Gardening glossary
Photosynthesis
Photosynthesis is the chemical engine that lets a plant turn sunlight into food. Inside the green chlorophyll pigment of each leaf, light energy splits water molecules and combines them with carbon dioxide from the air to produce glucose (a simple sugar) and oxygen. The sugar fuels new roots, stems, flowers, and fruit; the oxygen is released through the leaf's stomata as a byproduct.
Three inputs control the rate of photosynthesis: light intensity, carbon dioxide concentration, and temperature. Indoors, light is almost always the limiting factor. A plant labeled "bright indirect" needs roughly 200–800 foot-candles; a fruiting vegetable needs 2,000+ foot-candles to photosynthesize fast enough to set fruit. Too little light and the plant burns more sugar in respiration than it produces — the leaves yellow, growth stalls, and stems stretch.
Temperature matters because the enzymes that drive photosynthesis work best between roughly 65–85°F (18–29°C). Below 50°F most houseplants effectively pause. Above 95°F the stomata close to conserve water and photosynthesis grinds down even when light is abundant.
Practical implications for gardeners:
- Dirty leaves photosynthesize less. Wipe dust off broad-leafed plants monthly. - Yellow lower leaves on a healthy plant are often the plant recycling chlorophyll from shaded leaves that no longer photosynthesize efficiently. - Winter dormancy in many houseplants is really just a photosynthesis slowdown driven by short days — water and feed less, not more. - LED grow lights work because they emit the red and blue wavelengths chlorophyll absorbs most efficiently.
If a plant is declining and you've ruled out water and pests, light is almost always the next variable to check.