How To Plant Seeds Indoors (7 Key Steps To Start Seeds Indoors)

When you plant seeds indoors, it helps to have a guide so you don’t skip any steps (or go out of order). The steps are a little different, depending on where you live (and different seeds have their own particular needs for soil pH, temperature, and so on).

So, what are the steps for planting seeds indoors? To plant seeds indoors: first, carefully choose and buy your seeds. Then, choose a container for a seed tray and fill it with seed starting mix. Plant the seeds according to frost dates. Keep the soil damp – but not soaked – and keep the seeds warm enough (usually a heat mat is the best method).

Some seeds can be started indoors, well before the last frost date. Others (such as carrots) should only be planted outdoors (direct sowing).

In this article, we’ll talk about how to plant seeds indoors. We’ll also go into detail on what to do (or what to avoid) at each step.

Let’s get started.

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How To Plant Seeds Indoors

There are several important steps when you start seeds indoors. We’ll start at the beginning: choosing seeds to buy.

pumpkin seeds — Follow the steps below to get the best results when planting seeds indoors.

1. Buy Your Seeds

Before you buy seeds, you have to decide a few things:

What type of plants do you want to grow? (for example: tomatoes, peppers, lettuce, etc.)
What variety of each plant do you want? (for example: Sun Gold cherry tomatoes, San Marzano paste tomatoes, Brandywine heirloom tomatoes, etc.)
What traits are important? (for example: disease resistance, heat or drought tolerance, fast maturity, etc.)

seed package — Decide what type of plants you want before ordering seeds. Consider disease resistance and other factors.

The best way to find seeds that have what you want is to look through a seed catalog. You can also get a look at the mature plants and the fruit to see if it is something that you are interested in.

You can learn more about how to read a seed catalog here.

2. Get Your Container (Seed Tray)

You have a lot of options when it comes to seed trays. You can buy manufactured trays – or you can make your own from repurposed household items.

The tray has to be deep enough to hold some seed starting mix. It is a good idea to have drainage holes at the bottom of the seed tray – just in case you happen to over water by accident.

Personally, I like seed trays with individual cells (one for each plant). You can also opt for “open” seed trays with no cells – but plant roots are more likely to get tangled together that way.

3. Fill Your Container With Seed Starting Mix

Once you’ve chosen a container for your seed tray, it’s time to fill it up with seed starting mix. Seed starting mix is a sterile, soilless blend of ingredients.

seedling — Seed starting mix is sterile and soilless, helping seeds to germinate with less threat of disease, lack of water, or too much water.

Seed starting mix holds water pretty well (but not too much). It helps seeds to germinate better. There are lots of premade seed starting mixes available online or at garden centers.

You can also make your own seed starting mix. Generally, it will be a mix of things like:

Peat Moss
Coconut Fiber (coco coir)
Perlite
Vermiculite
Sterile Additive (sand, compost, etc. – it should be finished compost and free of weeds, herbicides, and pathogens!)

sphagnum peat moss — Peat moss is a key ingredient in many soil starting mixes, although there are other things you can use.
Image courtesy of user:
Ragesoss via:
Wikimedia Commons: https://commons.
wikimedia.org/wiki/File:Schultz_Sphagnum_
Peat_Moss.jpg

You can learn more about seed starting mix (and how to make your own) here.

After you get your seed starting mix, fill the tray with seed starting mix (pretty close to the top). If you have a seed tray with cells, fill up each cell with seed starting mix.

4. Check Frost Dates

Next step: wait until the right time to plant your seeds indoors! If you start seeds too early, your plants will outgrow their seed trays or transplant pots, get root bound, and have a poor season when transplanted outdoors.

If you start seeds too late, your plants might not grow to maturity before cold and frost ends the growing season in the fall. (This is a bigger danger in places like Northern Vermont, which have a shorter growing window).

The first step is to find your last frost date (in the spring). This is the date after which you don’t expect a big chance of frost (although waiting 1 or 2 weeks after this date reduces the chance of frost even more).

You can use this tool from the Old Farmer’s Almanac to find last frost date by zip code or city and state.

Based on the last frost date for your area, you can find indoor seed starting dates for various types of plants. Mark them on your calendar so you don’t forget!

For example, you can plant tomato seeds indoors 4 to 6 weeks before the last frost date. In Boston, Massachusetts, the last frost date is April 8.

tomato seedlings — Tomato seeds should be planted 4 to 6 weeks before the last frost date (it will vary, depending on where you live).

Working backwards 5 weeks (5*7 = 35 days), we get a date of March 4 (27 days in March + 8 days in April = 35 days, or 5 weeks). So, we would start our seeds indoors on March 4 in Boston, MA.

You can do a similar calculation with your last frost date and the seed starting date for your specific plant. Remember that some seeds (such as carrots) are not usually started indoors, due to difficulty transplanting.

If you want an app that gives you basic information on plant care (including seed starting), check it out here.

5. Plant Your Seeds

Ok, the big day is here – it’s time to plant your seeds! Often, the seed packet will tell you how deep to plant. A good rule is to plant seeds at a depth of twice their diameter (you can learn more about seed planting depth here).

winter melon seeds — A good rule is to plant seeds at a depth of twice their diameter. So, a 1/2 inch seed is planted at a depth of 1 inch.

Make a hole in the soil with your finger (or a pencil, or something similar) to the proper depth. If you have an open seed tray (no cells), make sure to leave enough space between seeds to avoid crowding.

After you put the seed in the hole, cover it up with seed starting mix. Generally, you will want to water right after planting (more on watering below).

For really small seeds, you might just want to press them into the soil surface. For example, some seeds (like lettuce, celery, dill, petunias, etc.) need light to germinate, so don’t cover them or bury them too deep!

lettuce seedling — Lettuce seeds and some other types of seeds need light to germinate.

Any seed that is buried too deep may not germinate – or if it does, it will emerge slowly. So, pay attention to planting depth!

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6. Water Your Seeds

Now that your seeds are planted, it’s time to water them. A spray bottle is a great way to water without drowning your seeds or washing away soil.

spray bottle head — A spray bottle lets you keep soil moist enough for seeds without over watering.

It’s also helpful to add just a little water if the soil is slightly drier than you would like.

Remember that seeds need water and air to germinate. So, the soil should be damp, but not totally soaked.

If your soil is too dry, the seeds will not germinate. On the other hand, if the soil is too wet, seeds will “drown” (excess water leaves no space in the soil for air).

seed trays — Put a seed tray inside a larger tray with water to “bottom water” your plants.

Another way to keep soil moist is to bottom water a seed tray. This means putting the seed tray (with holes in the bottom) within a bigger container that has some water in it. The soil soaks up water, which works against gravity to absorb moisture.

A humidity dome also helps to retain water in soil, especially if you have dry air in your house. Remember to take off the humidity dome after seedlings emerge.

A humidity dome also helps to retain warmth for seeds and soil. Speaking of warmth…

7. Provide Warmth For Your Seeds

Now your seeds have almost everything they need for germination. The one final thing that is missing is warmth!

I’m not talking about being nice to your seeds (although I do encourage that). I mean that you should keep the soil temperature warm enough so that seeds will sprout!

lettuce seedlings in growing medium — Put a seed tray on a heat mat to keep the soil at the ideal temperature for fast germination.

Every seed has an ideal range for soil temperature. Within this range, the seeds will germinate faster.

Outside of this ideal temperature range, seeds will take more time to sprout. In extreme cases (too cold or too hot), the seeds may never sprout at all.

Generally, a soil temperature of 70 to 75 Fahrenheit (21 to 24 Celsius) is a good soil temperature range for most seeds. Some (like lettuce) can germinate fast in soil as cool as 60 Fahrenheit (16 Celsius). Others (like okra) can germinate well in soil as warm as 95 Fahrenheit (35 Celsius).

okra pods — Okra seeds can still germinate well in soil temperature of 95 Fahrenheit (35 Celsius) or higher.

You can use this free app I made to get a sense of the best soil temperature range for germination of various vegetables seeds.

There are lots of ways to provide warmth for seeds. One of the best ways is to use a heat mat (germination mat).

A soil thermometer will tell you the exact soil temperature so you know where you stand. You can learn more about keeping seeds warm (a n d w a y s t o d o i t ) here.

Later On: Transplant Outdoors

After germination, the seedlings will need light to continue growing – and also nutrients. Usually, soilless seed starting mix doesn’t have enough nutrients to keep seeds going for long. So, you will need to either fertilize or transplant the seedlings into a larger pot with soil and nutrients.

You will eventually need to transplant your seedlings outdoors. The time it will take depends on the plant and the frost date where you live. You can get a sense of transplant dates for various vegetables with this free app I made.