Damping off is a fungal disease that kills seedlings by attacking stems at soil level, causing them to collapse suddenly. Prevent it by using sterile seed starting mix, ensuring good drainage, providing air circulation, avoiding overwatering, and not planting too densely. There is no cure for affected seedlings—prevention is the only effective approach.
What Is Damping Off?
Damping off is caused by several soil-borne fungi:
- Pythium species
- Rhizoctonia solani
- Fusarium species
- Phytophthora species
These fungi attack seedlings at or below the soil line, causing stems to rot and plants to collapse.
Types of Damping Off
| Type | When It Happens | What You See |
|---|---|---|
| Pre-emergence | Before sprouting | Seeds rot, never emerge |
| Post-emergence | After sprouting | Seedlings collapse at soil line |
Recognizing Damping Off
Symptoms
| Symptom | Description |
|---|---|
| Thin, water-soaked stem | At soil level |
| Seedling falls over | Unable to support itself |
| Rapid collapse | Healthy one day, dead the next |
| White/gray fuzzy growth | Mold on soil surface |
| Cotyledons touch soil | Stem too weak to hold up |
What It Looks Like
- Seedling is growing normally
- Stem appears pinched or thin at soil line
- Seedling falls over
- Often multiple seedlings in same area affected
- Dead seedlings have rotted stems at base
Not Damping Off
| Condition | How It Differs |
|---|---|
| Leggy seedlings | Tall but still standing |
| Underwatering | Dry, wilting, recovers with water |
| Physical damage | Broken stem, not rotted |
| Nutrient deficiency | Yellow but standing |
Why Damping Off Happens
The Perfect Storm
Damping off occurs when:
- Fungal spores are present (in soil or environment)
- Moisture is high (wet conditions)
- Air circulation is poor (stagnant)
- Temperatures favor fungus growth
Risk Factors
| Risk Factor | Why It Contributes |
|---|---|
| Non-sterile soil | Contains fungal spores |
| Overwatering | Creates wet conditions fungi love |
| Poor drainage | Water sits, roots stay wet |
| No air circulation | Humidity stays high |
| Dense planting | Increases humidity, spreads disease |
| Reused containers | May harbor spores |
| Cold, wet conditions | Fungi thrive, plants struggle |
Prevention (Most Important)
There is no cure for damping off—prevention is your only defense.
1. Use Sterile Seed Starting Mix
Not garden soil. Not potting soil from questionable sources.Seed starting mix:
- Sterile (pathogen-free)
- Well-draining
- Light texture
2. Ensure Good Drainage
Every container needs:
- Drainage holes
- Not sitting in water
- Well-draining mix
- Elevation (allow air under containers)
3. Don't Overwater
| Watering Approach | Damping Off Risk |
|---|---|
| Keep constantly wet | High |
| Water when dry | Low |
| Bottom water | Lower (leaves stems dry) |
4. Provide Air Circulation
Options:
- Small fan on low
- Open vents in greenhouse
- Don't crowd seedlings
- Remove humidity domes after germination
5. Remove Humidity Domes
After seeds germinate:
- Remove plastic covers
- Increase air circulation
- Reduce humidity
Humidity domes are for germination, not growing.
6. Don't Plant Too Densely
| Spacing | Damping Off Risk |
|---|---|
| Seeds touching | High |
| 1" apart | Medium |
| 2" apart | Lower |
| One per cell | Lowest |
7. Use Clean Containers
If reusing:
- Wash with soap and water
- Sanitize with 10% bleach solution
- Rinse thoroughly
- Dry completely
8. Thin Seedlings
After germination:
- Keep one seedling per cell
- Cut (don't pull) extras at soil line
- Reduces humidity and competition
Treatment Options
For Affected Seedlings
Unfortunately: Once a seedling has damping off, it cannot be saved. Remove immediately:- Prevents spread to other seedlings
- Remove affected soil too
- Dispose (don't compost)
For Remaining Seedlings
- Increase air circulation immediately
- Reduce watering (let soil dry more)
- Remove humidity covers
- Space out seedlings if possible
- Consider preventive treatment (see below)
Preventive Treatments
| Treatment | How It Works |
|---|---|
| Chamomile tea | Antifungal properties |
| Cinnamon | Antifungal, sprinkle on soil |
| Hydrogen peroxide | 3%, diluted, antifungal |
| Copper fungicide | Chemical prevention |
- Brew chamomile tea
- Let cool completely
- Use to water seedlings
- Repeat weekly
- Lightly dust soil surface
- Especially around stems
- Reapply after watering
Environmental Control
Temperature
| Soil Temp | Effect on Damping Off |
|---|---|
| Below 60°F | Higher risk (seedlings weak, fungi active) |
| 65-75°F | Lower risk (optimal for seedlings) |
| Above 80°F | Lower risk (too warm for some fungi) |
Light
Good lighting helps because:- Stronger seedlings resist disease
- Helps soil dry appropriately
- Reduces humidity at surface
Humidity
| Humidity Level | Risk |
|---|---|
| Very high (>80%) | High |
| Moderate (50-70%) | Medium |
| Lower (<50%) | Lower |
Starting Over After Damping Off
If you lose seedlings to damping off:
- Don't reuse the soil — Contains spores
- Clean containers — Sanitize thoroughly
- Start fresh — New sterile mix
- Improve conditions — Fix what caused it
- Try again — Time permitting
Species Susceptibility
More Susceptible
- Tomatoes
- Peppers
- Basil
- Impatiens
- Marigolds
- Most fine-seeded plants
Less Susceptible
- Squash/cucurbits
- Beans
- Nasturtiums
- Plants with large seeds/sturdy seedlings
FAQ
What causes damping off in seedlings?
Soil-borne fungi (Pythium, Rhizoctonia, Fusarium) that attack seedling stems at soil level. They thrive in wet, poorly drained conditions with stagnant air.
Can you save seedlings with damping off?
No. Once the stem is rotted, the seedling cannot recover. Focus on prevention and saving unaffected seedlings by improving conditions immediately.
How do you prevent damping off naturally?
Use sterile soil, don't overwater, provide air circulation, don't plant too densely, and remove humidity domes after germination. Chamomile tea and cinnamon are natural antifungals.
Why are my seedlings falling over at the stem?
If the stem is thin and rotted at soil level, it's damping off. If the stem is thin but intact (leggy), it's insufficient light. The key difference is whether the stem is healthy or rotted.
Should I use fungicide to prevent damping off?
For home gardeners, good practices (sterile soil, proper watering, air circulation) are usually sufficient. Fungicides are available but rarely necessary if conditions are right.
Can damping off spread to other plants?
Yes, through contaminated soil, water splash, and shared containers. Remove affected seedlings immediately and improve conditions for remaining plants.
The Bottom Line
Preventing damping off:
- Sterile seed starting mix — Non-negotiable
- Don't overwater — Let soil dry between waterings
- Air circulation — Fan or open area
- Good drainage — Holes and well-draining mix
- Don't crowd — Space seedlings appropriately
- Remove domes — After germination
There is no cure—only prevention. Create conditions that favor seedlings, not fungi.
Related: Seed Starting Indoors Guide | Best Soil for Seed Starting