With the indoor seed starting season right around the corner, and direct planting soon to follow, I figured it was a good time to share how to start seeds indoors. When starting your own seedlings as long as it is done correctly, it can save you tens to hundreds of dollars. Not only that, but you’ll also have healthier plants, and increased options for varieties since you choose the seeds!
This is a complete guide on how to start seeds indoors. I have started just a few seeds indoors and up to hundreds! But to tell you the truth, after many years of starting seeds indoors, I have completely moved away from this practice. I explain this more in detail later in this blog post. But it basically comes down to the fact that I start everything in an unheated outdoor tunnel now.
It took me 14 years of gardening before I purchased one, and before that I started seeds indoors. I explain below some of the things that no one tells you about indoor seed starting, like when it is a good idea and when you should look into other options.
This post contains affiliate links. Full disclosure can be found here.
What is Indoor Seed Starting?
Indoor seed starting is the process of germinating and growing seeds indoors in a soil medium. The basic concept is the same as outdoors, you need seeds, light, soil and water in order for them to grow.
Why Start Seeds Indoors?
#1 To get to a harvest faster
Indoor seed starting is a way to get to a harvest more quickly than direct sowing outdoors, in most cases. This is due to the fact that seeds germinate best at certain soil temperatures.
If you try to start a tomato seed in the ground in March and the ground is cold, along with the air temperature still getting below freezing occasionally, that tomato seed wouldn’t sprout until the conditions were right. It could be a few weeks to a few months depending on your climate.
However, if you were to plant that same tomato seedling indoors, with a heat mat, creating the optimal soil temperature, it would sprout quickly and get started growing. In theory, it will save you time to get to harvest because you’ve done the germination and the sprouting indoors.
And yet the time you spend growing the seedlings indoors will not directly save you that exact same amount of time outdoors. The reason for this being that seedlings grown indoors need to have time to get used to the amount of light that is outdoors, and the elements like wind. This is called hardening off.
In addition to hardening off a seedling, you also need to transplant. When transplanting, the seedlings go through some shock, called “transplant shock”. Even a small amount of transplant shock, will set back the plant in growth for several days if not more. So both these factors would lead to a certain amount of lag behind a plant that is directly sown in the ground.
But overall, starting seeds indoors can allow you to mimic optimal soil conditions and temperature, thus getting a head start on growing.
#2 To get proper plant spacing
Larger plants that are transplanted into the ground means the plants can be spaced exactly with little to no thinning involved.
When you don’t need to remove excess plants, you save money on the seeds and you save time, because you don’t need to go back and remove extra plants.
#3 To get ahead of weed growth
Another reason to start seeds indoors would be to get ahead of the weeds. You can weed just prior to planting, then transplant large plants into the ground. This practice allows you to easily seed where your crops you want to grow are and where the weeds are.
When weeding can be done without as much effort, it is more likely to get done in the first place. Anything that makes it easier and faster for you to weed is a good thing!
Recommended Supplies for Indoor Seed Starting
- A high quality seed starting soil mix. I make my own, and the recipe can be found HERE, along with where to find all the ingredients. Other options are Promix or Johnny’s 512. It is likely that you can find Promix locally at a garden center or nursery. Whatever you do, do not undercut yourself before you even get started. I don’t recommend the majority of the common brands found at big box stores (Ferry-Morse, Burpee, etc.)
- Seed tray. Your seed tray choice depends on your personal preference of how to start seeds. I love soil blocks, along with a homemade wooden tray. There is also plastic trays with cells, I only use those for things like flowers that are not particular about their conditions. For instructions on how to use soil blocks, check out THIS blog post. The only cell tray I’ve ever really liked comes with a larger price tag, it can be found HERE.
- Seeds! This one is obvious but quality seeds matter too. If you’re planning on growing organically, you may want to buy organic seeds as well. For a list of all the seeds I’m growing this year, click HERE.
- Heat mat For many years, I went along thinking I could get along without a heat mat but it would take forever for seeds like peppers to germinate and the germination was also spotty. It’s worth the investment. I really like THIS large one, because it fit my shelving unit perfectly.
- Lighting Lighting is an incredibly vital to indoor seed starting. It can make or break all your effort with indoor seed starting. In all the years I grew seeds indoors, I used plain fluorescent T8 bulbs. Technology is now getting better, and LED’s are the preferred option due to the light quality and energy efficiency.You can find 2ft LED grow lights HERE and 4 foot long LED grow lights HERE.
- Shelving THIS one is my favorite. When we stopped using it for seeds, we put it in our cold room. It’s very versatile. Use the 4 foot long lights, I mention above with this unit! It takes at least two per shelf.
- Want to take the work out of it? Try a small kit. This would be more ideal for a smaller amount of seed starting indoors. My siblings and I all went in on a gift for my Dad and bought him THIS stackable grow light system that can be added on to later.
They also sell large kits HERE and HERE. But I would discourage it. If you are starting a large amount of seedlings indoors each year, in my opinion it would be wiser to invest your money in an outdoor seed starting setup like a small greenhouse, unheated tunnel, or high tunnel.For the cost of a large all in one kit, you could buy an entire caterpillar tunnel (almost!).
Are indoor seed starting supplies worth the money? Like every project, there is some upfront costs to get the equipment. The idea is that over time, you’ll pay it off with the savings you reap, and then save money every year there after. If you know you won’t use the materials long enough to make back the money you spent, then by all means, buy transplants from someone else!
When Does it Make Sense to Start Seeds Indoors?
First off, I want to share when it makes sense as a gardener to start seeds indoors. When I say “makes sense”, I’m referring to if it is worth your time and energy to start them indoors. I haven’t heard many people talk about this but it is very important. Here are my two criteria for if It makes sense to start seeds indoors.
- You have a small garden (anything less than 500 square feet of growable area)
- You have a short growing season, in my book this would be less than 100 frost free days/nights.
Let me explain the why behind reason #1. When I bumped up my garden size, I increased the number of plants I would grow indoors. Specifically what comes to mind is tomatoes, starting 20 tomatoes indoors is no biggie, but if you get up to 120 tomatoes like me, it starts to get crazy.
First off, you need to have enough lights to grow that large amount, you need to have extra shelving, and you need to have a ton of extra time to maneuver all of them around when it comes time to harden them off. If you’re starting other seedlings too like peppers, herbs, flowers, it adds even more time, lights, shelving. You get the idea.
Any mess ups, like not enough space under lights and not watering sufficiently can lead to weak plants. When you have a lot to manage, it becomes more difficult to keep ideal conditions.
It is not my intent to discourage indoor seed starting, merely to point out when it makes sense to do so. The alternative would be either direct sowing seeds or seed starting outdoors in a greenhouse, high tunnel, caterpillar tunnel, or any smaller covered area, even in recycled plastic milk jugs or cloches (winter sowing).
Since switching over to outdoor seed starting I’ve noticed a huge jump in the health of the plants right off the bat, so it is now my preferred method. I do realize, however, that it is not ideal for some of you! There is definitely no “one size fits all” in seed starting. I’ll talk more about outdoor seed starting in my next blog post, but for now, back to how to start seeds indoors.
When to Begin Indoor Seed Starting
My number one advice tip for when to start seeds indoors is DO NOT begin indoor seed starting too early! This is a huge mistake among new gardeners. Plants started too early can become leggy, root-bound, or flowering transplants can cause stunting and reduce early production.
The best time to start seeds indoors will often be stated by your seed packet. Personally, I always lean toward the shorter of the time frames.
For example, on black cherry tomatoes from the Baker Creek Heirloom Seeds website it says:
“Growing Tips: Start indoors 6-10 weeks before last frost. Heat mat helps to warm soil and speed germination.”
I would never, ever start tomatoes ten weeks early, they may be flowering by then. I would start them six weeks before I plant to transplant them. Which is actually a week or two past my last frost!
This is my personal opinion, obviously, you can start your tomatoes whenever you wish 🙂 I have a longer season, so I don’t have to worry about not getting a harvest if I plant them too late.
If you don’t know your average last frost you can find it out HERE. Keep in mind, it is an average, not an absolute. You could still get frost after that date, but it’s easy to keep an eye on it with the weather forecast being about 14 days in advance.
How to Start Seeds Indoors
As I mentioned, I love using soil blocks for starting seeds indoors. It is not for everyone though, so below I’ll tell you how to start seeds in both cell trays and soil blocks.
How to Sow Seeds Indoors in Cell Trays (plastic, recycled, or compostable material)
#1 Moisten your soil mix.
Some seed starting soil mixes come with a “wetting agent” on them allowing you to easily moisten them after sowing seeds. If your soil mix doesn’t include this, you must wet down your mix prior to putting it in the tray!
The reason behind this is that most soil mixes are made with peat moss. When peat moss is dry, it is hydrophobic, meaning it repels water. The best way to get peat moss in a soil mix wet all the way through is to moisten it in a container and mix it around until it is all moist, almost like mixing cake batter!
#2 Fill your Cell Tray with Indoor Seed Starting Soil Mix
This is pretty straight forward but some tips I would add are to make sure all the edges of the tray get the same amount as the middle, and that they are filled completely to the top.
#3 Poke a Hole for your Seeds
Add a depression to the soil will help your seeds to germinate. The depth of the hole definitely matters! Check your soil packet and see how deep the seed needs to go.
If you feel unsure, I would go more towards too shallow rather than too deep.
Southern Exposure Seed Exchange has a good general guide that says:
“Seeds need air as well as water to germinate. Sow no deeper than 3 to 4 times the diameter of the seed or in the case of oblong seed no deeper than 1 to 3 times the length of the seed.”
#4 Place seeds in the holes
#5 Cover seeds with soil mix or vermiculite
I cover any seed that is sown at a quarter inch or more with indoor seed starting soil mix. Some people prefer vermiculite since it is lighter. But vermiculite is just one more thing to buy and to be honest, I get just as good of results from covering with moistened soil mix.
Anything that should be surface sown, like lettuce, I do not cover with anything. You’ll need to keep these seeds moist at all times. And I said moist, not soaking so a fine mist of water sprayed a few times will do.
#6 To Cover with plastic or Not to Cover?
Some seed trays come with plastic covers or you can cover a seed tray with plastic wrap, in order to keep the moisture in. With every good thing, comes the ability for it to become something bad too.
The intention is to use these only until the very first seed is emerging. Don’t wait until they all emerge. Take the cover off ASAP when you see the emergence of a seedling. Any longer and you’ll increase the risk of fungal diseases, especially damping off. This fungus will kill the seedlings, usually at the soil surface it will turn brown and die.
I use a cover with I am surface sowing seeds like lettuce.
I rarely use a cover if I have covered the seedling with the soil mix. I would especially not use it if you are sowing peppers, tomatoes, or any other nightshades, which are prone to fungal diseases.
#7 Moisten, Put Tray Under Lights
If you didn’t pre moisten your soil in step #1, make sure all the soil is moist at this point. It should not be soaked. Hopefully, whatever tray you chose has holes in the bottom to prevent overwatering 🙂 Then place it under grow lights.
An important word about lights, if using fluorescent lights, the lights should literally sit within 2 inches of the tray. This will prevent the seedlings from getting leggy.
LED grow lights are a different story. Some are very intense and need to be placed farther away from the seedlings. You’ll need to experiment with trial and error here since all LED lights are not the same. If the seedlings turn white they are getting too much exposure, if they have long stems and look light they are reaching toward the light they are not getting enough light.
Usually, most people will not put the light close enough. Start close and work your way away from the trays if needed.
It’s always best to mimic nature with gardening. So 12-14 hours with lights on, the rest of the day, lights off! Get a timer to plug your lights into and it will make your life easier 🙂
#8 Add a Heat Mat for Heat Loving Crops
For seeds that require a higher soil temperature for germination, add a heat mat underneath the tray. Usually anything categorized as a summer crop like tomatoes and peppers should use a heat mat.
If you have an area right next to a warm wood stove or heat source in your home, this would work as an alternative. Just make sure you can set up your lighting there too.
#9 After Germination Occurs, Add a fan.
A fan mimics a breeze that would naturally be found outdoors. It will help to keep your plants strong and sturdy and also keep fungal diseases at bay.
#10 Keep the Soil Moist at All Times
Again, do not overwater. Ideally, you should be bottom watering the trays to keep them moist after the seedling emerge. I use a wicking mat placed under the soil tray for this. It takes water in a tray and wicks it up under the tray (since 99% of plastic trays have holes this is where the water is soaked in). The CELL TRAYS I like come with this set up. Or you can create your own.
The reason for watering from the bottom is it helps to prevent fungal diseases.
If the soil dries out completely, so will your plants. There isn’t a lot of soil there, so you really need to keep an eye on it!
#11 Pot Up Plants as They Grow
If you see roots poking out of your trays, or the plants are getting large and crowded, move them to a larger container and add more soil!
This is a critical step to success, the plants will stagnate in growth if you wait too long to pot them up. When in doubt move them to a larger pot size earlier, rather than too late.
#12 Feed as Needed
Very likely, your seedling will need additional fertilizer until the time they can get outside. My very favorite is a water soluble fish fertilizer. It can be found HERE. Follow the directions for dilution rates. It is very stinky but works so good.
#13 Harden Off Seedlings
Once outdoor conditions allow you to do so, (warm enough soil and air temperature), it’s time to harden off the seedlings.
This process gets them used to the outside light and air. Slow and steady is the absolute key!
- On the first day, move all seedlings outdoors for 30 minutes, then they go back inside under the lights.
- On the second day, move all seedlings outdoors for 1 hour, then back inside under lights
- Day 3, 1.5 hours outdoors
- Day 4, 2 hours outdoors
- Day 5, 3 hours
- Day 6, 4 hours
- Day 7, 5 hours
- Day 8, 6 hours
- Day 9, 7 hours
- Day 10, 8 hours
- Day 11, 9 hours
- Day 12, 10 hours
- Day 13, 11 hours
- Day 14, 12 hours
- Day 15 12 hours + overnight
- Day 16 Transplant
Keep in mind, this is not an exact science unless you get clear skies everyday. If you get a cloudy day, the seedlings will get less light, on a sunny day, they will get tons of light. Watch the weather closely and adjust accordingly.
#14 After the Harden Off Period is complete, Transplant Seedlings
Most seedlings are placed in the soil at the same depth as the tray. Tomatoes in particular should be buried deep in the soil a few inches away from the first set of leaves, or placed sideways so the stem is parallel with the soil, the leaves still remain above the soil level. Peppers can also be buried slightly deeper, as can corn- if you’ve ventured into growing that in a seed tray.
How to Sow Seeds Indoors in Soil Blocks
The process for starting seeds indoors in soil blocks is similar to cell trays, but has some differences. I’ve noted them all below.
#1 Moisten your soil mix
When making soil blocks, the only way to make them is to wet down the mix. If the soil is too dry and they fall apart, if the soil is too wet they slip out before you’re able to compress the soil. When in doubt aim towards the more wet side. It should look like brownie mix when it’s the right moisture level
#2 Make your Soil Blocks
To make soil blocks you need a fairly deep tray or container, 8-12 inches to pile the wet soil and make blocks. I have a very in depth guide on how to do this HERE. Prior to starting, pick the correct dibble (I think that’s what it’s called), it’s a little plastic piece that creates the depth in the block.
Most soil blocks come with the standard size, I believe it’s a 1/4 inch deep but you can buy other sizes too. This works for most seeds but if I’m planting a large seed I have the 1 inch size dibble for the 2×2 soil blocks too.
As for a tray, I build my own wooden ones, using dimensions and instructions from Eliot Coleman’s Book HERE. I also add wicking fabric directly underneath, with an extra few inches and place two trays over a big half sheet baking pan then fill the pan with water and let the fabric sit in it.
A plastic tray can be used too, but they have their issues if your using it for containing soil blocks. The main problem being that most plastic trays don’t have the structure to hold very heavy blocks. They wobble all over the place when you need to carry them.
#3 Place seeds in the holes
This doesn’t require much explanation, just get it down in the bottom of the hole 🙂
#4 Cover seeds with Soil Mix
I cover any seed that is sown at a quarter inch or more with moist indoor seed starting soil mix. Some people prefer vermiculite since it is lighter. I’ve haven’t had great results with it, so I don’t use it anymore.
Anything that should be surface sown, like lettuce, I do not cover with anything. You’ll need to keep these seeds moist at all times. And I said moist, not soaking so a fine mist of water sprayed a few times will do.
#5 To Cover with plastic or Not to Cover?
You can cover a soil block seed tray with plastic wrap or some kind of clear plastic cover, in order to keep the moisture in. With every good thing, comes the ability for it to become something bad too.
The intention is to use these only until the very first seed is emerging. Don’t wait until they all emerge. Take the cover off ASAP when you see the emergence of a seedling. Any longer and you’ll increase the risk of fungal diseases, especially damping off. This fungus will kill the seedlings, usually at the soil surface it will turn brown and die.
I use a cover with I am surface sowing seeds like lettuce.
I rarely use a cover if I have covered the seedling with the soil mix. I would especially not use it if you are sowing peppers, tomatoes, or any other nightshades, which are prone to fungal diseases.
#6 Moisten, Put Tray Under Lights
I do a light misting over the soil blocks with water, then place it under grow lights.
An important word about lights, if using fluorescent lights, the lights should literally sit within 2 inches of the tray. This will prevent the seedlings from getting leggy.
LED grow lights are a different story. Some are very intense and need to be placed farther away from the seedlings. You’ll need to experiment with trial and error here since all LED lights are not the same. If the seedlings turn white they are getting too much exposure, if they have long stems and look light they are reaching toward the light they are not getting enough light.
Usually, most people will not put the light close enough. Start close and work your way away from the trays if needed.
It’s always best to mimic nature with gardening. So 12-14 hours with lights on, the rest of the day, lights off! Get a timer to plug your lights into and it will make your life easier 🙂
#7 Add a Heat Mat for Heat Loving Crops
For seeds that require a higher soil temperature for germination, add a heat mat underneath the tray. Usually anything categorized as a summer crop like tomatoes and peppers should use a heat mat.
If you have an area right next to a warm wood stove or heat source in your home, this would work as an alternative. Just make sure you can set up your lighting there too.
Even with the wicking trays, I put the heat mat under them, it heats water water and in turn heats the soil above when the water is wicked up.
#8 After Germination Occurs, Add a fan.
A fan mimics a breeze that would naturally be found outdoors. It will help to keep your plants strong and sturdy and also keep fungal diseases at bay.
#9 Keep the Soil Moist at All Times
Again, do not overwater. Ideally, you should be bottom watering the trays to keep them moist after the seedling emerge. I use a wicking mat placed under the soil tray for this, mentioned in step #2
The reason for watering from the bottom is it helps to prevent fungal diseases.
If the soil dries out completely, so will your plants. There isn’t a lot of soil there, so you really need to keep an eye on it!
#10 Pot Up Plants as They Grow
If you seed roots poking out of your trays, or the plants are getting large and crowded, move them to a larger container and add more soil!
This is a critical step to success, the plants will stagnate in growth if you wait too long to pot them up. When in doubt move them to a larger pot size earlier, rather than too late.
For soil blocks, I move my 2×2 inch block up to a 4×4 inch block. They take a ton of space and soil but grow so well. An alternative is moving the 2 inch block into a 4 inch plastic pot.
#11 Feed as Needed
Very likely, your seedling will need additional fertilizer until the time they can get outside. My very favorite is a water soluble fish fertilizer. It can be found HERE. Follow the directions for dilution rates. It is very stinky but works so good.
#12 Harden Off Seedlings
Once outdoor conditions allow you to do so, (warm enough soil and air temperature), it’s time to harden off the seedlings.
This process gets them used to the outside light and air. Slow and steady is the absolute key!
- On the first day, move all seedlings outdoors for 30 minutes, then they go back inside under the lights.
- On the second day, move all seedlings outdoors for 1 hour, then back inside under lights
- Day 3, 1.5 hours outdoors
- Day 4, 2 hours outdoors
- Day 5, 3 hours
- Day 6, 4 hours
- Day 7, 5 hours
- Day 8, 6 hours
- Day 9, 7 hours
- Day 10, 8 hours
- Day 11, 9 hours
- Day 12, 10 hours
- Day 13, 11 hours
- Day 14, 12 hours
- Day 15 12 hours + overnight
- Day 16 Transplant
Keep in mind, this is not an exact science unless you get clear skies everyday. If you get a cloudy day, the seedlings will get less light, on a sunny day, they will get tons of light. Watch the weather closely and adjust accordingly.
#13 After the Harden Off Period is complete, Transplant Seedlings
Most seedlings are placed in the soil at the same depth as the tray. Tomatoes in particular should be buried deep in the soil a few inches away from the first set of leaves, or placed sideways so the stem is parallel with the soil, the leaves still remain above the soil level. Peppers can also be buried slightly deeper, as can corn- if you’ve ventured into growing that in a seed tray.
The beauty of soil blocks is that they have so much less transplant shock because the roots are air pruned.
Conclusion
Indoor seed starting is a great way to have flexibility in starting many varieties of plants and save money while you’re at it. All you need is the supplies: soil, light, water, seeds, and your time. Starting any seed is a fun process. Keep in mind your circ*mstances to decide whether or not this method of starting seeds is for you.
