Effect of light on plant growth under shade cloth · Concept Studio

The idea

Plant dry matter comes from photosynthesis, so plants under heavier shade gain less dry mass, even when they grow taller.

What you need

40 lettuce or radish seedlings of the same age and size
40 identical pots of the same potting mix
four frames: open, and covered with shade cloth rated 30%, 50% and 70%, the rating being the percentage of light the cloth blocks
light meter (lux) or a calibrated phone light sensor, with its maximum reading known; thermometer
balance reading to 0.01 g, paper bags, drying oven set to 90–95 °C (the SAPS radish dry-mass method)

How to do it

Pot one seedling per pot and assign 10 pots to each light treatment, arranged as 10 replicate groups of four pots (one per treatment) with positions randomised within each group.
Measure light under each frame and in the open at midday on three days, and the air temperature under each frame, and express each light reading as a percentage of the open reading. A shade cloth's rating is the percentage of light it blocks, not the percentage it passes, so the measured transmission and the rating should together come to about 100: about 30% of open light is expected under the 70% cloth, and a reading near 70% under it means the cloth, the meter or the arithmetic is wrong. Read the rating from the cloth's own label or the supplier's specification. Check first that the open reading is below the meter's maximum: a sensor showing its maximum in full sun is saturated and makes every shade percentage too high, so on such days take the readings when thin cloud dims the sun.
Water all pots with the same volume.
After 4 weeks record plant height and leaf number, then cut the shoots, dry them in bags at 90–95 °C until the mass is constant and weigh.
Calculate the mean and standard deviation of shoot dry mass for each treatment, plot dry mass against measured light and test the open and 70% treatments for a significant difference.

What you should see

Shoot dry mass falls as shade increases; plants under the heaviest shade may be as tall or taller but are thinner and paler, so height alone misleads. The light meter shows what fraction of open light each cloth passes, which is the complement of the cloth's rating: a cloth rated to block 70% is expected to pass about 30%, so the measured transmission and the rating together come to about 100. A learner who measures about 30% under the 70% cloth has confirmed the cloth, not contradicted it. The learner knows it worked when each cloth's measured transmission and its rating add to about 100, and when replicate pots within a treatment vary much less than the difference between the open and 70% treatments.

What changes

What you change: light level (open frame, and frames covered with cloth rated to block 30%, 50% and 70% of the light)
What you measure: shoot dry mass (g), height (mm), leaf number
What you keep the same: seedling variety and starting size
pot and mix
water volume
randomised positions within replicate groups
harvest date

Common misconceptions

Each of these ideas is wrong, and the activity is a chance to test it.

The tallest plants have grown the most.
A plant's mass comes mainly from the soil rather than from carbon dioxide in the air.
Shade only affects plants by keeping them cooler.
A cloth rated 70% lets 70% of the light through.

Safety card

Low riskLearners carry it out

Hazards

dust and bacteria (including Legionella) in potting mix
hot drying oven

Controls

dampen potting mix before handling; gloves and a P2 mask
wash hands afterwards
teacher loads and unloads the drying oven

Note

No hazardous chemicals. Oven heat and general laboratory rules: NSW Department of Education, Chemical Safety in Schools (CSIS) 2021 Technical Update: Section 1 (general information for all staff) and Volume 2 Appendix D (generic risk assessment advice for each chemical). Complete a CSIS or RiskAssess site-specific risk assessment before the lesson.

Curriculum references

The NSW syllabus outcomes and Australian Curriculum v9 codes this activity supports. They are references, not a verified or complete curriculum alignment.

Agriculture Stage 6 Syllabus (2013)H2.1 H4.1
Agriculture 11–12 Syllabus (2026), new syllabus, implementation from 2028AG-12-03 AG-12-05
Australian Curriculum v9No Australian Curriculum v9 code is listed.

Sources

The pages the author read to write this activity.