Hardening off plants is an important process for transitioning seedlings from the perfect conditions indoors to the harsher outdoor environment. But what actually happens if you skip this step? Let’s explore the science behind hardening off and troubleshoot the potential issues that can arise.
Why Harden Off Seedlings in the First Place?
Indoor seedlings live a pampered life with warm temperatures, proper moisture, gentle light, and protection from the elements. When transplanted straight into the exposed garden, the difference in conditions can shock them.
Potential risks of not hardening off include:
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Sunscald – Leaves scorched by intense UV rays. This causes browning, discoloration, and desiccation.
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Wind damage – Fragile stems and leaves tattered by gusts This can weaken or snap seedlings,
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Transplant shock – Root damage and moisture stress This stunts growth
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Temperature shock – Seedlings wilting from temperature swings, This makes them vulnerable
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Pest vulnerability – Lack of hardy cuticle makes them a target for pests.
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Disease susceptibility – Defenses are compromised so diseases can take hold.
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Stunted growth – Harsh conditions impair photosynthesis and expansion.
Gradual hardening toughens cells and activates protective mechanisms in the plant, making the transition less traumatic.
What Actually Happens if You Skip Hardening Off?
Now let’s discuss what you can expect if you forego hardening off your seedlings.
Immediate Sunscald and Leaf Damage
If tender seedlings are thrown directly into full sun, the foliage can become scorched, desiccated and discolored by the sudden ultraviolet bombardment. Leaf tips and margins brown, progressing inward if the exposure continues.
Leaf texture also changes under intense light. The waxy protective coating (cuticle) hasn’t developed enough in a greenhouse setting. So leaves are more prone to moisture loss and sun damage.
Delayed Growth and Reduced Vigor
Even if no obvious sunscald occurs, the stress of sudden light intensity slows photosynthesis and growth. When sun exposure exceeds a seedling’s capacity, light becomes excessive rather than beneficial.
The shock essentially sets the plant back as it marshals defenses. During this recovery period, leaf expansion, stem elongation, bud development and root growth are impaired.
Greater Pest and Disease Susceptibility
Stressed, stunted plants are more appealing targets for insect pests like spider mites, thrips and cabbage worms. They lack the structural and chemical integrity of hardened-off plants to fend off attacks.
Shocked transplants are also more prone to fungal and bacterial diseases. Defenses are compromised, so pathogens gain a foothold more easily. Diseases like root rot, powdery mildew and botrytis pose bigger risks.
Higher Transplant Loss
In extreme cases, the composite shock of sudden light, wind, temperature change and moisture stress can overwhelm tender seedlings, leading to transplant death. Die-off ranges from a few individuals to the entire batch if conditions are severe enough.
When is Hardening Off Absolutely Essential?
While the above tactics help reduce transplant shock, some situations call for proper hardening off:
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Transitioning winter-started seedlings in spring – Seedlings need incremental exposure to acclimate to intense spring sunlight.
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Moving from controlled indoor environments to outdoor conditions – Greenhouse and indoor grow room plants require gradual temperature, light and humidity adjustments.
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Growing fragile, thin-leaved vegetables – Lettuce, spinach, celery, basil and other delicate greens likely require hardening for best results.
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Extending the outdoor growing season – Hardened off transplants can withstand cooler and hotter temps when planted outside normal zones.
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Rapid weather shifts expected – Pre-adaptation helps minimize stress from whipsawing weather.
Final Thoughts
The risks of skipping hardening off range from delayed growth to complete transplant loss. Pay attention to the specific threats your climate and growing conditions pose. With experience, you’ll learn which plants truly benefit from the process versus those able to successfully make the transition without it. Just be ready to troubleshoot issues either way! Proper hardening off is more of an art than strict science.
Frequently Asked Questions
How long does hardening off take?
Generally 2-3 weeks, but up to a month if starting conditions are very warm. Hardy plants acclimate faster than half-hardy or tender varieties.
When should I start hardening off?
After the last frost date in your area. Wait for overnight lows around 50°F for cool crops and 55°F+ for warm crops. In some cases you can start earlier with frost protection.
Do I need to harden off nursery plants?
Not usually. Greenhouse propagation and transplanting toughens up most commercial seedlings before sale. But provide some shade for smaller or delicate plants the first few days.
What’s the best way to harden off?
Gradually increase outdoor time over a week. Filter sunlight first, building up to full exposure. Bring seedlings in at night and don’t let them get dried out.
Are there shortcuts to hardening off?
Yes. Transplant on cloudy days, use shade cloth, or place seedlings in portable greenhouses outside. But take care with shortcuts if weather shifts abruptly.
How can I grow seedlings that don’t need hardening off?
Use LED grow lights or strong T5 fluorescents close to the plants. This builds hardy, dense foliage adapted to intense light from the start.
What are signs seedlings weren’t hardened off properly?
Wilting, stunted growth, leaf scorching or sunburn, pest damage, disease, and transplant loss. Immediate issues or lack of vigor indicates inadequate hardening.
Should I dig up shocked transplants and re-harden them?
No. Transplanting again will further stress the roots. Keep them warm and hydrated. Healthy seedlings will recover with time and protection from extremes.
With proper hardening off technique or alternatives like shade cloth, most seedlings transition successfully from indoors to outdoor growing conditions. Pay close attention the first weeks after transplanting, and be ready to implement emergency measures like netting if issues arise. With experience, the process becomes second nature!
How to Harden Plants
The best way to harden plants is to put them outside for a few hours the first day and then bring them in at night. Each day, increase the number of hours they are exposed to sunshine until finally after 3 or 4 days you can leave them out all night.
Partly cloudy days are perfect for hardening plants. If it’s a strong sunny day, set them in the shade, or ideally in part shade, for the first day. Using Reemay cloth, or floating row cover, to shade the plants is a good approach to hardening them on a sunny day.