The three things "light" actually means

When a care tag says a plant "needs more light," it usually means brightness. Maryland's extension service describes three separate properties of indoor light: intensity, quality and duration, and each one affects a plant differently (University of Maryland Extension, 2023). Keeping them apart clears up a lot of the confusion around grow lights, window placement and care tags.

Intensity: how much

Intensity is the amount of light: how many photons, the individual particles of light, land on each patch of leaf per second (Poorter et al., 2019). In our view it's the property that most often falls short indoors, because indoor light is often very low, above all in winter (Steil, 2023b). It matters a great deal. A meta-analysis that pooled some 500 experiments on 760 plant species found that some traits, including the actual rate of photosynthesis, rose more than tenfold between very dim and very bright growing conditions (Poorter et al., 2019). Intensity is also the property your eyes judge poorly (UF/IFAS Gardening Solutions, n.d.), which is what Lesson 3 is about.

Quality: which colors

Quality is the mix of colors, or wavelengths, in the light (Weisenhorn & Hoidal, n.d.). Wavelength is measured in nanometers, each a billionth of a meter. The band from about 400 to about 700 nanometers is the one that drives photosynthesis (Poorter et al., 2019). That band is called photosynthetically active radiation, or PAR.

Plants don't use every color equally, and how much the difference matters depends on how bright the light is (Liu & van Iersel, 2021). A whole leaf absorbs blue light most strongly, then red, and green least, and the green it doesn't absorb is why leaves look green (Liu & van Iersel, 2021). But "least" still means most. In one study, lettuce leaves absorbed about 93 percent of blue light, 92 percent of red and 81 percent of green (Liu & van Iersel, 2021). The old idea that plants barely use green light comes from measuring chlorophyll dissolved in a test tube, which the same authors say underestimates how much green a whole leaf absorbs. In their measurements, green and red light drove photosynthesis about equally well in bright light, and green did worst only in dim light, because less of it was absorbed (Liu & van Iersel, 2021).

In practice, sunlight brings all the wavelengths a houseplant needs, so on a windowsill the quality of light doesn't need much thought (Steil, 2023a). Films and coatings on the glass can shift the mix, though (Steil, 2023a), and glass type matters for how much gets through: in a Danish study of apartment windows, two-layer high-transmittance glass let in about 15 percent more visible light than three-layer energy-saving glass, and only the two-layer glass let UV-B through (Volf et al., 2024). The mix of colors is more of a concern when you grow under electric lights than under sunlight (University of Maryland Extension, 2023). On a windowsill, the questions that matter are how much light arrives and for how long.

Fig. 3. How much of each color a whole leaf absorbs. A leaf takes in most of the light across the whole band, blue and red a little more than green, so it sends back slightly more green, which is why it looks green. The curve follows measurements on lettuce leaves (Liu & van Iersel, 2021). Light from 400 to 700 nm is usually defined as photosynthetically active radiation, or PAR (Liu & van Iersel, 2021).

Duration: how long

Duration is the photoperiod: the number of hours of light in a 24-hour day (Weisenhorn & Hoidal, n.d.). It matters in two ways. First, it turns intensity into a daily total. Growers call that total the daily light integral, or DLI: the photosynthetic light delivered to an area over 24 hours (Steil, 2023b). Because the total is intensity multiplied by hours (Steil, 2023b), a moderately bright spot lit for 14 hours can deliver more in a day than a much brighter spot lit for four. Lesson 4 defines DLI properly, and Lesson 8 puts it to work.

Second, many plants use changing day length to time their flowering (Song et al., 2015). The poinsettia is the classic example: it forms flower buds only after about four weeks of roughly 14 hours of total darkness a day, which is why it colors up in winter (Smith, 2009). The foliage plants most Apprentices start with rarely need that cue, so for them we pay attention to the daily total.

Fig. 2. The three properties of light a plant responds to. Intensity is quantity, quality is spectrum, duration is hours per day. On a windowsill, intensity is usually the one that falls short, sunlight takes care of quality, and duration sets the daily total.