How to Start Seeds Indoors: Complete Guide

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Starting seeds indoors as a kid fueled my love of plants.

It was fascinating how tiny seeds could sprout and grow into plants that I could watch grow all summer.

I remember being excited to check them each morning before school to see how much they had grown.

I enjoy the entire process, from selecting the seeds to the satisfaction of planting them outside.

From a practical standpoint, starting seeds indoors is the most economical way to grow flower and vegetable plants.

For the small cost of seed packets and a few supplies, I can grow plants that would cost hundreds of dollars at a garden center.

Quite a return on my investment!

Tomato Seedlings Growing Indoors in green container.
These tomato seedlings will develop into plants that will yield a lot of vegetables!


Starting seeds gives me access to vegetable and flower varieties that I cannot find at my local garden center.

This is particularly important when selecting plants that are more disease and insect-resistant.

In addition, as my love for gardening has developed, I’m always looking for unique plants, different varieties, and heirloom plants.

Those are most easily found from seed companies. 

The back of a packet of English Lavender seeds with an arrow pointing to when to start the seeds.
The seed companies give great information about when to start each type of seeds.

When to Start Seeds Indoors

Most seed packets will give a time frame of how many weeks the seed should be started before planting outdoors.

To know when it’s safe to plant outside, know the average frost-free date for your area.

This is the average date that it’s usually safe to plant tender flowers and vegetables outside without the chance of frost killing them.

Count backward from this date to estimate when to start each seed. 

For instance, I live in Northeast Ohio and my frost-free date is the first week of May.

If a seed packet says to start the seed 8 weeks before planting outside, I would start it no later than early March.

I could start them in late February to give the plants a little more time for any problems or issues.


Supplies for Seed Starting

The supplies for starting seeds are simple but the quality is critical to the success of the seeds growing into healthy plants.

a seed starting kit with a tray, cell packs and clear cover to create a mini greenhouse.
This is one of my seed starting kits with a clear dome that creates a mini greenhouse. The tray doesn’t have holes to hold water. Less mess too!

Seed Starting Containers

I recommend using a seed starting kit that provides a tray, cell packs (pots), and a germinating dome.

These kits are easy to find online or at your local garden center.

I like to use trays that don’t have holes in the bottom because they keep the watering mess down inside my house.

However, it does require that I carefully “tip out” excess water so that the seedlings don’t become waterlogged.

These are the types of containers that professional greenhouse growers use for growing plants.

Peat pots, paper pots, and recycled or repurposed containers may work however I don’t recommend these for beginners.

The key to successful seed starting is to have clean, sterile containers that are free from any debris.

If the containers aren’t new, clean them in a 10% bleach solution.

Seedlings are very prone to fungal diseases and growing sterile containers will help prevent disease.

A bag of seed starting soil mix.
This is my bag of seed starting mix. It’s organic and ideal for vegetables.

Soil for Starting Seeds

I recommend using either a seed starting mix or a potting mix specifically made for containers (here is an example).

Don’t use soil from your garden as it is not ideal for use in containers.

I don’t recommend making your soil mix unless you are extremely experienced.

The professional mixes that are available are formulated at the ideal pH and texture for starting seeds.

Seedlings Growing Indoors with Plant Labels.
You will be so glad you labeled your pots!


Labels for Marking Containers

If you are starting more than one type of seed, you need labels and a permanent marker to insert into the cell packs to keep track of your seedlings.

I have made the mistake of thinking that I will “know” which trays had what particular seed sown in them, only to be confused days later!

In addition to writing the plant’s name, add the date the seed was sown.

This will be valuable information later if you need to make adjustments next year.

Heat for Starting Seeds 

Most gardeners in the north start seeds in their basements simply because that is where they have space.

Unfortunately, it can be the coldest part of the home.

The reality for successful seed starting and growing of seedlings into mature plants is that you will need a warming mat.

These are specifically designed for horticultural use and will make a huge difference when starting seeds like peppers.

Warming mats can raise the plants’ soil temperature 10-20 degrees Fahrenheit which improves the seed germination rate and will improve root growth.

Most seedlings need an ambient air temperature of at least 70 degrees Fahrenheit to thrive.


Bright Light for Growing Seedlings

A bright light source is one of the keys to growing healthy, strong seedlings.

I have been successful using LED fluorescent “shop” lights that are hung on an adjustable chain.

Seedlings need the light to be about 3-4 inches away from them, so it’s important to have the ability to move the light as the seedlings grow.

Ideally, purchase bulbs that are grow lights as these are designed to provide light in the photosynthetic spectrum.

Full spectrum lights or those with more blue light will be best for seedlings.

However, I have grown thousands of seedlings with traditional fluorescent lights if you don’t have grow light bulbs.

A smart timer will come in handy as well so that the lights can automatically turn on and off each day.

a collection of flower and vegetable packets from different companies.
I buy seeds from many different reputable seed companies- both mail order and at my local garden center

Seeds- Which Kind?

If you are new to seed starting, I recommend visiting your local garden center to begin your quest for seeds.

They will have fresh seeds from reputable seed companies.

There are also many reputable online and mail-order seed companies to purchase from.

Customer reviews and star ratings are a great way to guide you through the selection process.


Step-by-Step Instructions for Planting Seeds

After filling the pots with soil, I use a small watering can to moisten the soil.

Allow water to flow through the drain holes until the soil is moist, but not water-logged.

Empty excess water from the drip tray so that the pots aren’t sitting in water.


How to Sow the Seed

Place 1-2 seeds in each cell pack and cover gently with soil.

Plant at the depth as noted on the seed packet.

Note that each seed type will have specific instructions for planting depth.

In addition, some seeds may have special requirements or treatments to get them to germinate.

Reading the instructions on the packet is important.

If you love starting seeds indoors, I highly recommend the book Park’s Success with Seeds.

It is a wealth of information.

Another tip is for small seeds like tomatoes, I often use a pair of tweezers to place each seed in the cell pack.

After planting, use a household mister to add additional moisture to the soil and activate the germination process.

The mister is a gentle way to add water until the seedlings are established.

I avoid using the watering can to water the seeds because it’s too much force and will shift the seeds in the container.

Use a permanent marker to write the plant name, variety, and date on a plant label to identify the plants in each tray.

You will be glad you did in a few weeks!


Germinating Seeds Indoors

Most seed starting kits come with a dome that covers the tray to retain moisture.

This creates a small greenhouse.

Keep the dome on the tray until most of the seeds have germinated.

Check daily to make sure that the soil is still moist and use the mister to add moisture as needed.



Another key for germination is to place the pots in an area that is warm in your home.

Some gardeners put them on top of the refrigerator to capture rising heat.

If growing in a cool area of your home, consider placing a warming mat under the tray.

It provides bottom heat to get seeds to germinate quickly.

Note that you can germinate the seeds in one area of your home and then quickly move them to the area with lights once they germinate.


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How Much Light Do Seedlings Need?

After the seeds germinate, the key is to place the seeds in an area of your home with a lot of light.

It’s virtually impossible to grow quality seedlings without supplemental lighting.

Unless you have a greenhouse and live in an area that gets an adequate amount of sunlight, most seedlings will be leggy.

Over the years, I have improvised by using shop lights on chains that I can raise and lower as the plants grow.

Seedlings in Square Plug Tray.

There are now quite a lot of different “grow lights” on the market- some are basic for beginners and others are more sophisticated for the avid gardener.

When the seedlings are small, I keep the lights approximately 2″ above the tops of the leaves.

This encourages compact, sturdy growth.

I keep the lights on 8-12 hours a day.

I use a timer, so I don’t have to remember to turn the lights on and off daily.


How to Water Seedlings

Continue to use a mister to water the seedlings until they have at least one set of leaves.

At that point, the plants will need more moisture.

A small watering can with a narrow spout is perfect for the job.

One of the keys to keeping the plants healthy is to not overwater.

I keep them slightly moist but allow the top of the soil to dry in between watering.

A professional tip is to try to keep the leaves dry, using the narrow spout of the watering can to concentrate the water onto the soil.

Leaf and soil diseases are less common when the leaves stay dry and the soil surface dries between watering.


Seedling in soil.
This seedling has its first set of mature leaves sprouting.


How to Fertilize Seedlings

Approximately 4 weeks after germination, I add a small amount of liquid fertilizer to the watering can for each watering.

I use about 1/4 of the recommended rate.

This provides essential nutrients to the plants and keeps the leaves green, but doesn’t burn the tender roots.

Seedlings in Plug Tray.

How to Transplant Seedlings

If the plants become too large for their pot, they can be transplanted into a larger container or larger cell pack.

They must have several sets of mature leaves before they can withstand the stress of being transplanted.

Before transplanting, water the plants thoroughly.

Then gently squeeze the sides of the container to help release the roots.

If possible, tip the container while holding your hand over the top.

Tap the bottom of the container so that the plant begins to come out of the pot.

Gently remove and place in the new container.

Most seedlings can be planted deeper in the new pot.

Plant the seedling to the depth of the first set of mature leaves.

This will resolve the issue of a seedling that is leggy (too tall without leaves).

Add soil as needed so that the plant is well situated in its new home.

Water thoroughly to settle and moisten the new soil.

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Acclimatization Process

Plants grown indoors need an acclimatization period before they can be planted outside.

This is also referred to as “hardening off.”

This is an important step in insuring that your plants thrive outside.

I take advantage of warm (60 degrees or more) days in April and early May to set the plants outside in a shady area for an hour or two.

This allows them to adjust to the increased light.

The plants will respond with compact growth that will prepare them for planting outside.

As they adjust to the outdoor environment, they can spend increasingly longer amounts of time outside.

 white and green annuals in flower bed next to solar light by curb.

Planting Outside

Once the chance of frost has passed (usually mid to late May in Northeastern Ohio) it is safe to plant outside.

Click here for a complete guide to planting annuals outside.

I hope you will try starting seeds inside!

It’s such a great way to expand the varieties of flowers and vegetables in your garden.

Many gardeners find deep satisfaction in watching their plants grow from seed to mature, healthy productive plants in summer.

I’m sure you will too!

For more information about starting tomatoes from seed, click here.



4 thoughts on “How to Start Seeds Indoors: Complete Guide”

  1. I have been growing kale in cell starter, but they have grown so tall, but are still very fragile. I moved a few of them from the cells to 3” pots but still keeping them indoors. Any advise of what I’m doing wrong, because the ones that were transplanted aren’t thriving.

    • Hi Lisa, I would recommend checking the following: make sure the seeds are planted a 1/4 inch deep and the temperature is 70-75 degrees. Once germinated, the light source should be within a few inches of the seedlings to keep them compact. The light source may not be bright enough if you are doing all of these things. Feel free to send me some photos at [email protected] Best wishes!-Shannon

  2. What fertilizer program do you use? I find that my petunias want to grow up and don’t stay low and hardy. I start indoors under grow lights an then move to a small home greenhouse.

    • Hi Lucille, I use a water-soluble fertilizer at a diluted strength. I use it sparingly when the plants are young as I don’t want to encourage them to grow too quickly and become spindly. As soon as the weather allows, I try to get them outside in the natural light and air as this does help keep the growth compact. Thanks! – Shannon


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