Indoor seed starting can give you a jump start on the growing season, help you grow special plant varieties, and be a fun hobby! This guide covers the whole seed starting process from equipment, to fertilizing and up-potting, mistakes to avoid, and one last step to never miss.
Ah, the smell of damp soil in the seed starting room. Packets of seeds piled up on the table, all containing the potential for thousands of pounds of harvest. Pots line up like soldiers under the bright grow lights, and if you look closely, you’ll might see a tiny speck of green emerging from the center of each pot.
Seed starting season is time-honored tradition for all gardeners, beginner and experienced alike, and I’m here to walk you through the seed starting process I begin anew at the end of each winter.
This is going to be a long guide, so settle in or use these links to jump to the section that most interests you.
- Why Start Your Own Seeds?
- When to Start Seeds
- What Kinds of Seeds to Start
- Equipment for Indoor Seed Starting
- How to Start Seeds Indoors
- Mistakes to Avoid
- How to Harden Off Seedlings (the Crucial Final Step)
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Why Start Your Own Seeds?
Starting your own seeds indoors is beneficial for many reasons.
- get a jump start on the growing season by starting plants early
- be able to succession plant and flip beds efficiently
- be in control of the growing process from start to finish
- have access to a greater range of plant varieties
- choose seeds that are better adapted to your own growing conditions
- flexibility on when to put your garden in
- have healthier seedlings
For me personally, I start seeds to grow interesting plant varieties 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 is a big topic that I’ll try to condense here for the sake of brevity.
First, start by finding 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 guide you further if you need it.
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 also want to start seeds for your cool season. I explain more about cool and warm season growing in this post on planning a garden if you’re not sure how to figure that out.
I also named commonly grown cool season vegetables as well as warm season vegetables in this list.
What Kinds of Seeds to Start
Some seeds grow better when direct-sown and shouldn’t be started in pots indoors. These include root crops and plants that grow a large taproot. This group also includes plants that are sensitive to transplanting.
I created a list of vegetables to never buy as seedlings which breaks down which vegetables should be direct sown and why. You can take a look there if you’re curious about that.
I’m sure you’ll probably come across some gardeners who successfully transplant these types of crops; and yes, it can be done, but the timing on it is very tight. There is far less wiggle room for mistakes.
Personally, I prefer direct sowing when I can, especially on fast-growing plants like squash. It’s just not worth the indoor space and cost for me to start those types of seeds.
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.
For example:
- tomatoes
- peppers
- eggplant
- broccoli
- kale
- onions
- chives
Your seed packet should give you guidance on where and how to start the seeds.
Some of my favorite places to shop for seeds online include:
- Botanical Interests
- High Mowing Organic Seeds
- Pinetree Garden Seeds
- Baker Creek Heirloom Seeds
- Johnny’s Selected Seeds
- Hoss Tools
- Park Seed
Equipment for Indoor Seed Starting
I’ve written a whole post just on seed starting equipment, which you can read later if you want, but here’s a quick list of what you’ll need at minimum:
- potting mix or seed starting soil (I use this one)
- plant labels
- strong grow lights or shop lights and a timer
- seed pots or containers (ideally both small and large) with drainage holes (these are the ones I’m using)
- drip trays (1020 trays or something else) to catch excess water and for bottom watering
- mister or spray bottle
- electric fan
- liquid fertilizer
It’s also nice to have a heat mat for germinating warm season plants and humidity domes that fit your drip trays.
If you’re looking to get a full kit for seed starting, I recommend this one from Bootstrap Farmer that has all the equipment you’ll need.
A few other helpful things that I like to have are a small dibber (you could use a pencil or stick), fine vermiculite, perlite, and mycorrhizae powder. I’ll explain why later in the post.
How to Start Seeds Indoors
Once you have your equipment and seeds ready and it’s the right time, begin starting seeds.
Step 1: Fill pots with pre-moistened soil.
Transfer your seed starting mix or potting mix to a water-tight container.
I prefer using potting mix and adding a handful or two of extra perlite to my 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. I strongly limit my use of synthetic plant food especially around plants grown for eating, so 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 your seeds at the required depth for germination. Most seeds are planted at a depth that is twice the width of the seed. 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. Some seeds like rosemary take forever and it can be easy to feel discouraged!
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: Aid germination.
Help seeds germinate by placing them on a heat mat, using a humidity dome, and placing the seed trays under light if needed.
You may need to check your seed packet (again!) or do a little research online to know whether your seed requires light or has special temperature needs to germinate.
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.
Step 4: Care for baby seedlings.
Once seeds have germinated, remove them from the heat mat and place them under lights.
Place them close, like a couple inches away from a strong light source to keep seedlings from growing leggy.
Seedlings will need light for 14-16 hours a day. More if your light is weak, and less if you have a strong grow light.
Make sure the seedlings’ soil does not completely dry out. It’s fine, and actually good if the very top layer dries out between waterings, but do not let the whole pot become dry.
You can check the soil by sticking a finger in and seeing how moist or dry it is. As you do this more frequently, you’ll also be able to tell by the weight of the pots whether it needs water or not!
Bottom watering is best for seedlings. To do this, pour water into your seedlings’ drip tray. The soil exposed in the drainage holes of your seedling pots will wick the water up until the whole pot of soil is hydrated.
Pour out excess water after 10 minutes or so. Do not let pots sit in water longer than that.
Your seedlings will also benefit from good airflow in the room they are in. Set up a fan to circulate air gently round the seedlings.
Step 5: Fertilize seedlings regularly.
Once your little baby seedlings have grown a second set of true leaves (leaves that look like the mature plants’ leaves, different from cotyledon leaves that all seedlings first emerge with), feed them with weak, diluted liquid fertilizer.
Check the back of your liquid fertilizer bottle to see how much to add per quart or gallon of water. If it includes instructions specifically for seedlings, use that rate. If not, divide the normal rate in half to make sure you are not giving the plant too much fertilizer.
Too much fertilizer will burn the plants and harm them. They may develop brown leaf tips or die.
I usually fertilize seedlings once a week. If they seem like the need more as they get larger, I increase the dose gradually each week.
Step 6: 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 twice as tall as the pot it’s in or more!
These are all signs that the plant is ready to graduate to a larger container.
Choose a container that is just a bit larger than the old container. Place some pre-moistened potting mix in the pot. Pop the seedling out of its old pot and plant it in the new container, filling in the gaps with more fresh soil.
Once the temperature conditions are ideal, you’ll be able to harden off your indoor seedlings and plant them in your garden.
Mistakes to Avoid
- Do not overwater. Too much water keeps the roots from getting oxygen and can cause excessive algae or fungal growth.
- Make sure containers are clean before planting in them. Disease can linger on used plant pots. Give them a quick cleaning with mild detergent or bleach before using for seed starting.
- Don’t plant seeds too deep or too shallowly. Use the recommended depth on the seed packet, or plant at a depth that is twice the size of the seed itself.
- Seedlings need strong light. Keep seedlings directly under your grow lights or shop lights. Raise or lower the lights so that your seedlings are no less than a couple inches away from the light bulb. This will keep plants from getting leggy and straining to reach for the light.
- Plants need nutrition (fertilizer) to grow, but too much will burn them. Dilute fertilizer every time you use it on your seedlings.
- Keep air circulating in the grow room and on plants. A light breeze will encourage plants to grow strong stems and keep disease from taking hold.
- Do not plant seedlings outside right away. Instead, harden off seedlings before transplanting. This final step is crucial to your success! I’ll quickly explain that process below!
How to Harden 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!
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!