Do I Need A Grow Light To Start Seeds Indoors?

Do I Need A Grow Light To Start Seeds Indoors

Starting seeds indoors is a great way to jumpstart your garden when the spring time rolls around. Not to mention it’s nice having some green nature growing inside the home.

But this made me wonder, do we need grow lights in order to start seeds indoors?

Even in my step-by-step guide on how to start seeds in just water, I used a grow light to grow my seedlings. However, I ran a new seed germination test without a grow light and found out the following.

A grow light is NOT needed in order to start seeds indoors. In fact, seeds can germinate and sprout in complete darkness. Seeds only need water and a growing medium such as soil, rockwool, or hydroton in order to germinate. However, seedlings do need sunlight or a grow light in order to continue growing after sprouting.

Now this might sound crazy to hear that seeds can grow indoors without a fancy grow light so I’ve outlined my experiment below to show you how I successfully started my seeds indoors.

How To Start Seeds Indoors WITHOUT A Grow Light

I wanted to see if I could start my seeds indoors during the cold winter months in Portland without a grow light.

After all, if seeds can germinate and grow indoors during the coldest months that have the least amount of daylight then seeds can be started at anytime of the year indoors. So I chose a east facing windowsill out of convenience and started by experiment.

Starting Seeds Indoors On A Windowsill

I placed cilantro seeds directly into a net cup full of hydroton. I then placed this net cup into a mason jar and filled it with regular tap water.

After a few days I saw the cilantro seedlings start to sprout and after 2 weeks the cilantro seedlings are leggy but growing as seen in the photo below.

Starting Cilantro Seeds In Windowsill

So this experiment proved that seeds can be started indoors without a grow light and that plants will grow when their basic needs are provided.

However, this experiment isn’t all rainbows and sunshine since the cilantro is noticeably very leggy.

Starting Seeds Indoors With Growlights

In fact, compared to the cilantro growing inside my garage greenhouse, the windowsill cilantro is leggy and hasn’t even started growing its cilantro shaped leaves yet.

Below is the cilantro planted 2 weeks earlier underneath growlights in my greenhouse.

You can see that this cilantro is far less leggy as there are plenty of cilantro shaped leaves near the base of the plant. The grow lights are providing sufficient light for the plant to grow its leaves.

Why Seedlings Get Leggy or Tall

The reason why the cilantro seedlings growing in the windowsill are leggy versus the more compact cilantro growing underneath the growlight comes down to light intensity or photosynthetic photon flux density (PPFD).

Whenever you look at purchasing grow lights, they all advertise their PPFD map or how intense the light is. Here’s a PPFD map from Spider Farmer that shows the light intensity at different areas underneath the light at different light heights.

The main takeaway is that plants that are given strong enough light, so near the center of the map, will not grow leggy or tall. Plants that are given enough light will start to spend energy on sprouting its mature leaves and getting bushier.

Plants that are not given enough light will get leggy or tall since they’re trying to stretch closer to the light.

So the cilantro seedlings in my windowsill are not receiving enough light which is why they are tall and leggy as they stretch closer to the window for more sunlight.

Verdict – Seeds Can Be Started Indoors Without Growlights

This test has proven that seeds can be started indoors without grow lights. In fact, these cilantro seedlings were started at the worst possible time in Portland (i.e. cold winter months with less sunlight than spring or summer).

Although my cilantro plants are getting leggy, you could transplant these seedlings well before they get leggy. In fact, if it was spring these cilantro seedlings would have already been planted outside.

These seedlings can also be placed underneath a grow light if you wanted to accelerate their growth and keep them from getting too leggy.

However, I’ll keep growing this cilantro in the windowsill to see if I can not only start seeds indoors without a grow light but completely grow a cilantro plant from seed to harvest in a windowsill.


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