Lighting cacti and planning supplementary illumination

How to read the home growing-area diagram

Dividing a room by distance from the window is illustrative. Light quantity varies with compass direction, season, the building's surroundings, glazing and shade. The centimetres shown on the plate do not define universal zones for every home. Likewise, a power of 20–40 W and height of 20–40 cm do not specify a single light dose for plants. What reaches the stem surfaces is assessed. [15-WWW]

Plants from open habitats, forest cacti, freshly rooting cuttings and seedlings should not receive the same exposure indiscriminately. A change of position requires gradual acclimatisation. Turning the plant, as generally recommended in the illustration, also changes the exposure of previously shaded epidermis. [1-WWW] [Fleischer and Schütz 1978, pp. 183–185]

PPFD and daily light integral DLI

PPFD, expressed in µmol m⁻² s⁻¹, describes the photon flux falling on a unit area within the conventionally defined photosynthetically active range. DLI, in mol m⁻² d⁻¹, is the daily sum of these photons. At constant PPFD, DLI = PPFD × number of hours × 0.0036. An example of 200 µmol m⁻² s⁻¹ for 12 hours gives 8.64 mol m⁻² d⁻¹. This is a calculation example, rather than a recommendation for the whole family. [15-WWW]

Measurements are taken at plant height in several places on the shelf. Daylight changes over time, so a single midday reading is not DLI. Watts describe energy consumption, while lux accounts for eye sensitivity. Converting lux to PPFD depends on the spectrum. The 400–700 nm range shown on the plate corresponds to traditional PAR, but the words “full spectrum” alone do not specify a lamp's spectral distribution. [15-WWW]

What the experiment with an edible cactus showed

Horibe and colleagues grew Nopalea cochenillifera, treated here as Opuntia cochenillifera, in a hydroponic system under red and blue light and mixtures of the two. Light colour changed the rate of elongation and the number and width of new stem segments. The fastest elongation did not also mean the greatest width or the same crop quality. The study concerned production of young stem segments of a particular species, rather than overwintering an entire cactus collection. [Horibe et al. 2019]

Changing the spectrum also affected spine occurrence. The authors interpreted reduced spine development in the context of vegetable use, where spines are undesirable. The same result is not automatically beneficial to a collector seeking the plant's natural appearance. The cultivation goal is part of assessing the experiment. [Horibe et al. 2019]

Why young plants require separate assessment

Delgado-Sánchez and colleagues compared Opuntia streptacantha seedlings under different light and water availability. Better development under shade was associated with physiological traits and chloroplast condition, and the drought response depended on lighting. The result shows interaction between factors. It cannot establish one safe distance from a lamp for seedlings of all genera. [Delgado-Sánchez et al. 2013]

Your own trial. Control rather than a random change

The design of the cited experiments can inform a comparative trial, with several similar plants in each treatment, different lighting and other conditions kept as similar as possible. Records include origin, developmental stage, PPFD, photoperiod, temperature, watering and observation dates. [Horibe et al. 2019] [Delgado-Sánchez et al. 2013]

New growth, stem proportions, colour, spine development and damage are assessed. Increasing height need not mean better development. Lamp, fertiliser, substrate and watering are not all changed at once if the aim is to establish the effect of light. A dark period and the plants' seasonal requirements are also maintained, rather than extending the day without limit. [Fleischer and Schütz 1978, pp. 183–185] [15-WWW]