Growing Seedlings Indoors + Care Instructions

If you haven’t already, check out my article on how to start seeds indoors. This article will pick up from there.

Now that your seeds are planted, you are going to have to learn how to take care of them. The basic requirements for proper seedling growth indoors are water, light, and fertilizer.

Water

Of course seeds need water. Water is what causes your seedling to emerge from the seed. The seed contains all of the essential nutrients the seedling will need to get going in it's little life, and water is what causes it to start growing. Water your seedlings when the soil seems dryer than your towel would after you’ve used it after a shower. The soil should always stay a little damp, but not soggy. Once your seedlings are about an inch tall, start bottom watering.

Bottom watering refers to watering your seedlings from the bottom. Water is placed in the bottom tray and is wicked up by the soil in your seed cell tray via the holes in the bottom of said tray. Watering this way encourages root formation. When soil becomes dry, roots grow downward in an effort to find water, thus allowing them to get longer. Watering from above discourages this, and although your seedlings will still grow roots, they will not be as good as they could be if you bottom watered. Only bottom water when your soil is slightly drier than damp; you will probably have to water two to three times a week.

To bottom water, fill the bottom tray with 1-2 cm of water. Wait 15 min, then check on them. If the top layer of soil in your seed tray is still a bit dry, add more water to the bottom tray and wait another 15 minutes. I suggest starting with less water, as emptying water from the bottom tray is a pain in the ass to do if you happen to add too much water. You know the seedlings have enough water when the soil on top is wet. If your soil is properly saturated and there is still water in your bottom tray, soak up the excess with paper towels. You do not want your seedings to sit in wet soil, as this can contribute to root rot.

Dry soil appears light in color, however sometimes soil is dry without it visually appearing dry. The darker areas of the soil on this plant were quite spongy and crumbly to the touch. The visible and physical cues indicate the plant needs to be watered.

Light

Having sufficient light is very important for growth. Plants convert light photons into energy which is necessary for growth. If they do not get the proper light, they will grow weak, leggy, and die. In my previous post on starting seedlings indoors I went over light requirements. I suggest ordering these lights from Toggled, as they have the proper requirements (6500k and 8000 lumens per fixture) for growth. You can also buy grow lights specifically made for growing seedlings indoors, but they are often quite expensive versions of what these shop lights are doing. If you would like a detailed post where I go into the science behind light requirements for plants, let me know!

How many grow lights do I need? As mentioned above. Your lights need to be 6500k and between 3000-5000 lumens. The Kelvin measurement is how the light looks to the human eye, which we want it to be measuring Daylight, and lumens measure how much light the bulb gives off. In short, you need between 3000-5000 lumens per square foot of growing space.

For example, say you have the Toggled lights I recommended, which give off 8000 lumens per fixture, and say your seed trays take up 4 square feet. Knowing that you need 3000-5000 lumens per quart foot of growing space, you would multiply 4 x 3000 to get your minimum light requirement, and 4 x 5000 to get your best light requirement. Your lumen range requirements for 4 square feet of growing space would be between 12,000 and 20,000 for 4 square feet of growing space. It is better to have more lumens than not enough.

One of the Toggled light fixtures has 8000 lumens, so in order to get within range of the required amount of light, you will need two light fixtures. Two light fixtures would give you 16,000 lumens, which is right in range of the numbers we found above.

Always hang your lights 1.5-2 inches above your seedlings to prevent leggy-ness and to provide the most directed light to your seedlings. If you notice your seedlings are browning at the tips of the leaves, just raise your lights a little bit. Keep in mind that as your seedlings grow, you will need to keep raising the lights to keep them within that 1.5-2 inch range. As they grow bigger, you can increase the light height from the plant to 3 inches.

Plants should have 12 hours of light, and 12 hours of rest each day.

Fertilizer

As I mentioned above, seeds contain all of the nutrients the seedling will need to get started in life, but as those nutrients are used up, they will need to be replaced. Seed starting mix has very little (if any) nutrients, so you cannot depend on it to give your seedlings the nutrients they need. This is where fertilizer comes in.

Fertilizer is composed of three parts: Nitrogen (N), Phosphorus (P), and Potassium (K), or NPK, and is listed on the bottle of every fertilizer as a ratio, known as the fertilizer grade. It tells you the percentage of each element within the fertilizer. Generally speaking, you are going to want a fertilizer that has a 1-2-1 ratio of NPK.

Nitrogen helps the seedling grow its leafy green leaves. Phosphorus promotes root development (which is what we are most interested in when growing seedlings indoors). Potassium helps with flowering, i.e., the tomatoes you see on the tomato plant.

While all of these elements are important for a strong, healthy plant, when growing seeds indoors, we are most interested in the first two elements, as we will be transplanting the seedlings outdoors before any fruit develops.

Seedlings should be fertilized once per week, starting when your seedlings develop their first true leaves. You will fertilize up until you transplant them into the garden.

I recommend using an organic, liquid fertilizer on seedlings, since they are so delicate. This is the one I use.

Once you have your fertilizer, you are going to want to dilute it to half strength until your seedlings are well established, meaning they are at least 3.5-4 inches tall and have several true leaves. Once the true leaves have some growth on them, you can gradually increase to full strength. Do all seeds grow the same?

For the most part, yes, however here are some differences to look out for:

Tomato seedlings can get long stemmed, and you’ll notice their stems look hairy. Those hairy bits are actually places where roots can grow from if you were to plant them deeper in the soil. When your tomato seedlings have about four leaves, you can, and should, up pot them from the tiny 1”x1” seed starting pods to a larger 2”x2” container, and when you do, you can plant them deep enough to where only their first set of leaves are showing. This helps make a strong tomato plant. Do not do this with other plants. Other plants do not want to be planted deeper than how they naturally grow.

The flat leaves in this pictue are the seed leaves. They are not true leaves. True leaves on tomato plants appear curly on the ridges. When you up pot tomato seedlings, you can plant them as deep as the seed leaves. This only applies to tomato plants!

Onion seedlings will grow fast and tall. You may notice that they have kinks in the stems as well as some browning at the tip. You can cut the brown tips off and right below the kinks so that they stand up straight. When stems reach more than 5 inches tall, cut them down to about 3.5-5 inches tall. This will encourage growth.

Hardening off

One to two weeks before it is time to transplant your seedlings outdoors, you should start hardening them off. This means gradually exposing your seedlings to outside weather to avoid transplant shock or plant death when exposed to real life weather conditions. Up until this point your seedlings have been in a very controlled environment and have never had a chance to adapt to real life weather. Hardening off is very important for seedling success.

How to harden off seedlings:

On day one, put your seedlings outside for an hour.

On day two, put your seedlings outside for two hours.

Increase outside exposure by one hour each day, until you have reached 8-12 hours of outside exposure. At this point, your seedlings will be ready to transplant. You will also notice that they have gotten much stronger during this time!

Other things to consider

If you have used a heat mat or humidity dome to help with germination and soil moisture, you can remove them once your seedlings emerge.

Having a fan blow air on your seedlings is crucial if you wish to have strong plants. By having air blowing on your seedlings, you are mimicking wind and strengthening the stems of your seedlings. You can also run your hand gently over the tips of your seedlings several times a day to strengthen them even more. The air blowing on your seedlings should be as gentle as it would be if you were to blow on the leaves. They should wiggle but not bend drastically.

If you have any questions please let me know in the comments below! Growing seeds indoors should not be complicated, and although it seems like a lot of information to take in, it is not difficult to put into action.

Your growing station does not need to be perfect in order to work. My lights are strung up with zip ties! If it works, it works, and I assure you your plants do not care what your grow space looks like. The most important thing to remember is to have fun! Growing seedlings is so much fun and should not be overwhelming. I would be happy to help you answer any questions you may have about seed starting or gardening in general!

See you next Monday, and happy growing!

Mary