Plant on the window sill

Have you noticed most plants or seed packages give some vague description of ‘place in full sun’ or ‘place in partial shade’, I keep wondering to myself ‘How partial?’ Like when I cook my favorite meal and the recipe says cook until brown, I keep wondering ‘How brown?!’

In the following article, I’ll tell you all about the light requirements for plants and why its one of the most important considerations when buying and placing a plant.

Plants need light to photosynthesize, certain plants only start flowering with the right amount of light, and others burn if they are in direct sunlight. In general, if your plants don’t get adequate lighting they shrivel and die. Let’s get scientific here, first let’s start off with a description of how light is measured.

There are 2 things to consider:

  1. Light intensity (how strong the light is)
  2. Light amount (how many hours)

Light intensity is measured in units called Lux. Lux is equal to one lumen per square meter. Of course, lumens is a measure of visible light as perceived by the human eye (Plants also use far red and far violet spectrums). However, lux is sufficient for measuring sunlight and is generally regarded as the standard way to measure light.

I bought a cheap lux meter and walked around the house to check how much Lux I was getting (see my results lower down).

Wikipedia describes the following values for various light scenarios:

Illuminance (lux)
Surfaces illuminated by
0.3
Full moon on a clear night
50
Family living room lights (Australia, 1998) – This is pretty dark in my opinion
80
Office building hallway lighting
320–500
Office lighting
400
Sunrise or Sunset on a clear day
1000
Overcast day (Same light they use in a studio)
10,000–25,000
Full daylight (but not direct sun)
32,000–100,000
Direct Sunlight

Getting the right light for your specific plant could mean the difference between life and death (death of the plant! – not your own death…unless your plant is a gift from a partner)

Now we start to get some interesting measurements, especially for Plants. Your living room plants are received 50 lux while a plant outside is receiving 100 000 lux, that’s quite a difference – It like sun tanning at the salon vs Bondi beach without sun cream.

Plant receiving sunlight

Here is a rough breakdown of the lighting needs of each type of houseplant (ideally for 10+ hours a day):

Low light plants
Medium light plants
Bright light plants
Very bright light plants
Lux 500-2500
Lux 2500-10000
Lux 10000-20000
Lux 20000-50000

Most plants will survive at 10x lower values than specified but they won’t thrive (hence why low light plants can grow in the 50 lux living room).

So let’s break it down a bit further because it depends on some more factors, namely what season of the years as to how high in the sky the sun is and how much sunlight your plant will get.

Ok, I said I was going to get a bit more scientific, so I bought a lux meter online and walked around the house and the nursery, and my friend’s house, and some random houses and even an office.

I always thought my plants were dying because of poor watering, turns out half of them aren’t getting enough light. You would be surprised how quickly light drops off from your balcony to your living room. You can get a cheap Lux meter on Amazon (+-$30) and measure the light. Or use the light meter in the PlantMaid

Now that i’ve told you all about lux I should probably mention a quick disclaimer, Lux is the measure of the brightness of light visible to the human eye. Plants actually need more red and blue light for photosynthesis, so if you really want to get accurate with your light readings for plants you should use a PAR(Photosynthetically Active Radiation) meter. PAR meters are a bit more expensive, but they accurately measure light in the 400-700 nanometer range and are designed to measure plant light.

Using a LUX meter for natural sunlight gives a good indication of light for your plants, but don’t use it to measure fluorescent or artificial light intensity with it. Also, PAR lights measure the red and blue spectrum of light, but they don’t measure the UV spectrum which plants also use. So there are trade off’s with either light meter.

Here are the light level across a typical apartment unit with light coming from a west window:

Panorama showing light level across the room

Where should I place my plant for optimum light

So, you bought a plant and the label says its needs mild filtered light (with a hint of afternoon sun), but what does that mean!

Since the sun rises I the East and sets in the west different rooms in the house will get different amounts of light. let’s break it down by the type of light:

Bright light – Place right next to a north or south facing window (In the northern hemisphere this is in a south or south west facing window that will get maximum light. In the southern hemisphere this is the north or north west facing window.

Bright Indirect Light – This is any place where the plant will get a few hours of sun during the day but not the whole day. Roughly 1-2 meters away from the window in an east or west window. Or 1 meter away in a south window. The East window is often considered the best window for houseplants as its cooler than the west window without the dangers of overheating. West windows received full sun for part of the day and bright light for the rest, great for flowering plants.

Medium light/Partly shaded – This is where the plant gets some morning sun or some afternoon sun. Morning sun is less intense than afternoon sun, so putting your plants a few meters in from an East facing window works for maximum sun while reducing the risk of burning.

Low light / Shady – This is where your plant doesn’t get direct sun. Place in a north facing windows (if you are in the Northern hemisphere) or a south facing (window if you are in Southern Hemisphere) these windows don’t get any direct sunlight. Or place somewhere in the middle of your living room. This is generally more than 2 meters away from windows, such as near a staircase, in the passage or corners of rooms away from windows. Only low light plants will thrive here.

Plant next to a record player

“Are you saying if I don’t have a North or South facing window I can’t have a plant that needs bright light?” …. You will struggle, buy the right plant for the light you can provide to keep it thriving.

So how much light does my specific plant need?

You can find the light requirements for you specifics plant by googling the specific brand, there are plenty of guides out there. For instance, a fiddle leaf is a bright light plant, a ZZ or Zanzibar gem is a low light plant.

In general, plants that flower needs at least 12-16 hours a day. Plants also need darkness (like Vampires, they start to work at night!), a lot of plant flowering and growth is triggered by the changes in daylight hours, some plants will start flowering when the number of daylight hours drops (i.e. more darkness…winter is coming). To keep your standard plant alive make sure your plants get at least 8 hours of darkness a day.

Living room with plants
Living room with plenty of medium to bright light

How can I measure my light?

From what I’ve seen with people’s houseplants the number one cause of premature plant death (PMPD … I made that up) is that they have the wrong plant in the wrong light position (The next cause of death is wrong watering amount). You just can’t put that nice Fiddle Leaf fig in the middle of your living room away from the window and expect it to live. Fiddle leafs need bright light, not low light.

Fiddle leaf fig plant in corner of room

If you want to be more precise with your light measurements, here are some products you can use to figure out the Lux levels.

  • PlantMaid – the light sensor gives you the relative current light which is perfect to know if its low, medium, bright or very bright, plus you can also see the average over 12 hours (which is really what you want)

PlantMaid front view watering device

  • Lux meter – there are some cheap ones on Amazon $30, or some good ones $200+. I bought the cheap one and compared it with a bunch of other products. It was fine to get a rough idea of the light.

Lux light measuring device

  • Parrot Flow Power gives you the lux reading and you can get a nice graph of the total output over the day. It’s more expensive and you need to connect it with Bluetooth to your phone.

Flower power plant sensor

  • Hydrofarm LGBQM Quantum PAR Meter– Affordable PAR meter with a digital output.

Hydrofarm PAR meter

That’s it! You should now know where to place your plants for optimum growth, every plant deserves to photosynthesize!

To give your plants to best chance of thriving you need to have them in the right light conditions for the specific plant type. The next thing to consider is their watering requirements…

Happy Growing.

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