Hardening Off Seedlings

What is Hardening off?

 

Hardening off is the process of gradually acclimating indoor-grown seedlings to outdoor growing conditions before transplanting. Seedlings raised indoors are not prepared for direct sunlight, wind, or fluctuating temperatures. A transition period allows plants to adjust physiologically and structurally.

Why is hardening off necessary?

 

Indoor seedlings develop under controlled environments:

  • Low light intensity
  • Stable temperatures
  • High humidity
  • No wind exposure

When moved outdoors abruptly, plants may experience:

  • Leaf scorch (sunscald)
  • Wilting
  • Stem breakage
  • Transplant shock
  • Growth delay

Hardening off strengthens cell walls, thickens leaf cuticles, and improves stress tolerance.

When should hardening off begin?

 

Begin the process:

  • 7–10 days before transplanting
  • After seedlings develop 2–4 true leaves
  • When outdoor temperatures approach crop tolerance levels

Avoid starting during extreme heat, cold, or windy conditions.

How do you harden off seedlings?

 
Gradual exposure schedule:
Days 1–2
  • Place outdoors 1–2 hours
  • Provide shade and wind protection
Days 3–5
  • Increase to 3–6 hours
  • Introduce morning sun
Days 6–7
  • Extend to most of the day
  • Reduce protection
Final days
  • Leave outdoors overnight if temperatures are safe

This progression allows plants to adjust without stress injury

What temperatures are safe?

 

General minimum nighttime thresholds:

Cool-season crops: ~45°F
Warm-season crops: ~55°F

Avoid frost and chilling injury, particularly for tomatoes, peppers, cucurbits, and basil.

How should sunlight exposure be managed?

 

Sunlight must be introduced gradually. Indoor foliage is highly sensitive to ultraviolet radiation.

Too much sun too quickly can cause:

  • Bleaching
  • Necrosis
  • Leaf drop

Begin with filtered or morning sun, progressing to full exposure.

What happens if hardening off is skipped?

 

Skipping hardening off may result in:

  • Transplant shock
  • Stunting
  • Sunburn
  • Reduced vigor
  • Lower yields
  • Plant mortality

Even when plants survive, establishment may be delayed.

Can hardening off occur in greenhouses?

 

Yes. This is often called environmental conditioning and includes:

  • Lowering greenhouse temperatures
  • Increasing ventilation
  • Reducing irrigation
  • Removing shade

These adjustments simulate outdoor stress gradually.

For more information, please see links below:

 
→  Hardening Transplants – Penn State Extension
→  Hardening Transplants – Nebraska Extension in Lancaster County
Hardening Off Seedlings

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