🌿 Does grass grow best under red, blue or green light?

My hypothesis

If I change the colour of the light the grass gets, then how tall the grass grows in 14 days will ______, because ______.

🔊 You fill in the blanks. Think about which colours of light a green leaf soaks up, and which it bounces away. A guess is a good because: the test finds out.

Change, measure, keep the same

Scientists call the thing you change the independent variable and the thing you measure the dependent variable. The things you keep the same are the controls: if one of them changed too, you wouldn't know which change made the difference. Try each of the 4 levels 3 times.

What you need

• 🌾 Grass seed
• 🥤 Paper cups with a hole poked in the bottom: 12
• 🪴 Potting soil
• 🚰 Water from the tap
• 🥄 Measuring spoons
• 🟥 See-through coloured cellophane: red, blue and green
• 🎞️ Clear plastic wrap: for the clear covers
• ✂️ Kid scissors
• 🩹 Tape
• 📏 A ruler

Steps

1. Fill twelve cups with soil and sprinkle a teaspoon of grass seed on each.🔊
2. Cover the seed with a thin layer of soil and give each cup two tablespoons of water.🔊
3. Cut sheets of red, blue and green cellophane and clear plastic wrap, twice as tall as a cup.🔊
4. Wrap three cups in red tubes, three in blue, three in green and three in clear, and tape them.🔊
5. Tape a piece of the same colour over each tube's top, so all its light comes through the colour.🔊
6. Stand all twelve cups on the same sunny windowsill.🔊
7. Every other day, lift each top and give every cup two tablespoons of water.🔊
8. On day 14, measure the tallest blade in each cup in millimetres.🔊
9. Write down how green and thick each cup looks, too.🔊
10. Find the mean height for each colour and draw a bar graph.🔊
11. Wash your hands after touching the soil.🔊

Every step is one you can do yourself, with a grown-up nearby. Write each number down straight away, in the table below, so nothing is lost.

Your results table

From 4th grade, add the tries for each level and divide by how many there were: that's the mean, and it goes in the last column. Then draw a bar graph, one bar for each thing you tried.

Why it works

🔊 Plants make their food from light, and green leaves soak up red and blue light best and bounce much of the green away, which is why they look green. Under red or blue covers the grass still gets light it can use well. Under green it gets less it can use, so it may stay short and pale, or stretch tall and thin looking for better light.

Going further (7th and up): Change a second thing: put a second set of all four colours on a shady shelf, and graph sunny and shady side by side.

Board layout

This project at your grade

Every grade this project fits works out a mean. See every project for 4th grade, or print a blank results table.

What a judge will ask

• What did you change, and what did you measure? I changed the colour of the light the grass gets and measured how tall the grass grows in 14 days, in mm.
• What did you keep the same, and why does that matter? I kept the same grass seed, a teaspoon in each cup and two tablespoons of water every other day the same, so the only difference was the colour of the light the grass gets.
• How many times did you test it? 3 times at each of my 4 levels.
• If you did it again, what would you change or test next? I'd put a second set of all four colours on a shady shelf, and graph sunny and shady side by side.

Make it yours

Keep this project on this device and Peabody counts down to your fair day, with a plan for each day. Nothing is sent anywhere.

Try next

• 🌱 Do plants bend toward the window?1st–5th grade · 12 days of testing · free🌦️ Growing and weather · 🌱 Outside and plants
• 🌱 Does planting a seed deeper make it slower to come up?3rd–8th grade · 14 days of testing · about $1🌦️ Growing and weather · 🌱 Outside and plants