It's no secret that plants need light to grow, but how they get that light, how much they get, and what kind of light they get depends on the plant you are growing.
Some plants are more forgiving and will tolerate nearly any amount of light while others have very specific lighting needs. Most discussions about light center on the intensity, or lumens.
We're also going to discuss light spectrums and how those influence plant growth and production, since plants use different wavelengths of light to grow healthy roots and foliage and to produce flowers and fruit.Let's dive right in,
How Light is Measured: LUMENS, Lux and Footcandles... Oh My...
LIGHT intensity is commonly measured in terms of LUMENS per square foot which you might hear this measured as footcandles OR lumens per square meter which is measured as lux. No matter which one were talking about were using them as a reference point for the amount of light AKA the lumens falling on a given surface. All plants need different quantities and durations of lumens per day for proper photosynthesis.
Up until now we've just used our eyes and common sense outdoors and the white paper test indoors to help us measure lumens subjectively, that means we're only using our natural vision to decern how bright a space is, another test is to imagine (or you can actually try this out) that you have a lit candle, and the light from that candle only does so much - it only lights a certain amount of space and the further you get from that candle the darker it gets, now imagine you have 10 candles, that's enough to light up a square meter, which is why we measure light in footcandles and lux, it's the number of lumens or candles it takes to light up that space.
MATH TIME - we know, yucky maths... Hang in there!
To simplify it's easier to round down the lux and lumens, yeah I know that's not rounding to the nearest whole number, or going to be exact math - but it certainly makes the math going forwards way easier for us all.
For reference natural moonlight is usually 1/10 footcandles = 1 Lux and direct sun is 3200-10,000 footcandles = 32,000 to 100,000 lux.
Most commercially sold lighting in the US lists Footcandles - we run on the imperial system whereas Lux is a metric measurement.
If you are using this as a guide for purchasing grow lights please pay attention here, the following two lists are the actual breakdowns of what light measurements are considered outdoors and indoors in layman's terms and their corresponding Lux and Footcandles. Use them to inform you're decisions on what lighting is best for you.
Natural Light Condition Outdoors Typical Lux ( = Footcandles ):
- Direct Sunlight: 32,000-100,000 lux (3,200-10,000 foot candles)
- Ambient Daylight: 10,000-25,000 lux (1,000-2,500 footcandles)
- Part Sun Part Shade: 1,000-10,000 lux (100-1,000 Footcandles)
- Overcast Daylight: 1,000 lux (100 footcandles)
- Sunset & Sunrise: 400 lux (40 footcandles)
- Full shade: 200-1000 lux (20-100 Footcandles)
- Moonlight (Full moon): 1 lux (0.1 footcandles)
- Night (No moon): < 0.01 lux (0.001 footcandles)
Natural light Conditions Indoors Typical Lux ( = Footcandles):
- Direct Sunlight: 10,000 + lux (1,000 + footcandles)
- Bright Indirect Light: 3,000 + Lux (300 + footcandles)
- Medium Indirect Light: 750-3,000 lux (75-300 foot candles).
- Low light: 50-750 lux (5-75 footcandles)
Of course we could just use a light meter to get a better reading but my goal here is to promote your ability to recognize how intense light actually is subjectively without needing any extra technology to do it. Our ancestors certainly didn't have a light meter in hand when they started gardening and later building conservatories, greenhouses and collecting houseplants - they used common sense, and not everyone can afford the extra expense so let's skip it.
Duration of Intensity
Making sure your plants are getting the right light for the entire day is important - plants follow a seasonal growth pattern where light is available from sun up to sun down, which is longer during the active growing seasons of spring, longest during summer, slowly decreasing during fall and shortest during winter dormancy periods. These changes signal plants to produce new growth of foliage and roots, flowers, fruit, seeds, and even when it's time to stop growing and reserve energy for perennials and when to die for annuals. Annuals such as fruits and vegetables require these changes to produce properly, ornamental perennials may be more forgiving, but if you're goal is flowers or edible parts pay close attention to the "seasons" whether you're growing indoors or out.
16 hours of light is usually the max for any plant naturally and would represent summer hours, which means 8 hours of dark is a requirement for a plants circadian rhythm. 11 hours of light (or less) seems to be the sweet spot for most flowering plants to bloom, and Most plants have a minimum need for light, but this varies greatly depending on their natural light requirements. Signs your plants missing vital light is poor or leggy growth, leaning towards a light source, excess soil moisture leading to root rot, etc.
- Check your seed packs or plant tags for outdoor plants recommended sun exposure, planting times, germination and time to maturity.
- For indoor houseplants and orchids we will put a very handy but by no means exhaustive reference list at the bottom of this page. See week 2 indoor lighting for references on general light levels indoors.
The Light Spectrum: What this means for Roots, Foliage, Flowers, Fruit, and Seeds.
Humans only see in a specific visible light spectrum - plants (and animals) are different, more precisely what they see that we can't is important. What's good light for humans isn't necessarily great for plants depending on what our goals are with them.
These colors that we don't really see in natural light are actually the wavelengths of the light spectrum. Plants "see" ultraviolet, UV rays we don't really think much about unless we're talking sunburns and cancer as well as "seeing" infrared, two extremes well past of our visible spectrum that we don't recognize so well.
The spectrum of light that plants use most is known as Photosynthetically Active Radiation (PAR) and includes wavelengths from Violet about 400nm to Red about 700 nm. In nature plants also recieve UV and Infrared radiation in addition to the visible spectrum between violet and red. Different wavelengths effect plant growth differently - but some may leave us with a nauseating headache indoors.
- Infrared: Infrared radiation produces heat (one topic in our next series). It also influences internode length, speeding up growth and influencing fruiting crop ripening uniformity.
- Far red: Far red photons while mostly reflected off plants cause significant stem, leaf, and/or petiole elongation - faster and leggier growth of stems, leaves and even flowers.
- Red: Flowers, fruits and seeds all need red light to produce well. Red light is highly effective for regulating and inducing blooms and subsequent fruit on plants.
- Orange: increases leaf and root growth while promoting fruit growth - not as effective as red light. Does not help carbohydrate production
- Yellow/white: while this can help a plant survive a winter dormancy period it's not going to promote growth or production of carbs, sugars, or maintenance of chlorophyll to support photosynthesis.
- Green: most plants appear green so we originally thought green light was reflected off them entirely - however we now know that green light is absorbed deeper into a plant than other wavelengths, green lights can effect the architecture of plant cells. Most similar to moonlight plants don't perceive green as much as other wavelengths, so they unconsciously soak it up.
- Blue: blue light is arguably the most important for clorophyll production, however it slows down production of auxen - a plant hormone responsible for root and stem elongation, leading to shorter smaller but stockier growth - think thick leaves here.
- Violet: generally accepted as good for vegetative growth when coupled with other wavelengths, violet light tends to be low penetration - meaning plants just don't absorb much of it.
- Ultraviolet: UVA and UVB (Skip the UVC unless we're sterilizing something sinister). UVA light for indoor plants offers various benefits, such as protection from fungi and molds with minimal risk of damage to the plants tissues. UVB is also beneficial in small amounts. It can inhance a plants natural beauty and traits, and can act as a natural pesticide by influencing the plant to protect itself from microorganisms.
No one wavelength by itself is enough to produce a happy healthy plant - in natural light the various wavelengths are all available in various proportions for plants to use, so we recommend using a broad spectrum grow bulb for supplemental or artificial lighting for indoor gardening, but if we're just talking about that one plant in the corner that isn't getting enough natural light you may be surprised to learn that a standard light bulb in a standard light fixture above that plant may be enough to bridge the gap.
Grow lights will always generate more power and bang for your buck than regular household bulbs. The more intense the grow lights are, the more energy they will emit—the higher intensity of a grow light will increase the rate of growth. But be careful, there is such a thing as too strong, and too close - light intensity diminishes the further you get from the source, so as much as you wouldn't want your lights right on top of a plant, you also want to make sure your plants are close enough to really benefit from the use of artificial lighting.
Thanks for joining us for our three part series on outdoor, indoor light, and the science behind them. Stay tuned for our next three part series on Heat, Humidity and Airflow.
We didn't forget what you're really here for - Common Houseplants & Orchids Lighting Needs:
