When To Start Planting Seeds Indoors – Seed Starting Guide

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Let’s talk about when to start planting seeds indoors for a spring garden.  Knowing when to plant your seeds indoors is important so that your plants are just the right size when it’s time to plant them out into the garden.  Having plants that are too big is just as bad if not worse than having plants that are too small.  Let me share with you my tips and tricks for understanding when and how to plant your seeds indoors.  Scroll down to see a video tutorial on when to start planting seeds indoors and download your free printable seed starting chart on which seeds prefer it to be light or dark in order to germinate.

small basil plants

2 Ways To Plant In The Garden

There are two main methods for planting crops into your garden, direct seeding and transplanting.  

Direct Seeding

Direct seeding is the process of planting seeds directly into the garden soil.  Some crops do better when they are seeded directly into the garden because their roots do not like to be disturbed.  Examples of seeds that like to be placed directly into the garden soil are peas, carrots, corn, beets, potatoes, and most root vegetables.

handful of seeds


Transplanting is the process of taking young plants that have been grown indoors under lights, and “transplanting” them out into the garden soil once the weather is warm enough.  The benefits of transplanting include:

  1.  Crops that need a longer growing season will need to be started indoors and transplanted if you live in an area where your growing season is short.  Otherwise, you might not be able to harvest fruit from them before your first frost comes in the fall.  
  2. It allows you to harvest produce from crops sooner because they are doing some of their growing indoors when it is still to cold for them to grow outside
  3. It helps the plants to get a head start on the weeds because the weeds are growing from seed.  If you put already started plants in the ground they will naturally be bigger than the weeds. This helps to shade out the weeds so they don’t grow as quickly.  

Examples of seeds that are usually started indoors and then transplanted outside are tomatoes, peppers, onions, celery, sweet potatoes, herbs, and most flowers.

There are some seeds that you can choose to either direct seed or transplant depending on your growing zone and how soon you want to harvest the produce.  Examples of seeds that can be direct seeded or transplanted include cucumbers, melons, squash, beets, lettuce, and some herbs like dill and cilantro.

For those crops that you will want to transplant, you can either purchase the plants at garden centers, or start your own seeds at home.  Starting your own seeds is less expensive and allows you to choose the varieties you want to grow and when to grow them.

Cool Season Vs. Warm Season Crops

It’s also important to know which plants like to grow when it is cool in the spring or fall and which plants like to grow in the heat of the summer.  This will help you know when to start these seeds indoors.  Broccoli, cauliflower, cabbage, radish, carrots, beets, potatoes, onions and lettuce like to grow when it is cool, and they can even handle some frost.  These are the crops I always plant outside in the early spring. 

tomatoes ready to be transplanted

On the other hand, tomatoes, peppers, sweet potatoes, cucumbers, squash and melons cannot handle any frost and don’t usually grow much when the nighttime temperatures are below 50 degrees.  These are the plants that like to grow in the heat of the summer, so you’ll to be sure not to plant them too early.

Tips For Knowing When To Start Planting Seeds Indoors

Growing Zone

One of the first things you are going to want to find out is what growing zone you live in so that you know when your climate will be warm enough to plant outside. The USDA has developed a map that shows the different climates across the country. It is based on the weather history over the last century and they have broken the country up into growing zones.  The zone you live in will tell you how cold your winters get on average, and when you can expect to see your last frost date in the spring and first frost in the fall.  

It is also important to find other people that garden near where you live and talk to them about their experiences. Sometimes there is microclimates within the USDA hardiness zones that can be warmer or colder depending on elevation and many other factors.  Over time you will get comfortable with growing a garden in your environment.  Keep in mind that the USDA hardiness zones are based on averages, so before I plant anything outdoors that is sensitive to frost, I always check the 10 day weather forecast to make sure it won’t be near freezing temperatures at night.

Average Last Frost Date

To find out your average last frost date, visit https://www.almanac.com/gardening/frostdates and enter your zip code to find out when you can expect to have your last frost in the spring.  After this date, you can safely plant out all of your heat loving plants like tomatoes and peppers.  Keep in mind that the last frost dates are based on averages over many years. Before I plant anything outdoors that is sensitive to frost, I always check the 10 day weather forecast to make sure it won’t be near freezing temperatures at night.

I grow in Nebraska in zone 4b and my last frost date in the spring is right around May 15th.  We grow a 1/2 acre vegetable garden and preserve all of our own food for our family.  We also take a lot of produce to town and sell at our local farmers market each week in the summer.


Once you know when your average last frost date is in the spring, you’ll want to put that information into Seedtime.  This is my favorite online gardening app and it takes all the guesswork out of figuring out when to start planting my seeds indoors.  The app takes into account cool season versus warm season crops and based on your average last frost date, tells you when to start your seeds indoors.  It also tells you when to transplant them outside and when to plan on cultivating and harvesting your crops.  Seedtime will also tell you when to plant the crops into your garden that prefer to be direct seeded like potatoes, carrots and corn.  Click here to start using this free online gardening app today.


Getting The Right Size Plants

It’s important to start seeds at the right time, so that your plants aren’t too big or too small when it comes time to transplant them out into the garden.  If your plants are too big, they will shock when you go to transplant them, and they will be hard to get to grow.  Plants that have to wait a long time before being transplanted run out of the nutrients available in their individual containers or soil blocks, and start to become root bound.  This stunts their growth and they take a long time to get growing in the garden.  Conversely, plants that are too small usually transplant okay, but still take a long time before they are ready to produce fruit.  The best way to make sure you have everything ready to transplant at the right time is by utilizing a planning tool like Seedtime.

small basil plants

I’ve been using seedtime for many years and it has not only saved me time in planning, but helped me be more accurate in knowing when to start my seeds indoors, and not forgetting which seeds need started each week.  Here is a link for you to try out seedtime for free!  It has a lot more capabilities than I’ve mentioned, so you’ll want to be sure and check it out.

Which seeds prefer to germinate in the light vs. the dark?

Most vegetable seeds prefer to germinate in the dark, while most flower seeds prefer to germinate in the light.  

To germinate seeds in the dark, you’ll want to cover them with a thin layer of soil and place them in a warm dark place like your furnace room until you see the seeds starting to germinate.  You can actually stack the seeded trays on top of each other and cover them with a tarp to help keep in the moisture and heat if you don’t have a warm dark place like a furnace room.  Just be sure to check them daily to see if the seeds are starting to grow.  You can also put your plastic trays on a heat mat designed specifically for keeping your soil temperatures warm enough to germinate the seeds.  This works really well for tomatoes and peppers.

To germinate seeds in the light, just sprinkle the seeds directly onto moistened soil. Then put a clear humidity dome or plastic bag over the top of your seeded trays. This will help the seeds stay moist while still having access to some light. 

Click here to download your free printable seed starting chart that shows which seeds prefer to be germinated in the dark, and which prefer to be germinated in the light.

Seed Starting Tips

Soil Blocks

I like to start most of my indoor seeds in soil blocks because they transplant out into the garden so much easier.  To see my full tutorial on how to make soil blocks, click here.  You can also use peat pots that you can purchase at your local garden center.

full tray of soil blocks

Small Seeds

Lettuce, some herbs and flower seeds are really tiny, and can be hard to see and handle well enough to put them into a soil block.  For these I just put a thin layer of soil in the bottom of a tray, moisten with water, and then carefully sprinkle these seeds right on top of the soil.  There is no need to cover these tiny seeds with soil, because most of them prefer light to germinate.  You can keep them moist by misting with a spray bottle daily, and covering with a plastic dome.  

potting up tiny plants

Once these new seedlings germinate, and start growing their first set of true leaves (which is actually their second set of leaves), then you can transplant them into the soil blocks.  You can do this by gently pulling the plants apart along with some of the soil around their roots, and placing them into the groove in the soil blocks.  This helps you have nice full flats of plants instead of having some empty soil blocks from poor seed germination if you plant directly into the soil blocks with the tiny seeds.  

Large Seeds

Cucumbers, tomatoes and peppers can be planted straight into the 2” soil blocks as the seeds are easier to handle.  Most of these larger seeds prefer to be germinated in the dark, so it’s best to cover them with a little bit of soil or peat moss before placing them in a warm dark place to germinate.  I usually stack my trays of seeded soil blocks and place them in my furnace room until I see the seeds starting to germinate.  Then I put them under the lights on my plant growing racks and begin watering them daily.


You don’t need a special grow light for growing your seeds indoors.  Click here to see my budget friendly DIY Indoor Seed Starting Setup.  It is important to keep the lights as close the growing plants as possible so they don’t get leggy from trying to stretch to reach the light.  You can put your plants in a sunny window, but it’s really hard to make sure they are getting enough light especially on shorter cloudy days.

microgreens growing on a plant rack


​I like to lightly sprinkle my seeds with warm water if they start to dry out during germination.  This helps to keep the seeds and soil warmer and encourage faster germination.

Seed Sourcing 

If you are wanting to start your own seeds, I would suggest you purchase high quality seed from reputable sources.  Do not go down to your local hardware store and pick out random packets of seed.  You don’t know how old the seed might be, and if those varieties will grow well in your area.  I suggest that you purchase seeds from True Leaf Market, Johnny’s Seeds or Berlin Seeds.  They all carry high quality seeds and they give information on each variety so you can choose ones that will work best for you.  Less is more when it comes to choosing the seed varieties for your garden.

Hardening Off Your Tranplants

Hardening off your transplants is the process of getting your plants acclimated to growing outdoors. This is a very important step in having successful plants growing in your garden.  To do this, you will put them outside in the shade during the cooler parts of the day. Then slowly expose them to the sun and wind over a few days so they get acclimated to it and don’t get sunburned or blown over once they are out in the garden for good.

​Best Time To Transplant

The best time to transplant your young seedlings out into the garden is in the evening or on a cool and cloudy day.

Supplies You May Need

  • Soil – I like to mix my own seed-starting mix because it’s less expensive and then I know exactly what my plants are growing in.  Click here to download your free potting mix recipe.
  • Seed trays – I suggest making soil blocks and putting them in these 2” deep heavy duty growing trays with drainage holes.  Once the seeds have germinated, you’ll want to put this deep tray into a heavy duty shallow tray so you can bottom water your soil blocks easily
  • Soil blocker
  • Seeds – I suggest sourcing high quality seeds from True Leaf Market, Berlin Seeds or Johnny’s Seeds.
  • Indoor Seed Starting Racks And Lights
  • Fan – it’s a good idea to have a fan blowing gently across your plants once they are a little older.  This helps them to grow stronger stems, and prepares them to be out in the wind.  It also helps to keep the air moving to reduce the chance for any mold growth on your soil blocks.

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