For about two to three weeks before they are planted outside, plants that were grown indoors or in a greenhouse need to get used to cooler temperatures, less humidity, and more air flow. This ‘toughening up’ process is known as hardening off.
The process of hardening off involves gradually transitioning tender seedlings from the perfect conditions indoors to the harsher outdoor environment. It’s a common gardening recommendation but what actually happens if you skip this step? Let’s dig into the science and troubleshoot potential issues.
Why Harden Off Seedlings?
Indoor seedlings live a pampered life – warm temperatures, proper moisture, gentle artificial light or filtered sunshine through windows When transplanted straight into the exposed garden, the difference in conditions can shock them.
Potential risks include
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Sunscald – Leaves scorched by intense UV rays.
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Wind damage – Fragile stems and leaves tattered by gusts.
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Transplant shock – Root damage and moisture stress.
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Temperature shock – Wilting from temperature swings.
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Pest vulnerability – Lack of hardy cuticle makes them a target.
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Stunted growth – Harsh conditions impair photosynthesis.
Gradual hardening toughens cells and activates protective mechanisms in the plant, making the transition less traumatic.
What Happens if You Skip Hardening Off?
Now let’s discuss what you can actually 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 be 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 cuticle, or waxy protective coating, hasn’t developed enough in a greenhouse setting. So leaves are more vulnerable 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 can gain a foothold. Diseases like root rot, powdery mildew and botrytis are 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 loss. Die-off ranges from a few individuals to the entire batch if conditions are severe enough.
Tips to Minimize Hardening Off Issues
While hardening off is recommended, the risks can be mitigated with proper care:
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Acclimate transplants in a cold frame or greenhouse before final garden planting.
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Provide temporary shade with fabric row covers, shade cloth, lattices or tree branches.
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Transplant on overcast, rainy days or in late afternoon to avoid peak sun.
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Mist seedlings and keep soil consistently moist during early days.
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Shield from drying winds with plastic walls, fabrics or plant groupings.
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Monitor for pests and disease. Remove infected plants immediately.
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Select partially shaded planting sites rather than full exposure.
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Choose wind and sun-tolerant cultivars better equipped to cope with variables.
When is Hardening Off Essential?
While the above tactics help reduce transplant shock, some situations call for proper hardening off:
Transitioning Winter-Started Seedlings in Spring
Seedlings grown under artificial lights or winter greenhouse conditions require incremental exposure to acclimate to intense spring sunlight.
Moving From Controlled Indoor Environments to Outdoor Conditions
Greenhouse and indoor grow room plants need gradual temperature, light and humidity adjustments.
Fragile, Thin-Leaved Vegetables Prone to Sunscald
Lettuce, spinach, celery, basil and other delicate greens likely require hardening for best results. Sturdier cabbage, kale or broccoli may fare better without it.
Extending the Outdoor Growing Season
Hardened off transplants can withstand cooler and hotter temperatures when planted outside normal zones.
Rapid Weather Shifts Expected
If whipsawing weather from hot sun to cold snaps is forecasted, pre-adaption helps minimize stress.
Final Thoughts
In essence, while hardening off is often recommended, your particular growing conditions dictate just how essential it is. With thoughtful planting strategies and protective measures, lack of hardening doesn’t automatically spell doom.
Pay attention to the specific risks – sun intensity, winds, pest pressure, etc. Then tailor solutions to shield tender plants. Monitor them closely, and don’t hesitate to make adjustments like emergency netting if damage appears.
With experience, you’ll learn which of your vegetation truly benefits from hardening off versus those able to successfully skip the process. Just be ready to troubleshoot issues either way!
When to harden plants off
Hardening off usually takes two to three weeks, but it takes longer if the growing conditions were warmer at the start. Hardy plants acclimatise faster than half-hardy or tender kinds.
Plant tender plants outside after the last frost date, which is usually late spring in the south of England and later in the north and Scotland. This is just to be safe.
Why harden off plants?
Young plants bought from nurseries or grown from seed or
A way of growing new plants from pieces of an old plant, like roots, stems, leaves, or buds When prepared correctly and planted in the right conditions, they can produce roots and eventually become independent plants. In different plants and at different times of the year, there are many ways to take cuttings.
Hardening off allows plants to adapt from being in a protected, stable environment to changeable, harsher outdoor conditions. If suddenly placed outside, the shock can severely check a plants growth. Although plants usually recover eventually,.
Getting tender or indoor-grown plants used to the outdoors slowly to make them tougher and keep their growth from stopping For a few weeks, put plants outside in a protected area during the day and bring them inside at night. Or place in a coldframe, opening the lid during the day.
The effect of hardening off is to thicken and alter the plants leaf structure and increase leaf waxiness. It ensures new growth is sturdy although growth will be much slower than in the greenhouse. But be warned: hardening off does not make frost-sensitive plants hardy.