“Hardening off” is the process of preparing tender indoor-grown seedlings for the significant environmental changes they will face outdoors, including wind, direct sunlight, and temperature shifts. Moving plants directly from the controlled environment of a home or greenhouse outside often results in “transplant shock,” which can stunt growth or even kill the young plants. The goal is to safely and quickly condition the seedlings to survive the harsher elements without compromising the plant’s long-term health.
The Standard Hardening Process
The traditional method of acclimatizing seedlings typically spans between seven and fourteen days. This schedule begins by placing the seedlings outdoors for a very short duration, usually one to two hours, in a sheltered area protected from direct sun and wind. Start this process when the outdoor temperature reaches at least 45 to 50 degrees Fahrenheit.
Each subsequent day, the time the seedlings spend outside is gradually increased by an hour or more, while slowly exposing them to more intense conditions. This gradual exposure allows the plant to develop a thicker leaf cuticle, which slows down water loss and protects the plant from drying out. By the end of the period, the plants are able to remain outdoors all day and night, ready for planting.
Techniques for Accelerated Conditioning
To reduce the hardening period to five to seven days, the gardener must introduce controlled stressors to prompt the plant’s defense mechanisms sooner. One effective technique is to slightly reduce the water supply a few days before starting outdoor conditioning, but never to the point of wilting. This minor water stress encourages the development of a more robust root system and helps the plant prepare for higher transpiration rates outside.
Increasing air circulation can be accomplished by placing a small fan on the seedlings indoors for several hours a day. This simulated wind causes the stems to sway, prompting the plant to build thicker, stronger stalks, a process known as thigmomorphogenesis.
When moving the seedlings outside, utilizing environmental aids like cold frames or cloches can compress the adjustment period by offering protection from severe temperature swings. A temporary shade cloth can also be used to allow for quicker exposure to full sun while mitigating the risk of sun scorch. The use of high-intensity LED grow lights indoors, especially those with UV light output, may also contribute to a faster transition, as the plants are already accustomed to higher light intensity.
Monitoring for Stress and Identifying Damage
Because accelerated hardening increases the risk of damage, close monitoring for visual cues is necessary to prevent permanent harm. The most common sign of light-related stress is sun scald, which appears as bleached, white, or silvery discoloration on the leaves. If sun scalding occurs, the seedlings must be immediately moved to a deeper shade location or brought inside to recover.
Another indication of shock is cold damage, which manifests as purple stems or leaf margins, particularly in plants sensitive to low temperatures like tomatoes. If an unexpected temperature drop is forecast, the plants should be sheltered immediately, perhaps in a cold frame or brought back indoors entirely.
Severe, non-recovering wilting indicates the plant is struggling with water loss and needs immediate deep shade and watering. Allowing the plants a few days to rest indoors after any sign of damage can help them recover before resuming outdoor exposure.
Post-Hardening Transplanting and Immediate Care
Once the hardening process is complete, the final transplanting requires specific actions to ensure long-term success. The ideal time for planting seedlings into the garden is during the late afternoon or early evening, or on an overcast day, to avoid the intense sun and heat of midday. This timing allows the young plant several hours of cooler temperatures and lower light to begin establishing its roots before facing the next day’s sun.
When moving the seedling from its pot, minimizing root disturbance is important to lessen the impact of the move. After placing the seedling into the prepared garden bed, a deep and immediate watering is necessary to settle the soil around the roots and eliminate air pockets. Avoid applying high-nitrogen fertilizer immediately after transplanting, as the plant is focused on root establishment and recovering from conditioning stress, not rapid leaf growth.