50-minute lesson
Year 6 Science: What plants need to grow
Lesson objective
- Identify the conditions plants need to grow well.
- Design a fair test with one independent variable.
- Measure and record plant growth over time.
Success criteria:
- 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.
Materials
- Fast-growing seeds such as cress, mung beans or wheat
- Four identical small containers (e.g. clean yoghurt tubs)
- Cotton wool or potting mix
- Water and a measuring spoon
- A cardboard box or dark cupboard
- A ruler with millimetre markings
Introduction
5 minIntroduce today's words:
- variable: Anything that can change in an experiment.
- independent variable: The one thing you choose to change in a fair test.
- dependent variable: The thing you measure to see the effect.
- controlled variables: The things you keep the same so the test is fair.
- germinate: When a seed begins to sprout and grow.
- photosynthesis: The process plants use to make food (sugar) from light, water and carbon dioxide.
Ask your child what they already know about what plants need to grow.
Explanation
10 minPlants 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
8 minPlanning a light test
Plan a fair test to find out if cress grows better with light.
Step 1: Independent variable: light (sunny windowsill vs. inside a box).
Step 2: Dependent variable: height of the seedlings after 7 days, in mm.
Step 3: Controlled variables: same seeds, same number of seeds, same containers, same amount of water each day, same room temperature.
Step 4: 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?
Step 1: Add the heights: 30 + 34 + 26 = 90 mm.
Step 2: Divide by the number of seedlings: 90 ÷ 3 = 30 mm.
Answer: The average height is 30 mm.
Guided practice (do together)
10 min1. In a test of how water affects growth, what is the independent variable?
(a) The height of the plants (b) The amount of water (c) The type of seed (d) The size of the container
2. What do seedlings grown in the dark usually look like?
(a) Short and dark green (b) Exactly like those in light (c) Tall, thin and pale (d) Covered in flowers
3. Which gas do plants take in for photosynthesis?
(a) Carbon dioxide (b) Oxygen (c) Helium (d) Nitrogen
4. Why use several seeds in each tub?
(a) To make it messy (b) To use up the packet (c) So the seeds grow faster (d) To make results more reliable
Independent practice
12 min5. Which plant is adapted to dry inland Australia?
(a) Tree fern (b) Spinifex (c) Water lily (d) Mangrove
6. Seedlings measured 12 mm, 15 mm and 18 mm. What is the average height in mm? (number)
7. Which gas do plants give off during photosynthesis?
8. When a seed starts to sprout, we say it g_________.
Questions to check understanding
- Can you name the variable I change, the variable I measure and the variables I keep the same?
- Can you measure seedling height accurately in millimetres?
- Can you draw a conclusion from my results?
- What was the trickiest part today?
Answer guide
1. The amount of water — The independent variable is the one thing you change: the amount of water.
2. Tall, thin and pale — Without light, seedlings stretch, look thin and lose their green colour.
3. Carbon dioxide — Plants use carbon dioxide, water and light energy to make sugar.
4. To make results more reliable — More seeds and repeats reduce the effect of one unusual seed.
5. Spinifex — Spinifex grasses grow on dry, sandy inland plains.
6. 15 — (12 + 15 + 18) ÷ 3 = 45 ÷ 3 = 15 mm.
7. oxygen — Photosynthesis releases oxygen.
8. germinates — Sprouting is germination.
Review
5 minRecap the success criteria together. Watch for these common misconceptions:
- 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.
Extension activities
- 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.
Suggested follow-up
- Revisit what plants need to grow tomorrow with two or three quick questions from memory.
- Try the online practice check for this topic and look at any questions that need another go.
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.