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How to Grow Seedlings under Indoor Grow Lights: Step by Step Guide

Many farmers choose to grow seedlings under indoor grow lights. Doing this saves them from spending money on the purchasing, heating, and maintenance of a greenhouse. It also allows them to grow out-of-season crops which they can later on transplant into the outdoor garden.

Keep in mind that every species has its own specific needs for growing indoors. Seed depth, light and water exposure, and type of growing medium will all vary based on the plant you wish to grow. But the good news is that the general process for germinating seeds and growing seedlings is the same.

In this article, we’re going to show you how to grow seedlings under indoor grow lights. To keep things simple, we’ve divided the step-by-step guide into 3 sections. The 1st section deals with selecting seeds, and the next two deal with the plantation of seeds and moving them outdoors when the time comes.

Section 1 – How to plan for growing seedlings indoors

seedlings growing indoors

Here’s what you need to do to start growing seedlings indoors:

Step 1 – Buy Seeds

sliced tomatoes on a cutting board

Seeds are available from innumerable sources, ranging from mail order catalogs to garden centers and your local building supply store. Their prices can vary too. Keep in mind that the newest hybrids will cost more, as do certified organic seed, as well as seeds of unusual or rare plants.

Here are a few tips to help you buy seeds:

  1. Only buy seed that you’ll use in 2 to 3 years. That’s because, the fresher the seed, the greater the chances of it remaining viable over time. As time passes, fewer and fewer packet-bound seeds will germinate.
  1. Store leftover seed packets in air-tight containers. Throw in a silica gel packet to keep the humidity manageable in the container. A paper towel will also do a fine job of absorbing moisture.

Step 2 – Choose Location

location pin icon in front of plants

Make sure the place you’re selecting for growing seeds is shielded from excess heat, cold drafts, pets, and heavy foot traffic. It must be big enough to accommodate upcoming seed sowings. It should also be big enough to allow seedlings to reach their full growth potential.

Here are a few more tips to choose a location for growing seeds indoors:

  1. Avoid the windowsill. The majority of seeds need warm soil to produce strong roots and to germinate. Cool air coming in from the windowsill might decrease their soil temperature, potentially leading to seedling disease.
  1. Choose basement, if possible. Basements don’t get excessively cold at night, neither do they become extremely hot during the day. They also attract less foot traffic than other areas of the home.

Step 3 – Choose Container

hands holding seeds

Don’t start multiple seeds in the same container. We recommend that you stick to the formula of ‘one seed per container’. Sowing many seeds in the same container will allow the roots to grow into each other, exposing them to injury during transplanting.

Keep these tips in consideration while choosing a container for your seedlings:

  1. Make sure the container has a drainage system. Most seeds need to retain water to stay healthy – otherwise they might wither and die. But that doesn’t mean you should saturate their roots with water, potentially exposing them to rot.
  1. Make sure the container has perforation/holes. Roots also need plenty of oxygen to survive and thrive. Ensure that the container has holes, but they mustn’t be big enough to expose your seed’s root to the elements.
  1. Make sure the container is on the bigger side. It won’t only be your seeds’ buds that will branch out – their roots will expand in all directions too. A narrow container might force the plant to become rootbound.

Step 4 – Choose Soilless Seed Starting Mixtures

hand holding a clump soil

Soilless seed-starting mixes, usually containing peat and vermiculite and lacking any true soil, are your best bet for starting seeds. They are lightweight, sterile and lack weed seeds. Also, their porosity and texture complements the needs of tiny seedlings and germinating seeds.

After buying the soilless seed starting mixtures, place the containers into a solid tray, add potting mix into them, and fill them with plenty of water. Add water and potting mix again, until you can see that the cells or containers are almost full.

Step 5 – Plant Seeds

a pair of hands holding seed and soil

Check out the instructions written on the seed packet or catalog, as each seed comes with its own planting requirements. For instance, tomato, eggplant and pepper are normally started in early spring. Broccoli and cabbage, meanwhile, may be planted in June or July.

Here are the best months for planting common seeds:

  1. Early February: Onions, leaks, geraniums, pansies
  2. Early March: Broccoli, cabbage, Brussels sprouts
  3. Early April: Tomatoes, cornflower, morning glory

Section 2 – How to plant and grow seedlings indoors

Here’s what you need to do after you have planted seedlings indoors:

Step 1 – Sow Seeds

pair of hands holding a heart-shaped piece of dirt with a sapling

Follow package directions and sow one seed per container. Make sure you’re sowing the seed four times as deep as its width. If you’re still unsure about planting depth, a rule of thumb is to sow four seeds (on top of each other) in the same container.

Follow these tips when sowing seeds:

  1. Mark each container with a permanent marker. Especially if you’re going to plant multiple types of seeds simultaneously. Marking the container will make it easy for you to identify which container contains which type of seeds.
  1. Cover seeds that need light with a thin layer of vermiculite. A thin layer of vermiculite won’t stand in the way of light trying to permeate it to get to the seed’s root. Yet it will also serve as a barrier to moisture trying to escape.
  1. Cover seeds that need darkness with several layers of newspaper. Alternatively, you can place them in dark plastic bags. Keep them this way until the seeds start sprouting.

Step 2 – Hang Grow Lights

plants growing indoors under purple lights

Given that most seeds will need light for more than 12 hours per day, you should only rely on the best grow lights, even if you’re growing seeds in a greenhouse. You might want to rely on LED grow light bulbs as they are inexpensive, rarely get hot, and provide emit light in the natural spectrum.

Aside from that, LED grow lights stay cool to the touch and cost minimal electricity. A shelf life of an LED bulb is equally impressive – you can count on it to keep going for more than 20,000 hours, on average. This will keep your running lighting costs manageable.

Once you’ve selected grow lights, use these tips while hanging them:

  1. Hang grow lights from chains. This will allow you to easily raise the lights as your plants grow.
  1. Ensure the light is less than 4 inches apart from the seedlings’ tops. Lack of adequate light is the number one cause of skinny, elongated stems.
  1. Don’t leave the grow lights on 24 hours a day. Almost all plants need period of darkness to rest and to develop properly. You can also use a timer to turn the lights on and off automatically.


Step 3 – Provide Bottom Heat

drawing of heat being provided

There are multiple benefits of providing bottom heat to seedlings. First, research has shown that seeds whose potting mix is warm produce healthier roots and germinate sooner. Bottom heat can also kill pathogens at the potting mix’s surface, preventing damping off.

Also, since temperature in the indoor containers’ potting mix is usually lower than indoor air temperature, seedlings need a heat source to keep warm. All of these reasons are why we suggest you buy electric heating mats that are specific for seed starting.

Step 4 – Provide Water and Fertilizer

plants being sprayed with water

Start with watering seedlings. You need to ensure that the potting mix stays moist during the seeds’ germinating stage. We suggest using a spray bottle, as it will let you gently water the surface while not washing the potting mix off the container. Drain excess water to ensure roots stay healthy.

As for fertilizer, keep in mind that seedlings get all the energy they need for germination from the nutrients in the seed. That means you don’t need to add fertilizer until the seed sprouts into various sets of true leaves. There’s, however, a catch.

While the above advice stands for seeds sowed in true soil, those grown in a soil-less mix could do with a water-soluble fertilizer. The composition of the fertilizer-water mixture should be 1 part fertilizer and 3 parts water. Fertilize only once per week.

Step 5 – Transplant Seeds

seeds in containers about to be transported

Once you see that the seedlings have outgrown their cell packs, it’s time to transplant them into larger containers. We suggest that you pick larger containers with holes in their bottom – the perforation would allow excess water to go out and oxygen to come in.

While transplanting, don’t make the mistake of holding the seedling by its stem. Doing so might harm its growing tip or you may end up crushing the seed. What you need to do is to lift them by the rootball, with the help of a plant tag or spoon.

Section 3 – How to move seedlings outdoors

Follow these steps when moving seedlings outdoors:

Step 1 – Accustom Plants to Outdoor Environment

Seedlings stored indoors haven’t faced full sun, fluctuating temperatures or wind. If you fail to gradually accustom them to the outside environment, their leaves might end up getting scorched by sun or the wind. They might even wilt before dying.

Here’s how you can ‘harden off’ seedlings:

  1. At least two weeks before you permanently move them outside, start putting the plants outside for up to 3 hours in the shade. Do this in the afternoon and make sure the plants are protected from the wind. Bring them back at night.
  1. With each passing day, increase the time you’re leaving the plants out, and gradually increase their exposure to sunlight too.
  1. After two weeks have passed, and unless the weather is freezing out there, move the seedlings permanently in a sunny area. Keep them there until you’ve the time to transplant your seedlings into the garden.

Step 2 – Transplant them into the garden

plants being transplanted

Once you’re sure the leaves have been hardened off, it’s time to transplant them into the garden. Make sure you transplant on a late afternoon or a cloudy day. Don’t set them out in the garden when the sun is at its peak.

Don’t worry if the plants wilt upon their first exposure to the sun. They will generally recover their previous swagger within a day or two. Still, if you want to take no chances, use row covers to protect your plants from temperature fluctuations and wind.

Lastly, if you’ve grown seedlings in peat pots, cow-dung pots, newspaper pots or any other pots made of organic matter, then you have to trim the pots’ collars down to soil level. This will prevent the containers from wicking the water away from the root’s water zone.


Frequently Asked Questions

How far should grow lights be from plants?

Grow lights should not be more than 4 inches away from the top of the seedlings, ideally the distance should be 2 inches. If you place the grow lights farther, your plants might end up developing skinny, elongated stems.

What is the best grow light for indoor plants?

HausBright LED Grow Light Bulb is the best for indoor plants. It emits full spectrum light, has a safety design to prevent temperature fluctuations in check and is extremely easy to install. Also, this grow light for indoor plants comes with a 3-year warranty.

What kind of light for growing weed?

LED grow lights are the best for growing weed. Apart from offering a full spectrum light, they transmit minimal heat to the weed, minimizing the risk of leaves getting scorched or burned. You can leave LED lights on for 16 to 20 hours each day and they still won’t get hot to the touch.