What is Bright Indirect Light? The Terrarium Gold Standard

If you keep more than a few varieties of houseplants, you’ll know the familiar challenge of swapping and changing plant locations to keep them happy. Just when you think you’ve got everything perfect, the season changes and the game resets. Nightmare.

Thankfully, terrarium plants – in all their wonderful diversity – actually tend to share similar lighting requirements.

Seeing as most terrarium plants are tropical in nature, they’re used to growing under dense tree canopies where only sparse, filtered light can find them. Making bright, indirect light the gold standard for terrariums.

It’s strong enough to supply plenty of energy for growth, but not so strong that it can damage the plants.

Sounds straightforward right? Except bright, indirect light is a bit of an enigma. It’s difficult to define, identity and measure.

So, in this article we’re going to break it down simply, so you know exactly how to identify the best light for your terrariums and where you should be putting them in your home.

Indirect light is essentially sunlight that has passed through, or bounced off, another surface before reaching your plants. In reality, the majority of natural light in your home is indirect – after all, the Sun can only be on one side at a time. 

Much like a stray ball bouncing off (or going through) a window, the process of hitting the surface weakens the intensity of the light rays to varying degrees. Depending on the strength of the initial strike and the properties of the window.

Indirect light examples include; ambient room lighting, filtered light through blinds or curtains, and reflected light from walls and surfaces.

Direct Sunlight vs Indirect Sunlight

How to identify indirect light? Well, it’s a bit like identifying a neutral smell. It’s easier to look out for what it’s not. So, I tend to look for the telltale signs of direct sunlight, then if none are present, assume it’s indirect. 

Can you: 

• See defined rays of light coming through your window?
• See any harsh shadows from objects in front of the window?
• Feel the warmth of the sun on your skin?

If the answer is no to all of the above, it’s probably indirect light.

How Does Terrarium Glass Affect Sunlight?

There seems to be a common belief that glass terrariums magnify sunlight and can scorch your plants, but has anyone actually found this to be the case?

It sounds plausible in theory, but everything I can find suggests that glass predominantly reduces light intensity. 

I imagine that if your terrarium were shaped in such a way that were to focus light then it might be possible. Just like a traditional magnifying glass, curved glass terrariums may focus sunlight in certain areas that can be a scorch risk to your plants.

Can’t say I’ve tested it though, I’m not sure I have the heart to try. If anyone with a physics background wants to crunch the numbers for me I’d appreciate it!

Plus, that’s not to say overexposure to sunlight won’t be detrimental, terrariums are still greenhouses after all and will definitely trap heat. Direct light poses various risks to terrariums, but burning them through light magnification seems unlikely.

It’s a gross oversimplification, that’s what it is.

“Bright” is so incredibly vague, but for the purposes of terrariums we’re usually talking about indirect light at a high intensity. Strong direct light that’s had the edge taken off, so to speak.

The intensity of indirect light will vary based on many different factors. 

• Distance from the plant to the window.
• Any objects between the plant and the light source. 
• Any obscuring fabrics like curtains or screens.
• Cloud cover.
• Time of day.
• Direction of the window.
• Time of year.

As humans, we’re not very good at judging the strength of light. 

That’s probably why we use terms like “partial shade”, “low light”, ““medium indirect light”, “moderate sunlight” and more. I’m sure there’s a reasoning to them all, but we’ll all interpret them differently. 

Which makes the most straightforward way to determine “bright” light – scientifically. 

Scientifically

Get yourself a light meter (like this one from Amazon) and you can start to interpret light in definitive numbers rather than fuzzy language.

75-150 foot-candles (800-1600 lux) is considered the range for indirect light by Greenery Unlimited, so “bright” being more towards the top of that range.

There are also a variety of apps that claim to be able to measure lighting, but I’ve yet to find one that I trust (if you know of one, hit me up!)

Environmentally

Perhaps the one thing we can rely on is for the Sun to rise in the East and set in the West. So, this at least gives a starting point for estimating the light intensity from different directions at different times.

Assuming you’re in the northern hemisphere, in terms of bright, indirect light – here’s what you need to know.

• North-facing windows are fantastic for terrariums. They almost never get direct sunlight, and provide the most consistent indirect light throughout the day.
• East-facing windows are likely to be the next best thing. Your plants will experience some direct sunlight throughout the early morning hours, but those rays are typically much softer and less of a risk for your plants.
• West-facing windows can sometimes work for terrariums. Your plants will likely experience some strong direct sunlight for a few hours in the afternoon. Only the more light tolerant tropical plants will thrive in this environment.
• South-facing windows usually aren’t ideal for terrariums, as the Sun occupies this area of sky for the longest amount of time in the day, and your plants will be in full sunlight through the hottest part. 

These guidelines are subject to change throughout the year as the Sun occupies higher and lower positions in the sky, but they’re a good starting point.

Right, so now we know how to identify a plant’s ideal light, but how do we keep it consistent?

After all, the only consistent thing about sunlight is the fact that it’s consistently changing. 

What’s the perfect spot at 11am may not be so perfect at 5pm. After all, the Sun never stays still, relative to the day or the season. So, in reality there’s no perfect natural solution.

Which is why most plant peoples’ interpretation of indirect light is more pragmatic than perfect. In the real world, it’s more about how we can ensure the plant gets as much of its ideal sunlight as possible, whilst minimising the risk of non-ideal sunlight (i.e. direct or too little).

Here’s an example. A terrarium with all tropical, shade-loving plants is placed on an East-facing windowsill. As the Sun rises in the East, it gets probably a bit too much sunlight for a few hours, but then gets consistent indirect light for the rest of the day. So it gets mostly what it needs, but has to bear with less than perfect conditions for a little while too.

That’s why it’s so important to gauge a plant’s (or collection of plants) tolerance for light. 

Most plants have some wiggle room, and can adapt to different lighting conditions. We’ve just have to find the limits…

Getting indirect light for your terrariums usually isn’t a challenge, but bright indirect light can be.

Thankfully, there are some strategies we can employ to increase the intensity of our indirect light.

1. Adding mirrors into the home can be a big help in helping light from windows penetrate further into the home. If your windows don’t capture much sunlight, you can add mirrors directly to the inside of the window. You can also prop up mirrors behind your terrarium to reflect any incoming light back again, to really make the most of it.
2. Painting your walls white can have a small positive effect in increasing ambient light by increasing the amount of light reflected off the walls. Plants always look great against white anyway…

There’s a lot to consider when it comes to identifying the ideal lighting conditions for terrarium plants. Honestly, writing this article has been a real challenge when the final answer to every question comes down to “it depends”.

I’ve tried to give as much actionable information as possible, but there’s no ideal solution here. Even using an accurate light meter isn’t perfect when conditions are constantly changing.

So, in the absence of a complete solution, here’s what the process boils down to.

1. Place terrariums outside of direct sunlight, but as close to the light source as possible, e.g. a few feet away from a bright window or behind a thin curtain. That way you retain the maximum amount of light energy that’s safe for your plants. 
2. In the real world, in order to get sufficient bright, indirect light, most terrariums are going to experience some direct sunlight through the day. Most tropical plants can handle small amounts of direct sun exposure, so use the Terrarium Plant Index and your own observations to determine the tolerance of your plants.
3. Be prepared to move your terrarium throughout the year. Part of being a plant parent is looking after our plants’ evolving needs. Generally, terrariums need very little maintenance, but this is one area we need to pay attention to.