What happens to your pumpkin after Halloween? Instead of tossing it into the trash, turn it into a fascinating life science experiment! This project lets kids explore the science of decomposition, discover the role of microbes in recycling, and make real-world connections to microbiology.
What is Pumpkin Decomposition?
Pumpkin decomposition is the natural process of a pumpkin breaking down once it is no longer living. It’s powered by microbes like mold, bacteria, and fungi. Kids can watch this process unfold week by week with just a leftover pumpkin or two.
🔎 Pair this activity with our Microbiology Science Fair Experiments to see how bacteria, fungi, and other tiny organisms break down organic material. Don’t miss the free printable observation journal at the end to track your pumpkin’s journey!
Science Concepts Explored
- Biology: Decomposition and the role of fungi, bacteria, and insects.
- Life Science: Cycles of matter, nutrient recycling in ecosystems.
- Environmental Science: Composting and soil health.
- Microbiology: Microorganisms such as mold, bacteria, and yeast as decomposers.
Grade Level: K–5 (younger kids can observe and sketch, older kids can track, measure, and compare).
Pumpkin Decomposition Experiment Supplies
1–3 pumpkins: (optional testing): Carved pumpkin (fully open to air). Uncarved pumpkin (sealed skin, slowest to rot). Uncarved pumpkin with top removed (middle ground—faster than whole, slower than carved)
- Garden space, flowerpot, or a large clear bin with soil
- Notebook or [printable observation journal] (link to your opt-in/freebie)
- Magnifier or microscope (optional)
- Camera (optional, for before/after photos)
- Gloves (for handling as it softens)
How to Set Up the Rotting Pumpkin Experiment
Step 1: Choose Your Pumpkins
Try all three setups (carved, uncarved, and top removed) to compare how exposure changes the rate of decomposition.
Step 2: Pick a Spot
Place pumpkins outside in soil (garden bed, pot, or yard). For close-up observation, bury part of each pumpkin in soil inside a clear bin.
Step 3: Observe Regularly
Check your pumpkins daily or weekly. Look for mold, color changes, sagging skin, and even insects.
Step 4: Record Data
Draw or write what you see in your observation journal. Take photos to create a time-lapse slideshow.
Step 5: Try a Microscope Extension (Optional)
Carefully scrape a small sample of mold (using gloves and under supervision) and examine it under a magnifier or microscope. Sketch what you see.
Step 6: Wrap-Up Discussion
After weeks, the pumpkins will collapse into the soil. Compare which one decomposed the fastest and why.
The Science Behind Pumpkin Decomposition
Decomposition is the process of breaking down once-living things into smaller parts. Microbes (tiny living organisms like fungi, bacteria, and yeasts) play the biggest role.
- Fungi and mold appear first, covering the pumpkin’s surface.
- Enzymes break down the pumpkin and release its nutrients.
- Bacteria and yeast move in next, breaking down sugars and fibers.
- Insects and worms may join in, speeding the process.
This microbial teamwork is called succession, where different microbes arrive at different stages of decomposition. Eventually, the pumpkin is recycled back into the soil, providing nutrients for future plants.
🔎 Connection to Microbiology: This experiment is a real-world example of how microbes drive nutrient recycling in nature!
NGSS Guiding Questions
- K–2: What happens to plants and animals when they die? (K-LS1-1)
- 3–5: How do living things get the materials they need for growth and survival? (5-LS2-1)
- 3–5: How do decomposers connect plants, animals, and the environment in a cycle of matter? (5-PS3-1, 5-LS1-1)
Fun Facts About Rotting Pumpkins
- Pumpkins are ~90% water, which speeds up decay.
- Mold is a fungus that spreads through microscopic spores.
- Microbes are everywhere—even if you can’t see them, they’re working!
- Decomposition is nature’s recycling system.
Extension Ideas for Pumpkin Decomposition
Make It a Pumpkin Mold Science Experiment
🔎 Encourage kids to form a hypothesis, choose variables, and test it! Learn more about the scientific method here.
Possible questions:
- Does mold appear faster indoors or outdoors?
- Does a carved pumpkin decompose faster than a whole one?
- How does soil type (garden, sand, potting soil) affect decomposition?
Safety Notes for Rotting Pumpkin Experiments
- Always wear gloves when handling moldy pumpkins.
- Wash your hands after observing.
- Do not eat or taste any pumpkin used for the experiment.
- If using a microscope, keep mold samples small and discard them safely.
Free Pumpkin Decomposition Printable Journal
Download your Pumpkin Decomposition Observation Journal to extend the activity.
Related Science Activities
- Pumpkin Jack Rotting Science Experiment
- Microbiology Science Fair Experiments
- Bread Mold Science Experiment
- Fall Life Science Activities for Kids
- Fall Plant Science
- Fall Weather Science
Printable STEM Project Lesson Packs
Visit our SHOP for easy-to-print STEM and science lesson plans for the whole year. Or join us in the Little Bins Club.
Pumpkin Decomposition FAQs
How long does it take for a pumpkin to decompose?
A carved pumpkin can start rotting within a few days and may fully collapse in 2–3 weeks, especially in warm, wet conditions. An uncarved pumpkin can last much longer, sometimes months, because the skin protects it.
Why does mold grow on pumpkins?
Mold is a type of fungus that spreads through tiny spores in the air. Once the pumpkin’s skin is cut or broken, moisture and air get inside, and mold quickly begins to grow and feed on the pumpkin.
Which decomposes faster: a carved pumpkin or a whole pumpkin?
A carved pumpkin decomposes much faster because it’s exposed on all sides. A whole pumpkin takes longer since the skin acts as a protective barrier. If you remove just the top, decomposition happens faster than whole but slower than carved.
What microbes help pumpkins rot?
Microbes like fungi (mold), bacteria, and yeast are the main decomposers. They break down the pumpkin’s sugars, fibers, and water, returning nutrients to the soil.
Can I use this as a science fair project?
Yes! Kids can compare carved vs. uncarved pumpkins, test how temperature affects decomposition, or observe how pumpkins rot in soil versus open air. This makes a great microbiology-inspired science fair project.
