Year 7 · Science · Biological sciences
What plants need to grow
Students design a fair test with fast-growing seeds to find out how light and water affect plant growth.
Awaiting educator review. This material was drafted by our content team with AI assistance and is waiting for review by a qualified educator. Please check it suits your child before using it. Created 10 October 2026.
These lessons were written around Australian Curriculum strands and matched to New Zealand year levels by age (NZ Year 1 is the first year of school). They are not yet mapped to The New Zealand Curriculum, so check them against your own learning programme.
Learning goals
Learners will
- Identify the conditions plants need to grow well.
- Design a fair test with one independent variable.
- Measure and record plant growth over time.
Success looks like
- I can name the variable I change, the variable I measure and the variables I keep the same.
- I can measure seedling height accurately in millimetres.
- I can draw a conclusion from my results.
The big idea
Plants need light, water, air (carbon dioxide), suitable temperatures and nutrients from the soil to grow well.
Plants make their own food by photosynthesis. Their leaves use energy from light to turn water and carbon dioxide into sugar, giving off oxygen.
Seeds can germinate in the dark using food stored inside them, but seedlings grown without light become tall, thin and pale yellow. They cannot make enough food to stay healthy for long.
In a fair test, you change one thing only: the independent variable. You measure the effect: the dependent variable. Everything else stays the same: the controlled variables.
For example, to test light, change only light (one tub in a sunny spot, one in a box), and keep the seed type, number of seeds, container, water and temperature the same.
Repeating a test, or using several seeds in each tub and averaging, makes results more reliable.
Australian plants show how conditions matter. Saltbush and spinifex grow in dry inland areas, while tree ferns need cool, moist, shady gullies.
Worked examples
Planning a light test
Plan a fair test to find out if cress grows better with light.
- Independent variable: light (sunny windowsill vs. inside a box).
- Dependent variable: height of the seedlings after 7 days, in mm.
- Controlled variables: same seeds, same number of seeds, same containers, same amount of water each day, same room temperature.
- Use two tubs for each condition so you have repeated trials.
Answer: Change only light; measure height; keep everything else the same.
Finding the average height
Three cress seedlings in the sunny tub measured 30 mm, 34 mm and 26 mm. What is the average height?
- Add the heights: 30 + 34 + 26 = 90 mm.
- Divide by the number of seedlings: 90 ÷ 3 = 30 mm.
Answer: The average height is 30 mm.
Practice check
Have a go, then check your answers. Each answer comes with an explanation.
Answer guide for parents
In a test of how water affects growth, what is the independent variable?
The amount of water — The independent variable is the one thing you change: the amount of water.
What do seedlings grown in the dark usually look like?
Tall, thin and pale — Without light, seedlings stretch, look thin and lose their green colour.
Which gas do plants take in for photosynthesis?
Carbon dioxide — Plants use carbon dioxide, water and light energy to make sugar.
Why use several seeds in each tub?
To make results more reliable — More seeds and repeats reduce the effect of one unusual seed.
Which plant is adapted to dry inland Australia?
Spinifex — Spinifex grasses grow on dry, sandy inland plains.
Seedlings measured 12 mm, 15 mm and 18 mm. What is the average height in mm? (number)
15 — (12 + 15 + 18) ÷ 3 = 45 ÷ 3 = 15 mm.
Which gas do plants give off during photosynthesis?
oxygen — Photosynthesis releases oxygen.
When a seed starts to sprout, we say it g_________.
germinates — Sprouting is germination.
Watch out for
- Some think plants get their food from the soil. Plants make their own food by photosynthesis; soil provides water and small amounts of nutrients.
- Students often change more than one thing at once. Check that only one variable differs between groups.
- Some think seeds need light to germinate. Many seeds germinate in the dark, but seedlings need light to keep growing healthily.
Tips for parents
- Use a calendar to record measurements at the same time each day.
- Ask your child to fill in a planning table: change / measure / keep the same.
- Safety: wash hands after handling seeds and soil; some seed packets are treated with chemicals, so use food-grade seeds such as cress or mung beans.
Go further
- Test the effect of temperature by growing one tub in the fridge and one at room temperature (both in the dark so light is controlled).
- Draw a line graph of average height against day number.
- Research how Australian farmers use irrigation in dry regions.