Seed Starting 101: How to Sow Seeds Indoors

So, you’re thinking of starting your garden (or at least a portion of it) from seed, are you? Good for you! While growing food, herbs, and flowers from already-started nursery seedlings is completely acceptable, growing from seed has some awesome advantages. Starting from seed gives you more selection and diversity of varieties in your garden, along with a jump start on the growing season. It is also extremely rewarding! On the other hand, I realize starting seeds indoors can also feel quite overwhelming and intimidating, especially if it’s new for you.

This article covers everything you need to know to feel comfortable and confident in starting seeds indoors (or in a greenhouse) to give them a strong healthy start. We’ll go over the supplies you’ll need, tips for timing, and step-by-step instructions – from how to sow seeds and also tend to young seedlings. I broke it down into three sections: a discussion of supplies, steps for sowing seeds, and ongoing care of seedlings.

For you visual learners, there is also a video at the end of this article about starting seeds that compliments the written material. Click here to jump to the video at the end. Also, if you are in need of tips for buying started seedlings (we still buy starts sometimes!), be sure to check out this post all about how to choose the best, healthiest seedlings at the nursery.

Okay, let’s start some seeds!

WHEN TO SOW SEEDS

When should I be starting what types of seeds, you ask? Well, different vegetables have optimal seed starting and transplanting dates at various times throughout the year. This will depend on where you live. Some types don’t like to be started early inside at all. Instead, some veggies like to be directly planted outdoors in the garden when the time is right, called direct-sowing. This is true for things like radishes, carrots, and beans.

To determine what veggies likes what kind of planting and when, get out your garden planting calendar. If you choose to subscribe to our weekly newsletter, you will be emailed a free 20-page garden planning toolkit – and in it includes planting calendars for every zone!

SUPPLIES NEEDED FOR STARTING SEEDS INDOORS

1. Seeds
2. Seedling containers/pots and trays
3. Seedling start medium
4. Labels
5. Bright light
6. Water
7. Heat
8. Airflow or Fan

We’ll explore each of these items in detail below, but feel free to browse our master list of top seed-starting supplies here.

Now one by one, let’s discuss.

1) Seeds

Grab what you want to grow for this season! When choosing seeds, pick things that sound tasty to you (duh). But even more, look for plant descriptions that sound well-suited for your climate and garden. For instance, varieties that are especially cold-hardy if you live in the north, or heat-tolerant and slow to bolt if you have scorching summers. We seek out varieties that are naturally resistant to powdery mildew (a big issue here) as well as warm season crops like tomatoes or peppers that will still ripen well in our cool foggy summer weather.

Not sure where to buy seeds? There are many excellent companies are selling seeds online. Click here to see the top 12 places that sell organic, heirloom, and non-GMO seeds. Our favorite seed supplier is High Mowing, who sells all certified organic seeds. You can also usually find some good options at your local nursery, farm supply store, or garden center too, though not as diverse.

2) Seedling Starting Pots & Trays

With time, gardeners experiment and develop a preference on the type and size of containers they use for starting seeds. Our favorites are described below. No matter the chosen container type, it is best if they sit inside some sort of tray that can catch any excess water runoff. Trays should be kept covered with dome lids after the seeds are planted, before sprouting. The lids help keep in moisture and warmth, assisting in germination. Some seed starting trays come with dome lids, or you can purchase them separately.

We have found a great combo of super heavy-duty seedling trays and dome lids we love. They’re holding up really well after many years of use! The trays can literally carry bricks without bending. Most other types we have used in the past crack or leak easily, creating unnecessary waste.

In regards to pots, we utilize several types and sizes of containers depending on what we are growing. For example, when we start a large amount of smaller plants, like dozens of greens, flowers, and herbs, we generally start them in 6-pack cell trays to save space. On the other hand, if we are starting seed for larger plants that we grow fewer of, such as just a handful of tomatoes or peppers, we start them in mini nursery pots. Those hold a bit more soil than 6-packs, yet also take up more space in the seedling tray.

Types of Seedling 6-Packs

Some seedling 6-packs fairly small individual cells, like 1.5 x 1.5 inch. We have found these are totally adequate for starting seeds for flowers, greens, herbs. They are also good for other plants you would get out of those cells fairly quick, either by potting up or transplanting out. We love these super durable BPA free 6-packs. Like the trays we use, these things will last for YEARS without cracking like so many others do.

To start seeds for medium size plants like broccoli or cauliflower, we like to give them a tad more space for roots, and room to grow into with time. For these, we use large-cell 6-packs (about 2.5 x 2.5″). The ones we have now are somewhat flimsy. We are continuing to get good use out them for many years, but some are starting to crack so I may not want to recommend them. If I find something better, I will add them here!

Nursery Mini-Pots for Seedlings

While 6-packs are great space savers, there are definitely times we prefer to start seeds and seedlings in slightly larger nursery pots, like 4 or 6″ containers. Our favorites are these 4″ round pots, which are holding up really well after years of reuse!

Benefits of starting seeds in mini-pots:

• More soil space means more moisture retention. This helps to reduce the frequency you’ll need to water and prevents the seedlings from accidentally drying out.
• Larger containers also mean more root space. Things like tomatoes get large fast, with the potential to get root-bound sooner. Giving them extra space from the get-go means they can live happily in this size for several weeks or longer after sprouting. Which leads us to the next point…
• Starting seeds in bigger containers reduces the need and urgency to pot up seedlings as soon. This equals less hassle. If you intend to plant out seedlings fairly small and soon after sprouting, they may not need to be potted up at all. (We’ll talk more about that in the care instructions below).

What about biodegradable seedling cups?

Starting seeds in biodegradable cups are great in theory, but it depends on the thickness and how long they take to degrade. If the “plantable” container is still intact when planted out, and hasn’t broken down by the time the roots reach its edge and want to grow beyond, the container will constrict them and limit growth. This has been our personal experience.

On the other hand, some are too flimsy and could degrade too quickly (like toilet paper rolls). They may fall apart while you’re still tending to them inside, making a mess or even damaging seedlings. Also, many biodegradable pots are made from peat, which receives some criticism for being not very sustainable. They’re also a single-use product. Some people love them though, and no judgment if you do! Whatever works for you.

Re-Using Seedling Pots & Trays

Personally, we don’t mind starting seeds in plastic containers because they can be sanitized and reused year after year! Meaning we don’t need to buy more every season – thus reducing waste and cutting costs. Especially if you invest in ones that last. To learn how we sanitize our seedling & garden supplies, check out this article.

3) Seed Starting Soil or Medium

It is very important to start seeds in fresh, sterile, bagged seedling starting mix like this one. You do not want to use straight “potting soil” or other bagged raised bed mixes. They’re often times too heavy and dense for good seed germination. They may inhibit growth of super fine new roots, and don’t have the ideal moisture retention properties that seedlings like to thrive. Avoid using old soil from your garden! It may have diseases or pests.

You do not need any fertilizer in the soil that the seeds are started in! On the contrary, you want to avoid it – fertilizer can burn fragile seeds and seedlings, inhibiting or killing growth. Seeds themselves are amazing little things! They contain all of the nutrients needed to allow the seed to germinate (sprout) and grow its new baby plant for at least the first two weeks of life! We’ll talk more about feeding seedlings in the “Instructions” section below.

If you’re feeling frisky, you can also make your own seedling mix! I have read various recipes that call for combining something like 40% well-aged, fine compost, 40% rehydrated coco coir, and 20% perlite, small volcanic rock, or sand for aeration, though we haven’t done this ourselves.

We generally use a majority organic bagged seedling mix, and mix in just a little potting soil and worm castings. Over the years, we have experimented with many different brands. As long as it’s labelled as a seedling start mix, it should work just fine. We’ll go over the mixture specifics in the sowing instructions section below.

4) Labels

You will want something that you can mark your seedling containers with, to keep track of what is what. This is helpful while starting seeds indoors, and you can also transfer those labels into the garden along with the plants at the time they’re planted out later.

For labels, there are tons of options out there, from using plastic forks to painting on rocks. We like these inexpensive plastic tags. When using sharpie on them, the writing remains visible throughout the season and doesn’t wash away or fade in the sun, AND once you have used them twice (written on both sides) you can remove the sharpie with rubbing alcohol and cotton swab to keep on re-using them for years to come! The popsicle stick idea is cute so we have tried that before, but found the writing faded quickly.

5) A Light Source

Unfortunately, most times, natural sunlight through even your sunniest window isn’t enough to keep seedlings happy. This is particularly true when you are starting seeds indoors during the short-day winter months. In that case, you’re definitely going to want to provide artificial light. If it is summer or early fall, then a sunny window, greenhouse, or other natural light outside may be sufficient.

Your seedlings will tell you if they’re getting enough light or not. Without adequate light, seedlings will get “leggy”. This is when they stretch out super tall and thin in search for more light. Taller seedlings do not equal healthier, better seedlings! This growth pattern will make them weak, susceptible to toppling and breaking. The shorter and stockier you can keep them, the better!

Grow light options

The two most common types of grow lights used for starting seeds indoors are either LED or T5 fluorescent lights. Either way, it is best to choose a light that is described as ‘full spectrum’ for starting seedling. Second best is ‘cool white’ grow lights, best for vegetative growth. Here is one affordable and well-rated T5 grow light, and this is one LED option.

Edit: Since moving, we no longer have a greenhouse for seed starting (yet), so we invested in this awesome 3-tier LED light shelf – and grew the healthiest seedlings we’ve every grown! For more grow light tips, see this article all about choosing and using grow lights. It goes over FAQ, best practices, and also highlights various popular and well-rated lights – for seedlings, houseplants, or other types of indoor plants.

Setting up grow lights indoors

Using wider styles of lights with reflectors will give your seedlings maximum coverage and light exposure. The wider the better, really. Using a skinnier set of lights can work too, but you’ll want to rotate your seedlings below it every couple of days to ensure the ones on the outskirts also get their time in the spotlight. Veggie seedlings need at least 12 hours of light (16 is best), and at least 8 hours of dark. We find using a programmable light timer is very convenient and helpful!

Lights should be hung in a manner that can be easily adjusted, raising them as the seedlings grow. Many people starting seeds indoors do so by using a shelving unit where they can hang the lights from the underside of the shelf below. For most light types, it is recommended to keep lights low and close to the seedlings (just a few inches above!) to help prevent leggy seedlings. If you have LED lights, read their specific instructions. They can burn your plants if they’re hung too close!

Using grow lights in a greenhouse

Unfortunately, our greenhouse doesn’t receive full sun all day so we need to use some artificial lighting. However, the grow lights are meant to be complementary to the natural sun provided. Therefore, we opt for skinnier styles of lights instead of huge boxy sets so they won’t block all of the natural light. Even if a greenhouse receives full sun, it may be necessary to provide supplemental light during the shorter days of winter to achieve the goal of 12 hours of light per day.

6) Water

Oh, the great debate about chlorinated or dechlorinated water, and its impact on plants. There are dozens of studies out there arguing whether or not it harms your plants. My opinion is this: chlorinated city water will not necessarily “harm” your plants. They will not turn over and die if that is the only water you have access to.

However, when chlorinated water is added to a healthy, robust soil (like that in our established garden beds) the living soil food web that is present  – the beneficial microorganisms in your soil or compost – act as a natural buffer to reduce the impacts of chlorine and protect your plants. In a seedling container however, you usually don’t have the benefit of an established protective soil food web. This means seedlings can be a little more chlorine-sensitive than mature plants in larger volumes of soil.

My suggestion would be to use dechlorinated if possible. It could make the difference between living plants and thriving plants. We use captured rainwater for our seedlings. If using water in the kitchen, you could run it through a Brita-type carbon filter first. For outdoor hoses, we like to use these hose carbon filters.

7) Heat

If your house is cool in the winter (less than 70°F) or if you’re using an outdoor greenhouse, you’ll want to use a seedling heat mat or other method to help maintain a consistent ideal temperature while starting seeds. Optimum germination temperature is when the soil is 70-80°F, for most things.

Some plants like lettuce do prefer cooler soil to germinate. Furthermore, most seeds can sprout in the 50-60s, though slower and with less success. If your house is just below 70°F and you can’t use a heat mat, find a warm place to keep your seed trays. This could include the top of a refrigerator or next to a sunny (but not cold and drafty) window.

We use a seedling heat mat in combination with a thermostat gauge. It has a probe that stays in the soil and only turns on when the soil needs heat. Set it at 75° and walk away! When the greenhouse heats up in the daytime, it turns off the heat mats. As the temperature drops in the evening, it kicks back on. Do not heat seeds over 95°F! This can sterilize and kill them.  

A note about greenhouse temperatures

We are able to start seeds in our greenhouse during the winter because it doesn’t freeze here. Even on the coldest winter nights, our greenhouse usually doesn’t get much below 40°F. Our greenhouse is not “heated”, though the heat mats do help keep the soil warm and also maintain the overnight air temperature several degrees warmer than outside.

If you live in a place with freezing winters, you’ll have to be more cautious about having a protected outdoor set-up, or just stick to starting seeds indoors. There are also ways to passively heat a greenhouse without using electricity. See this article to learn more about setting up and using a hobby greenhouse.

In the daytime, greenhouses are prone to getting significantly warmer than outdoor temperatures (up to 20 degrees warmer, or more!). This puts seedlings at risk of frying. Therefore, especially when you are starting seeds in the warm summer months, you may have the opposite issue and need to find ways to keep them cooler. Ways to cool a greenhouse include providing afternoon shade, shade cloth, opening the greenhouse door, and/or providing a fan – which leads us to…

8) Airflow, or An Oscillating Fan

Seedlings started indoors need movement to develop strength and prepare them for the great outdoors. Good air circulation also helps prevent disease, mold, and a condition called “damping off”. An easy way to do this is to provide a gentle breeze via an oscillating fan once the seedlings are a few weeks old. See more info in the seedling care section to follow.

Now that you have all your supplies gathered….

HOW TO SOW SEEDS

Step 1: Get your labels ready

…so you can pop them in later as you go! I like to sit down with a glass of wine the night before we are going to sow seeds, sort through seed packets, choose what we want to start, and create the labels ahead of time. You could totally switch it up and do this later.

Personally, I have found it super helpful to have all the labels ready and waiting rather than stopping to write labels in the middle of juggling seeds and soil. It is really easy to get confused on who is who (or which containers you sowed already or not) when you’re in the full swing of sowing seeds! Especially if you are starting many plants. This way, I put the ready label in, sow the seeds, then set it aside as “done”.

Step 2: Prepare the seedling soil mix

We like to mix up an organic bagged seedling soil (about 70% of the total bulk), with just a little organic potting soil or finished compost (20%) and some worm castings (10%) in a large tote, tub, or other container that can hold it all. You’ll see why in step 3.

Step 3: Pre-moisten your seedling start mix

It is best to sow seeds into damp soil. Add a little water in to the seedling start medium or soil at a time, mixing as you go, until the desired consistency is reached. Aim for the consistency of a wrung out sponge – damp, but not sopping.

Pre-moistening soil is good idea for many reasons:

• It reduces the need to heavily water immediately after sowing seeds – which in turn reduces the risk of disturbing where your seeds are (e.g. pushing them too deep, or making them float to the top).
• It also helps the soil evenly absorb water going forward. Sometimes when soil has dry spots, it can actually repel water instead of absorbing it.
• Lastly, pre-moistening the soil can make it a tad more dense and compact from the get-go. Not that you want dense soil – but containers full of dry soil will compact and shrink way down in the container after being watered for the first time, reducing space for root growth. If you have already planted your seeds, you can’t add more soil on top of the shrunken stuff because then your seedlings would be buried too deep.

Step 4: Fill your seedling containers with the damp soil mix

Working with your seed starting soil in a tub makes this step really easy and less messy! You can literally just use the cups to scoop up soil, or set the 6-pack cell containers down in the tote with the soil and paddle it in on top. Do not compact it. If anything, give the containers a little shake and tap on the table surface to help it settle and fill any large voids that may be left in there. Top off as needed, but don’t press it down. Seedlings like loose and fluffy soil!

Step 5: Sow your seeds!

To do so, follow instructions on the seed pack. The package should tell you the recommended depth for sowing. For example, it may call for ¼” deep, an inch deep, or maybe even  “surface sown” – where the seeds should be placed just on the top of the soil surface. We have found for surface-sown seeds, it is helpful to sprinkle just the tiniest bit of soil on top of them. Like literally barely any. This prevents seeds from drying out as easily than if they were fully exposed.

How many seeds per container? 

This varies, but the short and sweet is: never only one, and usually not more than 4-5. By planting only one seed, you’re putting all your eggs in one basket. If it doesn’t sprout, it’s sad and you’ve wasted a bit of time, space, and energy for that one failed seed. If you overdo it, you’re kind of wasting seed. That is, unless you plan to pull apart the baby plants to keep all of the ones that sprout in one container or cell (more on this in the “thinning section below”.

Your seedling pack should list the “germination rate” – meaning the success rate at which this particular batch of seeds sprouted during trials at the seed company. If the germination rate is 95%, that means about 95% of the seeds should sprout. In that case, you would only need a few seeds per container to guarantee a healthy seedling or two. Yet if the germination rate is lower, something like 70-80%, then it may be wise to sow more like 4-5 seeds, and so on. Also, sow a few extra if you’re using old seeds.

The size and amount of seeds you’re working with may also dictate how heavily you sow. For example, if you order a specialty squash (which have large seeds!) and there are only a dozen seeds in the packet, with an 85% germination rate, I would only sow two. With something like kale or mustard greens, where the seeds are itty-bitty and you get seemingly hundreds of them in one pack, it’s usually not worth the time and energy being super picky about counting the tiny seeds perfectly. I usually just add a small pinch, anywhere from 4-8.

How many seedlings (plants) should I start?

I suggest to start a few extra seedlings of each thing as a backup! You never know what may happen. Seeds are cheap and you usually get a ton of them – why not give yourself a little extra insurance?

For example, maybe you only have enough space in your garden bed for three tomato plants. You bought seeds for three different varieties you want to grow. I would still consider starting and raising at least 2 to 3 seedling pots of each variety anyways. If one dies, you’re still good. If they all live? Great! Now you have your pick of the litter. You can give some away to a friend or co-worker and help them start their garden!  Or get a container or grow bag to go ahead and grow the extras you ended up with. If worse comes to worse, just compost them.

A note about pre-soaking seeds

Some seeds will sprout best if they’re soaked in non-chlorinated water for a few hours or overnight prior to planting. This is usually recommended for bigger, hard-shelled, wrinkled seeds like peas, fava beans, pumpkins, squash, beans, corn, nasturtiums, and even beets or cannabis. That said, it’s not a necessary step, so do not fret it you don’t get to it! It can just help them bust out a little quicker.

After reading the instructions on what depth your seeds like to be planted, use your finger or the end of a similar small object to create a little hole (or maybe only a slight indentation) in the top of the soil. Then pop those seeds in! Lightly pinch the soil around the seeds to cover them to the desired depth, but do not pack down the soil over them – unless the instructions say so directly! For example, some flowers are surface-sown and tell you to press them into the top of the soil.

Step 6: Water

Your soil is already damp, right? So we don’t need to go crazy here. However, it is really important that the seeds and their soil don’t dry out during germination. Especially the upper portion of soil where the seeds are hangin’ out. We give them a light misting over the top of the soil with a spray bottle or our favorite gentle trigger mister. Avoid using a watering can, as the strong stream of water can disturb the soil and seeds. It is also easier to accidentally over-water using a watering can.

Step 7: Cover the seedling trays

It is best to cover the trays up until they’ve sprouted. This helps prevents the soil and seeds from drying out, and also helps keep warmth in. We use these heavy-duty humidity domes.  In a pinch you could also use plastic wrap with a couple holes poked in it, or, any extra black bottom trays you may have turned upside down on top. The seeds do not need light and can germinate in dark conditions, but be sure to check frequently and take off any non-transparent lids as soon as they sprout! Without good light right away, they’ll immediately start to get leggy.

Step 8: Provide warmth

As we already discussed in the supplies section above, the optimum germination temperature is when the soil is 70-80°F, for most things. So get those trays into a nice warm spot, or turn on your seedling heat mats!  I will reiterate how convenient and ideal it is to have one with an automatic thermostat to turn your mats on and off for you as the temperature changes throughout the day, keeping them in that perfect range with no effort!

ONGOING SEEDLING CARE

Now that you have learned how to start seeds indoors, how about a few tips on how to take care baby of seedlings just before and after they’ve sprouted?

Water Amount & Frequency

The goal is to maintain the soil fairly moist but not soggy during germination. This may mean misting the top of the soil with a gentle sprayer every day, or every other day, depending on what it looks like in there. After sprouting, you can cut back water a little bit, waiting until the soil dries out just slightly between watering, but do not allow it to completely dry out!

Your watering frequency will vary depending on your unique climate, humidity, heat, and other factors. It will also vary depending on your watering method. Personally, we find that watering 1-2 times per week for winter seedlings and 2-3 times a week for summer seedlings works well.

The trays that your seedlings are sitting in serve many purposes. One, it keeps them all together and organized, easy to move about or rotate as needed. Two, it catches any excess water runoff. Three, it can actually serve as a WAY to water. You can mist from the top to keep top soil moist before the seedlings have emerged, but after they do pop, we have taken to watering from below. If you prefer to water from the top, that is okay too, just be gentle not to pour in a huge slug of water that will disrupt or topple the seedling and its roots.

“Watering from below”

… is exactly what it sounds like! This is our preferred method to water seedlings, and is accomplished by pouring water into the tray the seedling containers are sitting in. The soil will draw water from the bottom, as much as they need until the soil is evenly saturated, and then stop.

To water from below, pour in enough water to evenly fill the bottom of the tray (with the containers still sitting inside of it) to about quarter of an inch deep, up to almost an inch, adjusting depending on the current condition of the soil. Meaning, if the soil is still semi-damp, scale down. If the soil has accidentally gotten quite dry, scale up. Larger containers will also soak up more water, so take that into account. It is important that your trays are sitting pretty dang level to ensure all the seedlings are getting a similar amount of water.

After adding some water to the tray, erring on the conservative side first, wait about half an hour to see what happens. Did they already suck it all up, but seem a bit dry still? You may need to add a little bit more. Or, are they totally saturated and there is still a lot of water leftover? Allowing seedlings to sit in soggy conditions is not ideal, so we remove any leftover liquid that wasn’t absorbed by the soil within a few hours. You can do this by either very gently tipping it out (if possible), or using a turkey baster or a large garden syringe thing to suck it out. Yes… it is easiest if you don’t have a bunch of excess, so that is why I suggest to go lighter at first.

Benefits of Watering from Below

• This practice helps reduce top mold on the soil, and prevent “damping off” – a condition when seedlings get really thin right at the soil line, go limp, and suddenly die. This is caused from a combination of either overly wet conditions, fungus, mold, or other infections.
• Watering from below also encourages seedling roots to travel downward to the moist soil, developing faster and deeper. Thus the plant also develops faster and taller.
• Finally, the seedling containers will draw up moisture evenly. It is really easy to under-water some containers but over-water others if you are dumping in water from above.

Light

Seeds do not need light until they become seedlings – after sprouting. At that time, if you are providing artificial light through grow lights, it is important to keep the light only a few inches above the plants, raising the lights little-by-little as the plants grow. If the light is insufficient – either too weak or too far away – your seedlings will let you know by stretching tall and becoming leggy in search of better light. This is not optimal.

A note if you are using LED lights: research the recommendations on height and spacing between plants for your particular light. I have read that using LED can be a bit different, and some can actually burn plants if they’re placed too close.

Veggie seedlings need at least 12 hours of light per day. 16 to 18 hours is even better. We love using this timer connected to our grow lights, so we don’t have to worry about remembering to turn the lights on an off.

If your trays extend out quite a bit from under the main canopy of the light, leaving some sad stragglers on the outskirts, you’ll want to rotate your trays and containers every few days to make sure everyone gets their chance in the spotlight. This will also help strengthen them by causing them to bend and move in various directions, more similar to what a natural sun pattern would cause.

Air Flow & Movement

Seedlings started inside do not get the benefit of “the elements” to help challenge them and make them strong. Seedlings grown outdoors are strengthened through exposure to wind, rain, or other conditions in the garden. Coddled, immobile seedlings make for weak seedlings.

An oscillating fan can be introduced to create “wind” and force them to increase in strength once the seedlings are a few weeks old. A gentle, occasional wind is good. Avoid anything too strong and turbulent for them, especially at first. Good air flow also helps reduce the risk of disease, pests, fungus, and damping off.

In lieu or addition to a fan (here is where the tickling I mentioned earlier comes in…) many gardeners like to gently run their hands over the tops of the seedlings, giving them a little tickle. This too induces the same strengthening response as wind. If you do this, be conscious to wash your hands if you are coming in from the garden outside, to prevent introducing disease to the seedlings.  

Fertilizing Seedlings

Baby seedlings do not need or like fertilizer in the first couple weeks of life, but once their first set or two of “true leaves” appear, you can start to feed them very lightly. What are true leaves? When a seed germinates, the first set of two leaves that emerge (often heart-shaped, and often looking exactly alike between dozens of varieties of veggies) are not the true leaves. These are the cotyledon leaves – their embryonic leaves. The two leaves that come after the cotyledon are their “true” leaves. Those leaves will more closely resemble what the mature leaves of the plant will look like.

Once the first set or two of true leaves appear on our seedlings, we like to occasionally feed our seedlings with dilute seaweed extract. To read more about why, how, and when to feed your seedlings with seaweed extract, see this post. Another way to “feed” your seedlings is to eventually up-pot them into a richer potting soil and compost. This will be discussed shortly.

Thinning your seedlings

As painful as it may be, it is best to thin your seedlings down to the one healthiest sprout per cell space or container after the first couple sets of true leaves appear, as the seedlings are starting to show you who is looking the best to keep around. Proper thinning allows the chosen strongest soldier to march on in a mighty fashion!

Seedlings that are not thinned compete with one other for food, water, air, and root space. They will take off after a good thinning, and will quickly become four times the size of the ones that were left un-thinned, in just a matter of weeks! Watch the video below to see what I mean.

To read more details about thinning, the pros and cons of various methods, and to see a demonstration video, click here! Don’t worry, the thinned seedlings don’t go to waste. Most of them can be eaten as microgreens!

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Potting up

A few weeks after germination, start keeping an eye on the bottom of your seedling containers. Are roots starting to poke through the bottom drainage holes? How big is the plant looking? Does it still look happy? Has it still been growing steadily, or has it slowed?

You may need to “pot-up” your seedlings once, maybe even twice, before planting them in the garden outside. This will depend on the size of the seedling containers you started with, and how long it is from the time they sprouted until it is time to plant them out.

When the roots start to poke through the bottom of the container a lot, or if you started them in super tiny containers but they won’t be planted out for another month, it may be time to pot up into a larger size. Seedlings in too-small a container can get root bound. This is when root growth is restricted and may even start to spiral around itself. We want to aviod this. Also, if you started in straight seedling mix, the plants will enjoy a slighter richer soil now.

See this article for tips and best practices for potting up seedlings.

Heat

Heat is most essential during the germination stage, but can be slowly dialed back thereafter. However, the seedlings will likely grow best if they’re supplied at least some warmth for the first few weeks to a month after sprouting, whether from your naturally warm house or continued use of a heat mat.

Usually, we’ll heat them on 75°F for the first few weeks after sprouting, then slowly start turning it down by 5 degrees a week. Eventually we switch to using it only overnight, continuing to slowly wean them off in the weeks prior to transplanting into the garden.  This is part of their “hardening off” process.

Hardening off

You’ve tended to your growing seedling babies for weeks or even months. You’ve checked your planting calendar, and your last frost date is approaching or just past. Are you feeling ready to get those seedlings planted out soon? Hold tight!

Do not take your seedlings from an indoor or otherwise protected environment and plop them down outside without a proper adjustment period to prepare them. If you take them from one extreme to the other, there is a good chance they’ll go through transplant shock. This can either stunt their development or even kill them. We clearly do not want that!

“Hardening off” is the process of slowly introducing seedling started indoors to the great outdoors, little by little, over the period of a week or longer. To learn how to harden off seedlings, click here!

After you finish the hardening off process,
you’re ready to plant!

Congratulations, new plant parent! It is time to enjoy healthy homegrown food for months to come!

After planting out seedlings into the garden, keep an eye on the weather forecast, especially if your area is known to be a bit unpredictable! Even if an unexpected frost warning rolls in after they’re out, there are still ways to protect them! Placing things like frost blankets, sheets of plastic, or even plastic bottles and halved milk jugs over young seedlings can create mini greenhouses and prevent frost damage.

Also keep an eye out for critters like squirrels or birds that tend to go after seedlings. Use fencing or netting to protect them as needed. There is nothing more frustrating than raising seedlings and then losing them all! I talk a bit more about pest protection in the “How to Start a Garden 101” post.

I hope you found this information helpful! If so, please feel free to share it!

Video

For you visual learners, here is a video that goes through just about everything we discussed here today: