights for Indoor Plants to narrow down your search.

Watering Can

Person holding a green watering can.

Having a quality watering can will help you keep your seedlings hydrated.

Instead of the typical outdoor watering can, search for watering cans with a narrow spout. This will help you access the base of your seedlings without pelting leaves with water. Houseplant watering cans are perfect for this.

You can also use a bottom tray to set your seed trays in. Fill the bottom tray with water, and the seedlings will soak up water from underneath. This is a helpful way to make sure the entire seedling block is hydrated.

When to Start Seeds Indoors

When should you begin starting seeds indoors? That depends on two things: your USDA hardiness zone and the type of seed you’re planting.

A vegetable garden in tiered raised beds.

Your Zone

What zone you live in determines when your last average annual frost occurs. You’ll want to time your seedlings to be ready a week or two after this date.

Take a look at the seed packet, or search for growing information on a specific vegetable. It should tell how many weeks ahead to start your seeds. (Or, it may tell you to direct sow the seed.)

Count back that many weeks from your last average frost date. That’s when you should be starting seeds indoors.

Here are the last average frost dates according to USDA hardiness zone:

  • Zone 4: May 1 – May 31
  • Zone 5: March 30 – April 30
  • Zone 6: March 30 – April 30
  • Zone 7: March 30 – April 30
  • Zone 8: February 22 – March 30
  • Zone 9: January 30 – February 28
  • Zone 10: January 30

Type of Seed

Comparison of the back of seed packets sold by Hoss Tools.

Every vegetable and every flower is different. While you can wait a few weeks before transplanting to start zinnia seeds, artichokes should be started eight weeks ahead. This can make starting seeds indoors a little hectic.

It’s best to do your research for every type of seed you have. Make a calendar, and highlight when each seed needs to be started. This will keep you from starting seeds too early or not early enough.

How to Start Seeds Indoors

1. Set Up Equipment

A timer for grow lights.

Before you begin filling seed trays with seed starting mix and seeds, set up your growing station. That way, you’ll be ready to slide your seed trays into place once you’re finished.

Hang your grow lights, and lay out any heat mats. At the same time, you can go ahead and get everything plugged in. This is also a great time to program an automatic timer, if you have one.

I also recommend setting out all the supplies you’ll need to fill your trays. This includes seedling trays, seed starting mix, seeds, labels, and a pen. Everything will be in your sights and ready to grab when you need it.

2. Fill Trays with Starter Mix

A starter tray filled with seedling mix.

Now, you’re ready to fill your seedling trays with seed starting mix. You may want to put on some gardening gloves for this portion of indoor seed starting.

Before you fill the trays, it’s helpful to wet the seed starting mix in a bucket until it’s moist.

This will make it easier to create a hole for your seed and avoid air pockets. Your seeds also won’t start out in a dry mix. So, you’ll be one step ahead of the game.

As you fill each cell will mix, pack it down to remove any holes. In each cell, the mix should be even with the top of the seedling tray.

3. Sow and Label Seeds

Once you’ve filled enough trays with seed starting mix, you can begin to sow your seeds.

A starter tray of seedling mix and seeds.

Take a look at the seed packet to determine how deep to plant the seed.

Take your finger or the end of a pencil, and press the seed into the mix at the appropriate depth. You can also make a small hole beforehand. Just make sure to cover the seed with a light layer of seed starting mix after planting.

After sowing each type of seed, stop and label the appropriate cells. It’s easy to forget what you’ve planted after you’ve cycled through several types of seeds.

A starter tray of labels for herb seeds.

For small seeds, you can add more than one seed to each cell and thin them out later.

4. Care for Seedlings

Luckily, there’s not too much care involved after starting seeds indoors.

Once the seeds have germinated, make sure that they’re consistently hydrated. It’s not good for young seedlings to dry out.

Sadie using a spray bottle to keep seeds moist.

If you sowed more than one seed in a cell, you may have two seedlings competing for space. Thin them out to one seedling once they have two sets of leaves.

As your seedlings grow taller, make sure to adjust your grow lights accordingly. Keep between six and 12 inches between lights and seedlings.

5. Harden Off and Transplant

The last step in indoor seed starting is moving seedlings outdoors. Wait until your last average frost date to begin this process.

You can then begin to harden off your seedlings. This process involves leaving your seedlings outdoors for increasing increments of time every day. It gives seedlings a chance to adapt to outdoor conditions.

For more information, be sure to read our post on Hardening Off Seedlings.

Once seedlings are hardened off, they can be transplanted into your garden. Then, it’s time to celebrate your hard work and get ready for a great growing season!

Seedlings hardened off in the garden.

FAQs on Starting Seeds Indoors

1. When should I move seedlings outdoors?

Seedlings should stay indoors until all danger of frost has passed, unless you’re growing cool-season vegetables.

You can then determine when to transplant seedlings by looking at the seedling. Is it rootbound or outgrowing its cell? That’s a good sign it’s ready to make the transition to the garden.

Seedlings showing signs of being rootbound and ready for transplanting.

2. When is it better to direct sow seeds?

Many warm-season vegetables germinate and grow faster than you can get back out to your garden.

Unless you’re planting in succession, there’s no need for starting seeds indoors with these vegetables. They love garden soil and aren’t too picky about germination conditions.

Examples of these vegetables include summer squash, cucumbers, beans, and corn.

Wrapping Up Starting Seeds Indoors

A starter tray of sugar snap pea seedlings.

By learning how to start seeds indoors, you’ll be able to grow anything your zone allows. It’s a great skill to have, especially if you love cooking with unique vegetables. And, you’ll have plenty to share with family and friends!

Take a minute to check out our Seed Starting page for more blog posts about seed starting practices and equipment. We highlight how to grow specific vegetables, along with the best to use. You’ll be ready to start this growing season with a bang!