Observing growth under three light conditions

Why this question

Lesson 3 of the beginner path says that light is the variable most often responsible for a struggling carnivorous plant, and lists the symptoms of too little light. This protocol is a way for a grower to see those symptoms, or their absence, in their own home, with their own windows, rather than taking the lesson on trust.

It is an observation rather than a controlled comparison, because the three positions differ in temperature and humidity as well as light, and no attempt is made to equalise them.

Hypothesis

Plants under the grow light will produce the most compact, best-coloured growth; plants on the south sill will be close behind; plants on the east sill will show the longest, palest leaves. For Nepenthes ventricosa, the east sill plant is expected to produce the fewest pitchers.

Setup and conditions

• Species: Drosera capensis as the primary subject because it responds quickly and visibly to light. Optionally Nepenthes ventricosa as a second subject if three similar plants are available.
• Plants: three plants per species, as similar as possible in size, each photographed and measured on day 0.
• Positions:
  1. South-facing windowsill, pot touching the glass.
  2. East-facing windowsill, pot touching the glass.
  3. Under a full-spectrum white LED grow light, 25 cm (10 in) above the plant, 14 hours a day on a timer.
• Water: each pot in its own tray of rainwater or reverse-osmosis water, refilled to the same depth on the same days.
• Medium: all plants repotted into the same batch of medium one month before the start.
• Duration: eight weeks, starting at least a month after the spring equinox so that day length is reasonably stable.

Measurements

Weekly, for each plant:

• Number of new leaves (or pitchers) since the previous week.
• Length of the newest fully open leaf, in millimetres.
• Colour of the newest leaf against a simple three-step scale: green, blushed, red.
• For sundews, whether the newest leaf carries dew along its full length.
• A photograph from a fixed position in the same light.
• Maximum and minimum temperature at each position if a thermometer is available.

What is and is not controlled

Water, medium and starting size are matched. Light is the variable of interest, but the three positions also differ in temperature, humidity and airflow, and these are recorded rather than controlled. For that reason the experiment can show what happens at each position in this home; it cannot by itself prove that light is the cause.

Updates

No updates. This protocol has not been started. When an experiment is running, dated entries appear here, newest at the bottom, under headings in the form ### YYYY-MM-DD — short title.

Results

None. This is a planned example and no plants have been set up.

Limitations

• One plant per position per species, or at most three, is too few to generalise.
• Positions differ in more than light.
• Window orientation means something different at different latitudes and in different houses; the result would describe one home.
• Eight weeks shows new growth but not the plant’s longer-term health.

Next steps

If run, a useful follow-up would be to swap the plants between positions for a second eight weeks and see whether the pattern follows the position or the plant.