Quick reference table
| Problem | Likely cause | Urgency | Solution summary |
|---|---|---|---|
| Not moving | Too cold, too dry, or resting phase | Low | Warm up, add moisture, wait 12-24 hours |
| Mold contamination | Bacterial or fungal spores on food or substrate | High | Transfer to clean plate immediately |
| Drying out | Low humidity, lid not sealed, agar depleted | Medium | Mist, seal container, transfer to fresh agar |
| Turning dark | Light exposure, stress, or sclerotium formation | Medium | Move to darkness, check conditions |
| Not eating | Wrong food, contaminated food, or recent feeding | Low | Try different food, replace with fresh oats |
| Sporulating | Light exposure, age, or environmental stress | Medium | Harvest spores or transfer healthy tissue |
| Escaping the container | Seeking moisture or food outside the dish | Low | Improve conditions inside, seal edges |
| Bad smell | Bacterial contamination of substrate or food | High | Transfer to fresh plate, remove rotting food |
| Slowing down or shrinking | Food depletion, old agar, or natural cycling | Medium | Fresh food, fresh substrate, patience |
Problem 1: Slime mold is not moving
What it looks like
The plasmodium is sitting on the agar without extending new veins. It may appear flat, pale, or slightly retracted compared to its last active state. No visible growth after several hours.
Possible causes
- Temperature too low. Below 15 °C, Physarum slows down dramatically. Below 10 °C, it may stop growing entirely.
- Just been transferred. After being moved to a new plate, slime mold often pauses for 2-6 hours while it adjusts to the new environment.
- Natural resting phase. Healthy slime mold sometimes pauses growth for several hours before resuming. This is normal behavior, not a problem.
- Substrate too dry. If the agar has dried out, the slime mold conserves moisture by reducing surface area and movement.
What to do
- Check the temperature. Move to a warmer location (20-25 °C) if needed.
- Mist the agar surface lightly with non-chlorinated water.
- Place a fresh oat flake 1-2 cm from the leading edge.
- Cover with foil or place in darkness.
- Wait 12-24 hours. In most cases, the slime mold resumes growth on its own once conditions improve.
Problem 2: Mold contamination
What it looks like
Fuzzy white, green, blue, or black patches on the agar, on old food, or near the slime mold. This is true mold (fungi like Penicillium, Aspergillus, or Trichoderma), not slime mold. It looks distinctly different: mold is fuzzy and cottony, while Physarum has smooth, vein-like structures.
Possible causes
- Old food (oats, fruits, vegetables) left in the dish too long
- Airborne mold spores settling on the moist agar
- Unclean equipment (unwashed hands, used Petri dishes, contaminated water)
- Oat flakes carrying mold spores from the bag
What to do
- Act quickly. Mold contamination spreads fast in warm, moist environments. Do not wait to see if it resolves itself.
- Transfer the slime mold. Using a clean spatula or tweezers, cut a section of clean, actively growing Physarum (as far from the contamination as possible) and move it to a fresh agar plate.
- Discard the contaminated plate. Seal it in a bag and dispose of it. Do not try to salvage it.
- Clean your workspace. Wipe surfaces with 70% isopropyl alcohol.
Prevention is easier than treatment
Remove uneaten food within 24-48 hours. Wash hands before handling cultures. Consider briefly microwaving oat flakes (10-15 seconds) before use to kill surface spores. Keep lids closed as much as possible.
Problem 3: Drying out
What it looks like
The agar surface looks dull, cracked, or shrunken. The slime mold may have pulled away from the edges, turned darker, or begun forming a hardened sclerotium.
Possible causes
- Lid not sealed properly (or not present)
- Room humidity too low (heated rooms in winter are especially dry)
- Agar plate has been in use for more than 1-2 weeks without refreshing
- Container is too large relative to the amount of agar (more air volume means faster evaporation)
What to do
- Mist the agar surface with non-chlorinated water. Add enough to re-moisten the surface but not flood it.
- Seal the lid with parafilm, plastic wrap, or a rubber band to reduce evaporation.
- If the agar is very old and dried out, transfer the slime mold to a fresh plate.
- In dry environments, place a damp paper towel inside the lid to act as a humidity reservoir.
Problem 4: Turning dark
What it looks like
The normally bright yellow plasmodium has turned orange, brown, or dark amber. In some cases, the texture changes from smooth and glossy to rough and papery.
Possible causes
- Light exposure. Prolonged light triggers pigmentation changes and can initiate sclerotium formation. This is the most common cause.
- Sclerotium formation. The organism is entering dormancy, either because of light, lack of food, or drying conditions. This is a natural protective response, not necessarily a problem.
- Stress. Temperature extremes, chemical exposure (cleaning products, chlorinated water), or physical disturbance can cause darkening.
- Age. Very large, old plasmodia sometimes develop darker coloration naturally.
What to do
- Move the dish to complete darkness immediately.
- Check that food and moisture are adequate.
- If the slime mold has fully formed a sclerotium, you can either store it or revive it on a fresh plate.
- If only partially darkened, improving conditions (darkness, food, moisture) usually reverses the color change within 24-48 hours.
Problem 5: Not eating
What it looks like
Food has been placed near the slime mold but it shows no interest. Oat flakes sit untouched while the slime mold grows in other directions or does not grow at all.
Possible causes
- Recently fed. A well-fed slime mold may not be interested in more food for several hours.
- Food too far away. The chemical signals from food are only effective within a few centimeters. Place the oat within 1-2 cm of the leading edge.
- Contaminated food. If oats have been sitting in the humid environment for too long, they may have begun to rot or grow mold, making them unappealing.
- Wrong food type. While Physarum eats many things, some foods are more attractive than others. Plain rolled oats are the most reliable food source.
What to do
- Remove old food and replace with fresh oat flakes placed close to the organism.
- Wait 12-24 hours. The slime mold may be in a rest phase.
- If using a food other than oats, switch to plain rolled oats as a test.
- Ensure the slime mold is not already forming a sclerotium (see Problem 4).
Problem 6: Sporulating
What it looks like
Small, dark, stalked structures growing upward from the plasmodium surface. These are sporangia (fruiting bodies), the reproductive structures of slime mold. They look like tiny mushrooms or ball-on-a-stick formations, ranging from 1-3 mm tall.
Possible causes
- Light exposure is the primary trigger for sporulation in Physarum polycephalum
- Combination of light, food depletion, and age
- Environmental stress (temperature fluctuations, chemical exposure)
What to do
- If sporulation has just begun (a few stalks): Transfer healthy yellow tissue (away from the sporulating area) to a fresh plate in complete darkness with fresh food. The transferred tissue usually resumes normal growth.
- If sporulation is widespread: The plasmodium may be past the point of recovery. You can collect the spores (let the sporangia mature and dry, then tap them into a container) for future germination, though growing Physarum from spores is significantly more difficult than maintaining an active culture.
- Prevention: Keep cultures in darkness or very dim light. The red light from a darkroom safelight does not trigger sporulation.
Problem 7: Escaping the container
What it looks like
Yellow veins creeping out from under the Petri dish lid, growing up the walls of the container, or appearing on the outside surface of the dish. Slime mold on the shelf, the table, or even neighboring containers.
Possible causes
- Seeking moisture. If the inside of the dish is too dry, the slime mold will explore outward looking for more humid environments.
- Seeking food. A hungry slime mold explores aggressively in all directions, including upward and outward.
- Natural behavior. Some Physarum strains are more exploratory than others. Even in ideal conditions, they may send scouting veins beyond the container edges.
What to do
- Gently scrape escaped material back into the dish or onto a fresh agar plate.
- Improve conditions inside the container: add food, increase humidity, ensure adequate substrate.
- Seal the container more tightly. Parafilm or plastic wrap around the lid edge prevents escape.
- Use a larger container if your culture has outgrown its current home.
Not dangerous
An escaped slime mold is not harmful to your home, furniture, or other organisms. It will simply dry out and die on non-moist surfaces within a few hours. It is a nuisance, not a hazard.
Problem 8: Bad smell
What it looks like
An unpleasant odor coming from the culture dish. Healthy Physarum has a mild, slightly mushroom-like smell. A bad smell indicates something has gone wrong.
Possible causes
- Rotting food. Old oats or other food left in a warm, moist environment will rot and produce strong odors.
- Bacterial contamination. Bacterial colonies growing on the agar produce various unpleasant smells.
- Dead slime mold. If the organism has died (from heat, chemicals, or severe contamination), the decaying biomass can smell.
What to do
- Open the dish in a well-ventilated area (not in a small room).
- Identify the source: is it the food, the agar, or the slime mold itself?
- If the slime mold is still alive and healthy-looking, transfer it to a clean plate with fresh agar and new food.
- If everything looks contaminated or dead, dispose of the culture (seal, bag, trash) and start fresh.
Problem 9: Slowing down or shrinking
What it looks like
The plasmodium is smaller than it was yesterday. Veins are thinner, the network is simpler, and growth seems to have stopped or reversed. The organism is alive but retreating.
Possible causes
- Food depletion. The slime mold has consumed all available food and is consolidating its network while waiting for more.
- Old substrate. After 1-2 weeks, agar loses moisture and nutrients. The substrate may no longer support vigorous growth.
- Natural cycling. Physarum goes through phases of expansion and consolidation. A period of shrinkage after rapid growth is normal.
- Environmental stress. Temperature changes, vibration, light, or chemical exposure can cause temporary retreat.
What to do
- Add fresh food immediately (2-3 oat flakes near the organism).
- If the agar looks old, dry, or depleted, transfer to a fresh plate.
- Check for environmental stressors (Is it near a window? A heater? A vibrating appliance?).
- Wait 24-48 hours. If food and conditions are good, the slime mold will usually resume growth.
When to start over
Sometimes a culture is beyond saving. Consider starting fresh if:
- Contamination has spread throughout the entire plate with no clean tissue remaining
- The slime mold has not responded to improved conditions for more than 72 hours
- The organism has turned completely black and brittle (as opposed to the firm, papery texture of a viable sclerotium)
- The smell is overwhelming despite transferring to clean plates
If you need a new culture, you can purchase a sclerotium from online suppliers, revive a stored backup, or find Physarum in the wild. For comprehensive growing instructions to prevent future problems, see our complete growing guide.