By slowly exposing tender plant seedlings to wind, sun, and rain, hardening off makes them stronger so they lose less water when they’re out in the elements. This helps keep seedlings from getting transplant shock, which is when they sit still, grow slowly, or die after being exposed to sunlight and sudden changes in temperature. Read on to learn how to harden off seedlings.
Hardening off is the process of gradually acclimating tender indoor plants to the harsher outdoor growing conditions. It involves slowly exposing seedlings or transplants to sun, wind, and fluctuating temperatures over 7-10 days before permanently moving them outside. Skipping this important step can seriously damage or even kill unhardened plants.
What Exactly is Hardening Off?
Hardening off serves two key purposes:
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It toughens the plant’s cell structure through controlled stress to make it more resilient.
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It allows the plant to build internal protections against sunburn and temperature shock.
Here’s a quick overview of the hardening off process
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Place seedlings in a sheltered, shaded spot outdoors for a few hours, then bring back inside.
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Each day, gradually increase the time spent outside by 1-2 hours.
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After 3-4 days, begin giving plants morning sun, then back to shade.
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Over 7-10 days, slowly expose to more sun, wind, and temperature changes
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Transplant hardened off plants to the garden on a mild, cloudy day if possible.
Without this gradual transition period, tender indoor plants will suffer great distress when thrust into the full force of the outdoor environment.
Sunscald
One of the biggest threats to unhardened plants is sunscald – leaf burn caused by sudden, intense exposure to direct sunlight. Indoor seedlings grow under lower light levels and have thin, delicate leaves adapted to that environment. Their photosynthetic mechanisms are not prepared to handle the solar energy bombardment of the midday sun.
Within hours, the leaves will take on a bleached, yellowish appearance. Soon dark brown necrotic lesions develop. The leaf margins shrivel and the blades become brittle.
Severely sunscalded leaves often die and fall off completely. The setback in photosynthetic capacity dramatically slows growth. Weakened plants are also more vulnerable to pests and diseases.
However, if hardened off gradually, the plant has time to thicken its leaf cuticles, produce more protective pigments, and ramp up antioxidant activity. This allows it to cope with intense ultraviolet and infrared radiation without incurring damage.
Freezing Injury
Getting caught off guard by a sudden cold snap can spell disaster for plants that haven’t been properly acclimated through hardening off.
Prolonged exposure to chilling temperatures causes intracellular ice crystals to form, disrupting cell membranes. Once thawed, the injured cells leak, collapse, and die.
Visual symptoms include dark watersoaked spots that later turn necrotic. Affected leaves and shoots blacken and shrivel up. Dieback of twigs and branches may occur. Entire plants can perish in an untimely late frost.
Through cold acclimation, plants undergo physiological changes that prevent freeze damage:
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Cell walls become more rigid and elastic to withstand ice expansion.
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Toxic ion concentrations decrease as ions exit the cell.
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Natural antifreeze compounds like sugars and proteins build up.
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Metabolic rates and water content are reduced.
This cryoprotection allows hardened off plants to shrug off freezing temps that would shock tender greenhouse transplants.
Dessication
Indoor seedlings grow in a protected, humid environment. When suddenly exposed to dry outdoor conditions, the leaves can desiccate and wilt rapidly before the roots can supply enough water to compensate for transpirational losses.
Leaves may turn dry and limp, with browned margins and tips. Without the ability to carry out photosynthesis, plant development comes to a standstill.
Hardening off permits the plant to adjust its stomatal response and cuticle transpiration barrier. This primes the plant to contend with lower humidity, wind, and dry soil.
Impaired Growth and Yield
Even if an unhardened plant manages to survive its rude introduction to the outdoors without sustaining outright tissue damage, the stress will take a significant toll.
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Growth comes to a halt for 1-2 weeks after transplanting while the plant copes with the shock.
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Fruiting and harvest are set back. Total yield for the season decreases.
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Vulnerability to pests and diseases rises.
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Vitality and lifespan are impaired.
Plants hardened off properly before transplant will take root and resume vigorous growth without missing a beat. Taking the time to properly acclimate your plants makes all the difference in their performance.
When Harden Off Is Not Needed
Not all plants require formal hardening off, even tender seedlings. Here are some cases where it may be unnecessary:
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If seedlings were grown outdoors in a cold frame or unheated greenhouse.
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Plants grown indoors under powerful grow lights that mimic sun intensity.
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Transplants being moved from one outdoor garden to another.
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If weather has been unseasonably mild, with overcast days.
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When transplanting into a protected environment like a greenhouse, hoop house, cloche, or cold frame.
Use discretion, monitor the forecast, and gradually introduce plants to new conditions whenever possible. But take advantage of natural opportunities to skip hardening off when you can!
There are certainly risks to taking shortcuts and not properly acclimating plants through hardening off. Stunted growth, leaf damage, and plant fatalities are all possible consequences. Following the simple hardening off protocol prevents most issues and results in smooth, vigorous transitions. For the small time investment, it’s well worth doing it right!
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When to Harden Off Seedlings
Begin the hardening-off process seven to 14 days before the date you will plant your seedlings outdoors. In spring, most seedlings are typically planted outdoors after the last frost date. However, cold-hardy plants can be planted outdoors two to four weeks before the last frost date.
The type of plants being grown and the temperature outside affect how long a seedling needs to harden off. Be flexible with your hardening off schedule and be ready to move your seedlings inside or cover them if a late spring freeze or snow is expected.
Here are two common methods to harden off seedlings:
- Seedlings grown indoors
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The Spruce / Steven Merkel
Harden Off Seedlings by Gradually Exposing Them to Outdoor Conditions
- Start Exposure to Outdoor Conditions: On the first day, put the seedlings outside in a spot that is out of direct sunlight and wind for an hour on a warm day that is above 45 degrees Fahrenheit. The Spruce / Steven Merkel .
- Bring the Plants Back Inside: When the seedlings have had enough time outside for the day, bring them inside and put them somewhere warm, like a heated garage or basement. The Spruce / Steven Merkel Tip: Don’t forget to keep animals, snails, and slugs away from young seedlings. Put them on a table or somewhere else animals can’t get to them.
- Increasing Exposure: Allow the seedlings to be outside for an extra hour every day to get them used to more dappled sun and wind. The key to success is gradual exposure to the outdoors. Do not put seedlings outside when it is windy or when the temperature is less than 45 degrees Fahrenheit. The Spruce / Steven Merkel .
- Increase the amount of sunlight: After two or three days in a somewhat shady spot, move the seedlings to a spot that gets morning sun and slowly move them to a spot that gets more direct sunlight. If you put very young seedlings in direct sunlight too soon, the leaves can get burned. Every day, the seedlings will be able to handle being outside for longer periods of time. The Spruce/Steven Merkel Tip: If you want to slowly get your plants used to spending more time outside, a wagon or wheelbarrow will make moving them inside and outside easier.
- Leave Plants Out All Night: If it stays warm (at least 50 degrees Fahrenheit) all day and night, the seedlings should be able to handle more sunlight and can be left outside all night. If the weather suddenly gets warmer, make sure the soil doesn’t dry out. The Spruce / Steven Merkel .
- Transplant: Your plants are ready to be moved into the garden or a container after seven to fourteen days of getting used to being outside. If possible, choose a cloudy day. Water well after planting. The Spruce / Steven Merkel .
Damage From Not Hardening Off Explained Garden Quickie Episode 134
FAQ
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