The plant's side of the deal

So far we've talked about supply: what the window offers. The other half is demand: what the plant needs. Most houseplants tolerate only a fairly narrow range of light (UF/IFAS Gardening Solutions, n.d.), and in the Guild's view, matching the plant to the spot is most of the job.

Leaves grown in sun and in shade are built differently

Leaves grown in bright light are built differently from leaves grown in shade. A meta-analysis that pooled some 500 experiments on 760 species found that leaf thickness almost doubled across the range of light studied, and that the top rate of photosynthesis a leaf could reach, per unit of leaf area, more than doubled (Poorter et al., 2019). Leaves grown in more light also carried more of the compounds that protect a leaf from strong light (Poorter et al., 2019).

A shade leaf tends to be larger and thinner, and it isn't as productive as a sun leaf (Trinklein, 2016). Don't count on it carrying more chlorophyll per square inch, though. Across the meta-analysis, chlorophyll per leaf area went up with light about as often as it went down, and in soft-stemmed plants it usually rose (Poorter et al., 2019). In a growth-chamber study of pothos and two other houseplants, leaves grown under the brightest of three dim treatments had more chlorophyll per leaf area (Sugano et al., 2024). Extra layers of palisade cells, the column-shaped photosynthesis cells under the upper leaf surface you met in Module I, turned up in brighter light in only about half of the cases (Poorter et al., 2019).

Leaves aren't completely locked in once they form. When sycamore maple seedlings were moved from shade into full light, the mature leaves of plants from moderate shade adjusted: their palisade cells grew longer and the leaves thickened somewhat. Leaves from the deepest shade bleached and were lost within a few days (Wyka et al., 2022). That's the risk when a shade-grown houseplant goes abruptly into sun, and it's why Lesson 8's acclimation step matters.

Fig. 8. Sun leaf and shade leaf in cross-section. Leaves grown in bright light are thicker, with a higher top rate of photosynthesis; shade leaves are larger and thinner. In the maple study in this lesson, mature leaves readjusted to more light only partly, and leaves moved from deep shade into full sun bleached (Wyka et al., 2022).

Every plant has a light budget

Two thresholds help explain a plant's light budget. The light compensation point is the light level at which photosynthesis just equals respiration, so the plant makes exactly as much food as it uses. Below it, the plant uses more than it makes: it stops growing and sheds leaves, and the shedding usually stops once the leaves it has left can cover its needs again (Trinklein, 2016). The light saturation point is the level above which more light no longer raises photosynthesis, and in a study of cucumber it shifted with temperature and carbon dioxide (Xin et al., 2019). Light beyond what a leaf can use can cause photoinhibition, a slowdown of the photosynthetic machinery caused by light damage, and how much harm follows depends on how fast the leaf repairs itself (Guidi et al., 2019).

Shade leaves have lower compensation points than sun leaves (Trinklein, 2016). In a study of two maple species, seedlings grown in full light tended to have higher saturation and compensation points than shaded ones, though each difference was clear in only one of the two species (Zhang et al., 2022). That's why one spot can suit one plant and not another. Pothos kept growing for six months in a growth chamber on a daily light integral of just 0.22 moles per square meter per day (Sugano et al., 2024), while Iowa State's extension puts herbs and vegetables at about 12 to 30 (Steil, 2023b). There's no such thing as a good light spot in the abstract, only a good match between a spot and a plant.

Fig. 9. The light-response curve. Below the compensation point a plant uses more food than it makes; above the saturation point more light doesn't help. Shade-grown leaves (left curve) break even at lower light and level off lower; sun-grown leaves (right curve) need more light to break even and keep gaining longer.

Why variegation usually costs something

Variegation is the white or cream patterning prized in collector plants. In most variegated plants the pale sectors lack chlorophyll and do little or no photosynthesis of their own. In variegated zonal geraniums, for example, the white sectors depend on the green ones for their sugar (Milić et al., 2023). Not all variegation works this way. In some pothos and begonias, silvery patterning comes from air pockets under the leaf surface and doesn't seem to affect the leaf's chlorophyll (Mitchell, 2023). Most variegated plants grow less vigorously than all-green forms of the same plant (Mitchell, 2023).

Low light makes the patterning harder to keep. In dim spots, the coloration on variegated leaves tends to fade (Pennisi, 2022). Pothos in particular can lose its variegation in low light (North Carolina Extension Gardener Plant Toolbox, n.d.), and Minnesota's extension lists a variegated plant reverting to solid green among the signs of too little light (Weisenhorn & Hoidal, n.d.). True reversion can also happen for another reason. An unstable variegated plant can spontaneously put out a vigorous all-green shoot, and that shoot can outgrow the variegated parts (Mitchell, 2023). If you want to keep the variegation, give the plant good light.

Fig. 10. The pale sectors of a variegated leaf carry little or no chlorophyll, so they add little to the plant's food supply. The green tissue carries the load, which is one reason most variegated plants grow less vigorously than their all-green relatives.