Few things are more thrilling than seed starting indoors in late winter. I can already smell the damp soil in the seed starting room. Pots lined up attentively under the bright grow lights and piles of seed packets falling all over themselves on the table.
Indoor seed starting gives us a jump start on the growing season, allows us to grow unusual plant varieties, and connects us to our garden even as it lays sleeping under the snow.
Seed starting season is time-honored tradition for all gardeners, both beginner and experienced, and I can’t wait to relive it with you in this post.
We’ll cover the whole seed starting process from planting to up-potting, mistakes to avoid, and one last step to never miss.
Table of Contents
Affiliate Disclosure: This post contains affiliate links. By purchasing through my links, I may earn a small commission at no extra cost to you.
Why Start Your Own Seeds?
- Get a head start on the season. Starting seeds indoors gives you transplant-ready plants the moment the weather is just right.
- Keep your garden producing all season. Succession planting is quick with no downtime, just continuous harvest.
- Control the process from day one. You know exactly what went into growing these plants because you were there for every step.
- Grow varieties you’ll never find at the store. Want fancy, striped tomatoes or pale peachy pink zinnias? Starting from seed is your only option.
- Find varieties that will actually thrive in your garden. When you start from seed, you can choose varieties bred for your specific climate, soil type, and growing challenges.
- Plant on your own schedule. Starting your own seeds allows you to control the timeline and adjust for extra-cold springs, late summer plantings, or whenever life actually allows you to get plants in the garden.
- Grow stronger, healthier plants. Your indoor grown seedlings can be potted up when they need it, and transplanted at peak, stress-free. Happy seedlings get established faster and stay healthier.
I start seeds for all these reasons and for the excitement of watching those first seedlings pop out of the soil. It’s truly a miracle every time!
When to Start Seeds
Knowing when to start seeds can be complicated because the dates are different for gardeners in different zones. But everyone can follow these steps to find the right time to start seeds.
First, find your last frost date. This will give you a good idea of when you’ll be able to plant your baby seedlings outdoors.
(My post on knowing when it’s safe to plant can help you find your growing zone and frost dates tell you more about this.)
Next, check the back of your seed packet.
It should tell you how many weeks before last frost to start seeds, or give you a span of time for your region in which you’ll want to plant the seeds.
If your seed packet doesn’t have that information, search online using the plant and variety name (i.e. “when to start Opalka Tomato indoors”).
Count that number of weeks back from your last frost date to get the date on which you’ll start seeds for that plant.
For example, if I were to start tomato seeds 6 weeks before my last frost date of April 25, I would start them on March 14.
You may be wondering how you’ll keep track of all this. Don’t worry, I’ve got you!
I’m sharing my free garden planning spreadsheet at the end of this post. On it, you’ll be able to put your seed inventory and info in and also create your seed starting schedule to keep yourself organized.
This is the exact spreadsheet I use to keep myself organized. (But with better colors.)
If you don’t want to scroll yet, you can also click here to go directly to the sign-up page.
What Kinds of Seeds to Start
Some vegetables you’ll always direct sow and never buy as seedlings—find them in this post.
But the best vegetables and herbs to grow from seed are ones that 1) take a long time to mature and 2) aren’t fussy about being transplanted.
A few examples of seeds to start several weeks before last frost are:
- tomatoes
- peppers
- eggplant
- broccoli
- kale
- onions
- rosemary
- celery
- artichoke
- lavender
- pansies
Look at the back of your seed packet or read the growing information in the seed catalog. If it recommends starting indoors anywhere from 4-12 weeks before last frost, it’s a great candidate for indoor seed starting.
Affiliate Disclosure: This post contains affiliate links. By purchasing through my links, I may earn a small commission at no extra cost to you.
Where to Get Seeds for Your Garden
My favorite seed companies are:
- Botanical Interests
- High Mowing Organic Seeds
- Pinetree Garden Seeds
- Baker Creek Heirloom Seeds
- Johnny’s Selected Seeds
- MIgardener
- Quail Seeds
- Botanical Interests
- High Mowing Organic Seeds
- Pinetree Garden Seeds
- Baker Creek Heirloom Seeds
- Johnny’s Selected Seeds
- MIgardener
- Quail Seeds
Equipment for Indoor Seed Starting
You will need:
- potting mix or seed starting soil (I use this one)
- plant labels
- grow lights (a sunny window isn’t enough in late winter)
- seed starting cell trays and pots (these are the ones I’m using)
- drip trays (1020 trays work great)
- mister or spray bottle
- electric fan
- liquid fertilizer
- heat mat (optional, but speed germination)
- humidity domes (plastic wrap will do in a pinch)
A few other helpful things that I like to have are fine vermiculite, perlite, and mycorrhizae powder. I’ll explain why in a bit.
Find a full breakdown of essential seed starting equipment in my post here if you need more info.
If you’re looking for a complete kit for seed starting, this one from Bootstrap Farmer has all the equipment you’ll need.
How to Start Seeds Indoors in 5 Steps
Once you have your equipment and seeds ready and it’s the right time, begin starting seeds.
Step 1: Fill pots with moistened soil.
Transfer your soil to a water-tight container. Remove large pieces if present.
I add a handful or two of extra perlite to my potting mix to make it extra fluffy. I also add mycorrhizae powder to give my seedlings a boost of beneficial fungi.
Potting mix and seed starting mix are sterile, meaning there are no bacteria, fungi, or soil organisms to help plants. Adding organic compost, fertilizer, worm castings, or mycorrhizae is how I like to boost my plants.
But back to our bin of soil.
Add enough water to the soil so that it is wet, but not soggy. If you squeeze a handful, it shouldn’t be dripping much and should be able to hold it’s shape.
Aim for the moisture level of a wrung out sponge.
Fill each of your seed starting pots with this pre-moistened soil.
Step 2: Plant seeds and label.
Plant 2-3 seeds per cell at the required depth and tuck them in with soil.
Most seeds are planted at a depth that is twice the width of the seed. Some seeds need to be sown on the surface and left uncovered.
Check your seed packet for details.
Then, label the pots or cells so you can remember what you planted!
I also add the date that I planted the seed so I can keep track of how long it’s taking to germinate.
Next, I sprinkle the soil with vermiculite.
The vermiculite helps retain moisture on top of the soil. This step is optional, but I find it most useful for tiny seeds like sweet alyssum which are surface sown and more susceptible to drying out.
Finally, I spray the trays with water before putting the humidity dome on.
Step 3: Help seeds germinate.
Help seeds germinate by placing them on a heat mat and placing the seed trays under light if needed.
You may need to check your seed packet or do a little research online to know whether your seed requires light or a certain temperature for germination.
In general, most vegetable seeds will do well with a little bit of warmth and darkness to germinate. The exceptions to this should be noted on your seed packet (I know, I know, how many times have we looked at that thing!).
Check your seeds regularly for germination. No shame in checking multiple times a day.
Once you see little green seedlings popping up, do a happy dance, remove the humidity dome and heat mat immediately, and provide plenty of light if you haven’t already.
When your seedlings get their first set of true leaves (leaves that actually look like the adult plant’s leaves), use scissors to carefully thin each cell or pot to one seedling per space.
I know it’s hard, but it must be done.
Step 4: Care for baby seedlings.
Little, baby seedlings grown indoors need you to provide them with four things: water, light, airflow, and fertilizer.
Once they’re outside, they may only need water and occasional fertilizing, but indoors they are completely dependent on you for these four things.
Water
Water when the soil surface feels dry to the touch and the cells or pots feel light when you pick them up.
The soil should be moist, but not soggy. Over-watering causes damping off, a fungal disease that annihilates seedlings.
I also recommend bottom watering. (Stop giggling.)
Bottom watering is when you fill drip trays with water and let roots suck up the water vs. watering from the top and splashing water everywhere. Not only is it neater, but it also encourages strong root development.
Just be sure to drain the drip trays after 10 minutes or once the soil is sufficiently watered.
Light
Seedlings need 14-16 hours of bright light daily.
Keep grow lights 2-3″ directly above plants and raise them as the plants grow.
Insufficient light creates leggy plants. A trait admired in supermodels, but not in seedlings.
Airflow
As seedlings grow taller, ensure they have plenty of airflow.
Moving air helps to:
- Prevent disease (high humidity is basically a spa day for fungi)
- Strengthen seedlings’ stems
I have an oscillating fan near my seedlings blowing air on low. You don’t want to blast seedlings with air, just move the air around them.
Feeding with Fertilizer
Once your little baby seedlings have grown a second set of true leaves (remember? leaves that look like the mature plants’ leaves, different from those initial cotyledon leaves), feed them with diluted liquid fertilizer.
I use Espoma Organic Liquid Tomato fertilizer for everything.
Follow the package directions for diluting, using half-strength for seedlings.
If your bottle includes instructions specifically for seedlings, use that rate. If not, divide the normal rate in half.
Feed plants just once a week.
Too much fertilizer will burn the plants and harm them. They may develop brown leaf tips or die.
If they seem like the need more as they get larger, I increase the dose gradually each week.
Step 5: Up-pot seedlings if needed.
Seedlings can grow fine in tiny pots, but at some point they will begin to outgrow the pots.
You may notice the roots coming out the bottom of the pots and growth beginning to slow down. Your plant may also be over twice as tall as the pot it’s in!
These are all signs that the plant is ready to graduate to a larger container.
If it’s not yet time to plant them outdoors, you’ll need to pot up into larger containers so they can continue growing stress-free. (See my post on seed starting pots for ideas on recycled pots to use.)
To pot up seedlings, choose a container that is slightly larger than the old container and fill it nearly to the top with moist potting mix.
Carefully pop the seedling out of its old pot and plant it in the new container, filling in gaps firmly with fresh soil.
Seedlings will live in these pots until transplant.
Hardening Off Seedlings (the Crucial Final Step)
Ok, now you’ve spent weeks nurturing these tiny seedlings from seed to plant and the time is right to get them in the ground.
But wait! You need to harden off indoor-grown plants before planting them outside.
This is true for any indoor plant, no matter what size it is.
Plants grown indoors haven’t experienced the harsh rays of the sun, and it will be a big shock for them if they haven’t acclimated to it gradually.
Choose a warm, sunny day that isn’t too windy to begin hardening off seedlings.
Keep the seedlings outside in dappled sunlight or light shade for an hour before bringing them back inside.
Gradually increase sun exposure and time outside by an hour over the next few days.
At the end of the week, seedlings should be able to handle being in direct sun all day. Remember to bring seedlings in at night if temperatures drop low and keep seedlings from being rained on if they are in a tray with no holes.
Finally, plant seedlings in their new home outdoors and watch them thrive!
Sample Hardening Off Schedule
- Day 1: 1 hour of sunlight
- Day 2: 2 hours in sunlight
- Day 3: 3-4 hours of sunlight
- Day 4: 5-6 hours of sunlight
- Day 5: Full day in dappled sunlight
- Day 6: Full day in direct sun
- Day 7: Ready to plant
Common Mistakes to Avoid
- Overwatering. Too much water keeps the roots from getting oxygen and can cause excessive algae or fungal growth.
- Make sure containers are clean. Disease can linger on used plant pots. Use mild detergent or bleach to sanitize before seed starting.
- Planting seeds too deep or too shallow. Use the recommended depth on the seed packet.
- Leggy seedlings. Keep seedlings only a couple inches beneath the grow light to keep plants from straining for more light.
- Fertilizer burn. Always dilute fertilizer with water and use it sparingly.
- Too humid. A gentle breeze from an electric fan will encourage plants to grow strong stems and keep disease from taking hold.
- Sunburnt seedlings. Harden off seedlings before transplanting so that they get used to outdoor growing conditions.
You’re On Your Way to Becoming a Seed Starting Expert
Starting seeds at home gives us gardeners so much flexibility on choosing what to grow and when to grow it. You’ll have healthier seedlings to plant and you’ll know exactly how the plant was grown.
I know for beginners, it can be a lot to learn all at once. But follow the steps to the best of your ability, and even if it isn’t perfect, you can adapt and learn from the process.
Seeds want to grow and the gardener in you wants to grow right along with them!
Have fun.
And when in doubt, check the back of your seed packet!
As promised, you can get my garden planning spreadsheet by signing up below.