How to Start Seeds Indoors

Updated on January 15, 2020
OldRoses profile image

Caren White is a Master Gardener and instructor at Home Gardeners School. She has been associated with Rutgers Gardens for over a decade.

Seed packets with planting directions
Seed packets with planting directions | Source

Most of us don’t have a long enough growing season that we can direct sow seeds outdoors. So we are left with a choice of either buying plants that we can put in our gardens after our last frost or starting our seeds indoors while our gardens are still in the grip of winter cold.

To start your seeds indoors, you must first create the proper environment. Choose a surface or table that is large enough to hold all of your seedlings and that you don’t care if it gets dirty (it will) or wet (it will).

The Best Light to Start Seeds

Aha, you think, that sunny window is the perfect place to start my seeds. Sorry, but it’s not. What looks like a sunny window to us appears much darker to plants. Outdoors, plants are exposed to light from all directions. In our homes, the only light is coming from one direction in only a small amount the size of the window.Trying to start plants in front of a sunny window will result in weak, spindly plants as they try to grow towards the small source of sunlight.

For best results, you want to use lights. There’s no need to invest a lot of money in special “grow” lights. Plain old fluorescent shop lights work just fine. You want to hang them on chains above the surface where your plants will be growing. The lights should always be one inch above your seedlings. The chains make it easy for you to raise your lights as your seedlings grow.

Invest in a timer. Even though your lights are one inch from your seedlings, they are still not as bright as sunlight so they will need to be on for 15 to 16 hours a day. To prevent you from forgetting to turn your lights off for 8 hours, use a timer to do it automatically for you.

The Best Containers to Start Seeds

Plastic is your best bet. Clay pots dry out too quickly. Plastic containers retain moisture better. You can buy seed starting flats or you can make your own from any kind of plastic container. Just make sure that your containers have a hole in the bottom for drainage.

Another popular option are biodegardable pots for plants like cucurbits that don't like to have their roots disturbed. The pots are usually made from peat or manure that allow you to transplant the resulting seedlings without disturbing their roots and will break down and feed your soil over the growing season.

Use a Heat Mat to Warm the Soil

Seeds germinate when the soil reaches a certain temperature. Heat mats can fool your seeds into thinking that the soil is warming because it is spring and time to grow. Heat mats are also a must if you are starting tropical plants such as peppers or impatiens. For tropical plants, you want to maintain a temperature between 70°F and 75°F. For all other seeds, a cooler temperature of 65°F to 70°F is fine.

The Best Soil to Start Seeds

Always use a sterile soilless seed starting mix. This is critical because you want to prevent disease and pests from destroying your seedlings. Soilless mixes, as their name implies, contain no soil which is can become a haven for pests and diseases. This also allows proper drainage so that your seedlings don’t drown. You can use regular potting mix, but be aware that there is a strong possibility of fungus developing and the dense soil will make it more difficult for roots to develop and water to drain properly. Also, many potting mixes come with fertilizer already mixed in. This is too strong for your young plants. And if you are an organic gardener, you want to control the types of fertilizer that you use on your plants especially if you are growing vegetables.

How to Tell if Your Seeds are Fresh

Fresher is always better, especially if you are growing flowers. Vegetable seeds will stay viable for 3 to 5 years with decreasing germination rates. Flower seeds are rarely viable beyond one year. Check your seed packets. They will always tell you how old the seeds are. For instance, seed packets this year will say “packed for (current year)”. You can also use seeds that you have saved from your garden last year. If you saved from open pollinated plants, you should get identical plants this year. If you saved seeds from hybrid plants, the resulting plants will not look like the parents. The parents were the result of crosses between two varieties so their offspring will have the characteristics of both original varieties. Be aware that some hybrids are sterile so the seeds will not germinate.

When to Plant Seeds Indoors

Timing is critical. The seed packet will tell you how many weeks before your last frost date that you should plant your seeds. NOAA has a chart that will tell you when your last expected frost will be. Remember, this is an AVERAGE. You could get a frost after that date. In fact one year in New Jersey (zone 6) where I live which has an average last frost date of April 15, had a frost a month later on May 15. Once you know your average last frost date, you can count backwards from that date to arrive at the date on which you can begin sowing your seeds. This date will vary depending on what you are planting. Using a calendar to keep track of your sowing dates is recommended.

How Deep to Plant Seeds

Also critical is planting depth. Again, your seed packet will tell you how deeply to sow your seeds. Some seeds require light to germinate, so you can sow them right on the surface of the soil and not cover them. I have seen a few seed packets that don’t tell indicate a planting depth. In that case, the rule of thumb is to plant your seeds 2 to 3 times as deep as the seed diameter. If the seeds are very tiny, almost dust, those need to be surface sown. Once you have planted your seeds, gently press down on the soil. You want to make sure that that the seeds have good contact with the soil. Remember, soil temperature tells them when to grow so you want them to not be stuck in an air pocket.

The Best Way to Water Seeds

Put your watering can away. You’re going to be using a mister or sprayer. Before you even plant your seeds, you want to moisten your soilless mix using warm water. Moisten, not wet, not drenched. Just moistened enough so that your seeds will have good contact with the soil. Keep the soil moistened with a mister or sprayer, using warm water, as your seeds germinate. By the way, using a watering can which dumps water will cause your seeds to float to the sides and corners of the containers. Once your seeds have germinated, you can switch to room temperature water. Don’t use cold water that can chill them.

How to Fertilize Seedlings

Even though the soilless mix contains no nutrients, you will not need to fertilize your seedlings until they develop their first set of true leaves. That’s because seeds themselves contain all the nutrients your seedlings need to germinate and then develop those first two leaves. After that, you want to use a water-soluble fertilizer at half-strength. This is just like feeding a baby. You don’t give infants adult food and you don’t give seedlings full-strength (adult) fertilizer.

Greenhouse with fans
Greenhouse with fans | Source

Why You Need a Fan for Your Seedlings

Have you seen those giant fans in commercial greenhouses? They make sense when it is warm outside, but why would they be using them when it’s cold outside? The fans are used for two reasons. The most important reason is to prevent the growth of algae and fungi which develop rapidly in a warm, moist environment. A light breeze will prevent them from getting taking over and destroying the plants.

The other reason why you would want to use a fan is to strengthen the stems of your seedlings. Think about it. Inside your house, the air is still. They don’t need thick stems to be able to stand up. When you put them outdoors where the air is constantly moving, they will just flop over because they are not strong enough to stand up to even the lightest breeze. So it’s a good idea to have a small fan running on its lowest setting to “strengthen” the stems of your plants.

How to Harden Off Seedlings

Congratulations! You have a whole, healthy crop of plants ready to go into your garden. But before you do that, you have to acclimate them to the outdoors. This is referred to as “hardening off”. Starting ten days before your planting date, bring your seedlings outdoors for an hour or so, increasing the length of time each day until they are able to be outside all day. Don’t start them out in direct sun. Like us, they can get sunburned. Put them in light shade at first, gradually increasing their exposure to the sun each day.

Starting seeds indoors has two advantages. It allows you to grow varieties that are not available at the Big Box stores or your local nursery and it’s much less expensive than buying plants. For a small initial investment, your garden can be the envy of the neighborhood.

© 2014 Caren White


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    • OldRoses profile imageAUTHOR

      Caren White 

      6 years ago

      I love old-fashioned flowers like foxglove, Canterbury Bells and columbines! they are so easy to grow and remind me of my grandmother's friends' gardens.

    • blueheron profile image

      Sharon Vile 

      6 years ago from Odessa, MO

      I will probably not do much with flowers this year--due to going a little nuts last year. But somce of last year's favorites were the lisianthus (Lizzies) and the newly developed annual foxglove (Camelot series).

      I also have some regular foxgloves--I think I like "The Shirley" best, but I haven't gorwn anything else, so who knows?--that won't bloom until this spring, and some Canterbury Bells that also take two years to bloom. I also grew some white-flowered columbines and perennial sweet peas that should bloom this spring. Keeping my fingers crossed.

      I think everyone should grow the hardy passionflower (the incarnatas). They are really easy to grow from seed--just a kind of low germination rate, and kind of slow. I grew some of the tender passionflowers (caeruleas) and brought them indoors for winter--where they are vining all over the kitchen--so I'm hoping I'll see them bloom this summer. They are also easy from seed.

      This year it's going to be mosly veggies, along with quite a few herbs. I'm going to try to get some schizandra and goji berries up from seed. The seed packet directions make them sound like quite a project.

    • OldRoses profile imageAUTHOR

      Caren White 

      6 years ago

      Glad you liked it DDE!

    • DDE profile image

      Devika Primić 

      6 years ago from Dubrovnik, Croatia

      A Guide to Starting Seeds Indoors has great ideas and is helpful indeed.

    • OldRoses profile imageAUTHOR

      Caren White 

      6 years ago

      Flourish, starting seeds indoors id a great way to bring a little spring into your home. thanks for reading, voting snd pinning!

    • FlourishAnyway profile image


      6 years ago from USA

      Oh, you make me want Spring to get here even faster! This will be helpful. Voted up and pinning.

    • OldRoses profile imageAUTHOR

      Caren White 

      6 years ago

      Thanks Eddy. i'm so glad that you found it useful.

      Preston & Kate, I can't wait to hear what you are growing.

    • Eiddwen profile image


      6 years ago from Wales

      Interesting and very useful.

      Thank you for sharing and voted up.


    • prestonandkate profile image

      Preston and Kate 

      6 years ago from the Midwest

      Excellent! Thanks for the tips! We are getting ready to buy our seeds.

    • OldRoses profile imageAUTHOR

      Caren White 

      6 years ago

      Thanks for reading, Jackie! I'm happy that you found it helpful. I've made every mistake possible with seeds.

    • Jackie Lynnley profile image

      Jackie Lynnley 

      6 years ago from the beautiful south

      Thanks for the tips here I am terrible with seeds, probably not using a mist has been the problem, makes perfect sense now.

    • OldRoses profile imageAUTHOR

      Caren White 

      6 years ago

      Thanks, Sage! Glad you found it helpful.

    • WiccanSage profile image

      Mackenzie Sage Wright 

      6 years ago

      Interesting! I live in the sunny subtropics and I thought that my big glass south-facing picture window would be enough. I had no idea clear glass blocks some of the light they need. Maybe that's why my seed starters have never performed as well as transplants I bought at the nursery. Great hub, thanks, just in time!

    • OldRoses profile imageAUTHOR

      Caren White 

      6 years ago

      Blueheron, you are so right! i'm partial to heirloom flowers so I grow mainly from seed. The local nurseries only offer the latest hybrids which don't interest me. thanks for reading.

    • blueheron profile image

      Sharon Vile 

      6 years ago from Odessa, MO

      Wonderful advice! Growing vegetables and flowers from seed is often the only way to obtain some of the plants that aren't offered at your local nursery. Tomato growers, especially, have been pioneers in bringing back heirloom and older varieties from all over the world. I have yet to see many of my own favorites offered as nursery plants. Many of my favorite herbs and flowers are seldom or never offered--and are getting less and less affordable. And many rather exotic plants are easy to grow from seed--such as passionflowers.


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