Plants need light to make food, but the amount of light can change how they grow. In this investigation, you will compare similar plants grown in a bright location and a shaded location while keeping other conditions as similar as possible.
What you will learn
By completing this investigation, you can:
- Make a fair comparison between plants.
- Measure plant growth using numbers and observations.
- Identify variables that can affect an experiment.
- Create a simple table and graph.
- Explain why results may differ from your prediction.
Light is only one factor in plant growth. Water, temperature, soil, pot size, seed type, and planting depth also matter. The goal is not to prove that one location is always best for every plant. Instead, the goal is to find out what happens to the particular plants and conditions you tested.
Question and prediction
Start with a testable question:
How does growing in light compared with growing in shade affect plant growth?
A prediction is an educated guess about what you think will happen. For example:
I predict that the plants in bright light will grow stronger and develop greener leaves, while the plants in shade may grow taller but have paler stems and leaves.
This prediction includes an important possibility. A shaded plant may become tall as it stretches toward light, but it may not be as sturdy as a plant receiving more suitable light. Your results might be different, and that is useful information. A scientific investigation does not fail when the prediction is wrong.
Decide what “growth” will mean before beginning. You could compare:
- Stem height in centimeters.
- Number of leaves.
- Leaf color.
- Stem thickness or strength.
- Number of new leaves produced.
- Overall appearance, such as upright, leaning, weak, or healthy.
Height is easy to measure, but it should not be your only measurement. A plant that is taller is not necessarily healthier.
Materials
You will need:
- At least six identical pots or cups with drainage holes.
- Seeds of the same type, such as bean, pea, radish, or fast-growing herb seeds.
- The same kind of potting soil.
- A ruler marked in centimeters.
- Water and a measuring cup or syringe.
- Labels and a pencil or waterproof marker.
- A tray to catch extra water.
- A notebook or data sheet.
- A bright growing location.
- A shaded growing location.
- Optional: a kitchen scale, camera, or light meter app.
Using six plants is better than using only two. You can place three plants in the light group and three in the shade group. Repeated plants make the comparison more reliable because one seed may be unusually large, damaged, or slow to sprout.
Choose a safe setup. Do not place plants where water could reach electrical equipment, and do not leave them in a dangerously hot car, greenhouse, or closed window area. If you use a grow lamp, follow the lamp manufacturer’s safety instructions and ask an adult for help.
Plan a fair test
The independent variable is the condition you change: the amount or location of light. The dependent variables are the measurements you collect, such as height and leaf number.
Keep these conditions the same for both groups whenever possible:
- Seed type and age.
- Number of seeds per pot.
- Pot size.
- Soil type and amount.
- Planting depth.
- Amount of water.
- Watering schedule.
- Temperature.
- Length of the investigation.
- Measuring method.
Label the pots clearly. For example, label three pots “Light 1,” “Light 2,” and “Light 3.” Label the other three “Shade 1,” “Shade 2,” and “Shade 3.” If you are testing several types of plants, keep each type in a separate comparison rather than mixing them into one group.
The light group should receive more direct or usable light than the shade group, but avoid changing several conditions at once. For example, do not put the light group in a warm room and the shade group in a cold basement if your question is about light. If the locations have different temperatures, record that limitation.
Setting up the experiment
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Prepare the pots. Fill each pot with the same amount of soil. Leave a little space at the top so water does not spill over.
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Plant the seeds. Plant the same number of seeds in each pot at the same depth. Use the seed packet directions when available. If you plant several seeds in one pot, space them evenly.
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Water equally. Give each pot the same measured amount of water. If the soil is already wet, wait rather than adding water simply to follow a schedule.
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Place the pots. Put three labeled pots in the bright location and three in the shaded location. Keep the pots away from extreme heat or cold.
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Rotate carefully, if needed. Plants near a window may receive different light on different sides. You can rotate all pots in a group by the same amount at each observation, or rotate their positions within the same group. Do not move a shade pot into bright light or a light pot into shade.
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Record the start date. Note the seed type, locations, temperature if known, and any other details that could affect the result.
If seeds are still underground, do not dig them up to check them. Record when each seed first appears above the soil. This is called the germination date and can be another useful comparison.
Collecting data
Measure the plants at the same time of day, such as after school, because stems can change slightly during the day. Begin measuring when a seedling appears, or begin on the same date for every pot if that is more practical.
To measure height, place the zero end of the ruler at the soil surface next to the stem. Measure to the highest growing point, not necessarily the tip of a bent leaf. Try to use the same method every time. Do not press the ruler into the soil or pull the plant upright while measuring.
A data table could look like this:
| Day | Light 1 height | Light 2 height | Light 3 height | Shade 1 height | Shade 2 height | Shade 3 height |
|---|---|---|---|---|---|---|
| 0 | 0 cm | 0 cm | 0 cm | 0 cm | 0 cm | 0 cm |
| 3 | 2.1 cm | 1.8 cm | 2.4 cm | 2.7 cm | 2.3 cm | 2.5 cm |
| 6 | 4.0 cm | 3.7 cm | 4.2 cm | 5.1 cm | 4.8 cm | 5.0 cm |
| 9 | record | record | record | record | record | record |
The numbers in the first rows are examples of how to organize a table, not expected results. Replace them with your own measurements.
Record observations as well as measurements. You might write:
- “Light group has dark green leaves.”
- “Shade group is leaning toward the window.”
- “One plant has a damaged leaf.”
- “Soil in the bright location dried faster.”
- “Two seeds in Shade 2 have not sprouted.”
Take photographs from the same distance and angle if possible. A ruler placed beside the plants can help show scale. Avoid using photographs as your only measurement because perspective can make one plant look larger or smaller.
Managing water and other conditions
Check the soil before watering. The top layer may feel dry even when deeper soil is still moist. Use the same rule for every pot, such as watering when the top centimeter feels dry. Measure the water so that one group does not accidentally receive more.
If bright plants dry out more quickly, giving them additional water may be necessary to keep them alive, but it changes the experiment. Record the extra water and explain that water availability was not perfectly identical. In a later investigation, you could test light while keeping soil moisture equal with smaller, more frequent measured amounts.
Do not add fertilizer to only one group. If fertilizer is needed, use the same product, amount, and schedule for every pot. Also watch for pests, mold, or overcrowding. If seedlings are packed tightly together, they may compete for light, water, and nutrients. Follow the seed packet instructions about thinning, and record any seedlings that you remove.
Analyzing the results
At the end of the investigation, compare the groups rather than focusing on only one plant. Calculate the average height for each group:
Average height = total height of the group ÷ number of plants measured
For example, if three light-grown plants measure 8 cm, 9 cm, and 10 cm, their average height is 9 cm. You can calculate the average number of leaves in the same way.
Make a graph to show the pattern over time. A line graph works well for height because measurements are collected on different days. Put time on the horizontal axis and height on the vertical axis. Draw one line for the average light-group height and another for the average shade-group height.
You can also create a comparison table:
| Feature | Light group | Shade group |
|---|---|---|
| Average final height | ||
| Average number of leaves | ||
| Leaf color | ||
| Stem appearance | ||
| Plants that survived | ||
| Important observations |
Look for patterns. Did one group grow taller? Did one group develop more leaves? Were the leaves different colors? Did any plants lean, wilt, or develop thin stems? A result is stronger when several plants show a similar pattern.
Explaining what happened
Plants use light to make sugars through photosynthesis. When a plant receives too little light, it may stretch toward a brighter area. This can produce a long, thin stem and leaves that appear paler. A plant in suitable light may have a shorter, sturdier stem and greener leaves, although too much intense light can also cause stress, wilting, or scorched leaves.
The phrase “light group” does not automatically mean “healthy group.” A bright windowsill may be hot, and a shaded place may still receive enough indirect light. The plant species also matters. Some plants naturally prefer strong light, while others grow well in partial shade.
Use cautious language when writing your conclusion. Instead of saying, “Shade always makes plants unhealthy,” write, “Under the conditions in this investigation, the shade group had thinner stems and paler leaves than the light group.” This statement matches the evidence and does not claim more than the experiment tested.
Troubleshooting
No seeds sprouted. The seeds may be old, planted too deeply, kept too wet, or exposed to an unsuitable temperature. Check the seed packet and try a few extra seeds in a separate backup pot.
One plant is much larger than the others. This can happen because seeds vary naturally. Keep recording it, but compare group averages and mention the unusual plant in your notes.
The plants fell over. They may be stretching toward light, overwatered, or growing in loose soil. Record the problem. Do not tie plants upright unless you apply the same support to every group.
Mold appeared on the soil. Airflow may be poor and the soil may be too wet. Ask an adult for help, improve ventilation safely, and avoid touching mold. If the pot is badly affected, mark it as damaged rather than quietly replacing it.
The light and shade locations were not equally warm. Measure or describe the temperature difference if possible. In your limitations, explain that temperature may have affected the result along with light.
The plants grew too large for the pots. End the test early or transplant all plants using the same method. Record the change because transplanting can temporarily affect growth.
Alternatives and extensions
If outdoor space is unavailable, use a bright indoor window and a location several feet away from the window. You can also compare indirect light with a safe grow lamp. Keep the lamp distance, daily lighting time, and watering schedule consistent.
For a shorter investigation, compare germination time, first leaf appearance, or growth over seven days. For a longer investigation, continue for two to four weeks and measure new leaves, stem thickness, and final fresh mass. Ask an adult before cutting or weighing plants, and use the same procedure for every plant.
Other questions you could investigate include:
- Does the color of light affect seedling growth?
- Does a plant grow differently with morning light and afternoon light?
- How does partial shade compare with complete shade?
- Does changing the amount of water affect plants in bright light differently from plants in shade?
- Which of several plant species grows best in the same light condition?
Change only one main factor in each new experiment. If you test light color, keep brightness, temperature, water, and plant type as similar as possible.
Limitations and safety
This investigation cannot show exactly what would happen to every plant in every environment. A small number of pots may not represent all plants, and a window can change in brightness during the day or across seasons. Soil moisture, temperature, seed quality, pests, and accidental differences between pots can influence the measurements.
Plants and soil should not be eaten unless they were grown specifically as food under appropriate conditions. Wash your hands after handling soil, keep water away from electrical devices, and ask an adult for help with grow lamps, cutting plants, or disposing of moldy material. Careful notes about these limitations make your investigation more scientifically honest and more useful to someone who wants to repeat it.