What Do Plants Need to Grow?

Overview

Every gardener and farmer knows that plants need sun, water, and soil. But young children don't know this — to them, plants just appear. This experiment makes the invisible visible by growing the same seeds under different conditions and watching what happens. One plant gets everything. One gets no light. One gets no water. The differences are dramatic and unmistakable, and your child will discover through observation — not lecture — exactly what plants need to thrive.

The Question

Do plants need sunlight and water to grow? What happens if we take one away?

Background

Plants need three things from their environment to grow: light (for photosynthesis), water (for nutrient transport and cell function), and soil (for nutrients and physical support). Temperature matters too, but at room temperature this isn't a variable.

This experiment tests light and water as independent variables. We keep soil constant across all pots. This is a genuine controlled experiment, simplified for a 3-4-year-old.

For your child, the core concept is: Plants are alive and have needs — just like people. If they don't get what they need, they struggle.

Hypothesis

Before setting up, ask your child to predict:

"We're going to grow some plants. Some will get sunshine and water. Some won't get sunshine. Some won't get water. What do you think will happen?"

Write down (or remember) their prediction. It doesn't matter if it's right — the act of predicting is the skill.

Some common predictions from 3-4-year-olds:

• "They'll all grow!" (Optimistic — plants are invincible)
• "The ones without water will die." (Often correct — kids understand thirst)
• "The dark one won't grow." (Sometimes — light is less intuitive than water)

Materials

See frontmatter list. The key is having at least 4 identical pots/cups with the same soil and the same type of seed, so the only thing that changes between groups is the condition you're testing.

Procedure

Setup (20 minutes)

Step 1: Prepare the pots.

Help your child fill all 4 pots with the same amount of soil (about 3/4 full).

Step 2: Plant the seeds.

Put 1-2 bean seeds in each pot, about 1 inch deep. Cover with soil and pat gently.

"We're planting the exact same seeds in the exact same soil. Everything is the same right now."

Step 3: Label the pots.

Write on popsicle sticks (or let your child decorate them):

"This is the most important part: we're going to give each plant something different. This one gets sun AND water. This one gets sun but NO water. This one goes in the dark but gets water. This one goes in the dark with no water."

Step 4: Water the first time.

Water Pot A and Pot C. Do NOT water Pot B or Pot D.

"Okay. Let's see what happens!"

Step 5: Place the pots.

• Pots A and B go on the sunny windowsill.
• Pots C and D go in a cupboard or closet (somewhere dark — check that it's truly dark when closed).

Experiment: Daily Check-Ins (5 minutes/day for 7-10 days)

Every day, at the same time (attach it to a routine — after breakfast works well):

1. Check Pot A (sun + water): "Let's look at our sunny, watered plant. Do you see anything?" Water it if the soil feels dry.

2. Check Pot B (sun + no water): "This one doesn't get water. Does it look different?" Touch the soil — it should be getting dry.

3. Check Pot C (dark + water): Open the cupboard. "This one has been in the dark. What does it look like?" Water it if needed, then put it back in the dark.

4. Check Pot D (dark + no water): "This one has nothing — no sun, no water. What's happening?"

5. Compare: "Which one looks the healthiest? Which one looks the saddest?"

What you'll likely see:

• Days 1-3: Not much visible difference. Seeds are germinating underground.
• Days 3-5: Pot A (sun + water) sprouts first. Green and sturdy.
• Days 4-7: Pot C (dark + water) may sprout, but the seedling will be pale yellow/white and leggy — reaching for light that isn't there. This is incredibly dramatic and visible.
• Days 5-10: Pot B (sun + no water) may sprout weakly but will quickly wilt. Pot D will likely show little to no growth.

Record (2 minutes during each check-in)

For 3-year-olds: Keep it verbal. "Pot A is green and tall. Pot C is yellow and bendy. Tell me which one looks best."

For 4-year-olds: Make a simple chart on paper with 4 columns (one per pot). Each day, your child can draw what they see — a small green line, a droopy line, nothing. Stick figure plants are perfect.

Alternatively, take a daily photo of all four pots side by side. Scrolling through the photos at the end makes the change over time vivid.

Analysis

After 7-10 days, bring all four pots to the table together. Look at them side by side.

"Let's look at all our plants together. Which one grew the best?"

Guide the conversation:

• "Pot A had sun AND water. How does it look?" (Healthy, green, tall)
• "Pot B had sun but no water. What happened?" (Wilted, dried, maybe dead)
• "Pot C had water but no sun. What happened?" (Pale, yellow, weak)
• "Pot D had no sun AND no water. What happened?" (Nothing grew, or dead)

"So what do plants need?"

Let your child tell you. Guide them to: sun AND water. They need BOTH.

The Explanation

For your child:

"Plants need sunlight to make their food — the light helps them turn green and grow strong. And they need water, just like you need water to drink. Without sun or water, they can't grow. They need both!"

Point to Pot C: "Look at this one — it had water but no sun. It tried to grow, but it's all pale and wobbly because it couldn't make its green food without sunshine."

For you (the parent):

This experiment demonstrates that light drives photosynthesis (converting CO2 and water into glucose) and water is essential for nutrient transport and cell turgor. Pot C is particularly instructive — etiolated seedlings (pale, elongated) are the plant's attempt to find light by growing taller, sacrificing structural integrity. This is a visible survival strategy.

Extensions

• Temperature test: Put one pot in the fridge (cold + dark). Does the cold slow growth even more?
• Soil vs. no soil: Can a bean grow on a wet paper towel? (Yes, initially — seeds contain stored energy. But it won't grow for long without soil nutrients.)
• Rescue experiment: After the observation period, move Pot C (dark + water) to the windowsill. Will it recover? (Often yes — the plant will green up within days. This is a beautiful lesson in resilience.)
• Outdoor version: Plant seeds in a sunny garden patch and a permanently shaded area. Observe over weeks.

Safety Notes

• Soil handling: Potting soil is not food. Supervise children to prevent ingestion. Wash hands after every soil interaction.
• Mold risk: Pots in dark, damp conditions (Pot C) may develop mold. If white or green fuzz appears on the soil, point it out as a science observation ("Mold is a tiny living thing that likes dark, wet places") but don't let your child touch it. Discard the pot if mold is extensive.
• Seeds are not snacks: Bean seeds, especially raw lima beans, should not be eaten. Keep seeds accounted for and away from mouths.
• Water spills: Daily watering at child height will lead to spills. Keep a towel handy. Use small watering containers to limit volume.
• Cupboard access: If the dark location is a cupboard with cleaning supplies, choose a different location. The pot should be in a safe, chemical-free dark space.