Hardening off seedlings that you started indoors the easy ways is different from the traditional method where you bring plants out on a nice and sunny day for 2 hours, then bring them back in, repeat that the next day for 4 hours and bring them in again. Adding a couple of hours each day, it takes 7 -10 days till plants are hardened off enough to be outside all day.
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Harden off plant is one of the most important steps in successfully transplanting seedlings from indoors to the garden. Many gardeners swear by it, saying it’s absolutely necessary for plant survival. But what really happens if you skip this step? Will your plants inevitably die or can they still thrive without a gradual hardening off period?
As an avid gardener and plant enthusiast I’ve experimented with hardening off over the years. In this article I’ll share my experiences and research on the effects of not hardening off seedlings. My goal is to provide a realistic picture of the risks and benefits so you can make an informed decision for your garden.
What is Hardening Off?
Hardening off refers to the process of gradually acclimating seedlings to the outdoors over 7-10 days before transplanting them into the garden This transition period allows the tender plants to adjust to the harsher conditions outside, compared to the regulated indoor environment they were started in
Specifically, hardening off prepares seedlings for:
- Full sun exposure
- Temperature fluctuations
- Wind
- Lower humidity
- Less frequent watering
To harden off seedlings, they are moved outdoors for just 1-2 hours on day one. Each day, the duration outside is slowly increased until the plants are spending full days (8+ hours) exposed to full sun and outdoor conditions by day 7 to 10. At that point, the seedlings are considered fully hardened off and ready for transplant.
The gradual process allows the plants to adjust and build tolerance so they aren’t shocked when permanently moved outside. Shock can stunt growth and even kill seedlings.
What Happens if You Don’t Harden Off Plants?
Many experts claim hardening off is absolutely vital for plant survival. But in reality, the effects of skipping hardening off depend on several factors:
Plant Type
Some plants are far more sensitive than others to direct transplant without hardening off. We’ll look at specific examples later, but tender plants with thin leaves typically struggle more than hardy, woody plants. Herbs and vegetables are very vulnerable.
Light Intensity
How much of an increase in light levels will the seedlings experience when moved from indoors to their outdoor home? Seedlings started in a very bright, south facing window may not need much adjustment to full sun. But plants started under grow lights or in low light will likely experience severe shock without incremental light exposure.
Outdoor Conditions
What are the weather conditions immediately after transplant? Are the temperatures fluctuating widely from day to night? Is it abnormally hot, windy, or dry? Severe outdoor conditions combined with no hardening off is a recipe for disaster.
Plant Health
Are the seedlings robust, dense, and vibrant green? Or are they leggy and purpling from inadequate light and nutrients indoors? Healthy, thriving plants can withstand stress far better than weak, stunted seedlings.
Transplant Process
How are you moving the seedlings outdoors? Giving them adequate water and shade for the first several days can help reduce shock. Transplanting in the evening vs mid day also decreases stress.
Your Zone/Region
Are you gardening in a extreme northern or southern region with highly variable weather and intense sun? Or is your climate relatively mild and humid? Zone also affects day length changes. Plants moved from short indoor days to 18+ hour summer days undergo added shock.
Now that we’ve covered the factors that influence how seedlings respond without hardening off, let’s look at some potential effects:
Effects of No Hardening Off
Sunburn
One of the most common reactions is leaf sunburn or scorch. When tender, shade-grown leaves are blasted with hours of direct sun they can scorch and die back. Some types of plants, like impatiens, are extremely prone to burning. Others, like basil, can burn quite easily if conditions are right.
Sunburn manifests as browning, crispy leaves with shriveled, dry patches. It may affect just portions of the leaf or the entire thing. Severe sunburn can kill young seedlings.
Wilting
Seedlings may rapidly wilt soon after transplant if they aren’t hardened off. The tender leaves lose moisture quickly in the dry, windy conditions outdoors compared to the humid environment indoors. Plants unused to wind also have a high rate of transpiration and water loss.
Persistent, severe wilting can starve the plant of water to the point it dies. However, mild temporary wilting is normal as the plant adjusts and shouldn’t kill it.
Slow Growth
The shock of sudden changes in light, temperature, humidity, and wind can stunt plants. So while an unhardened off plant may survive transplant, you may notice minimal new growth for several weeks.
It takes the plant time to recover from the shock and resume normal growth patterns. This delayed establishment allows weeds to crowd out seedlings in the garden.
Increased Pest/Disease Susceptibility
Plants weakened by shock also become more vulnerable to pests and diseases. Sudden exposure to new microclimates introduces the plant to different pathogens and insects too. So unhealthy seedlings often experience more problems with powdery mildew, aphids, slugs, and other invaders.
Flower/Fruit Abortion
Seedlings transplanted while actively flowering or fruiting often jettison the reproductive structures in order to focus resources on recovery. Your flower buds blast and small fruit simply drop soon after transplant.
Complete Death
In worse case scenarios, the combined shock is simply too much for tender seedlings. They decline rapidly soon after transplant and die. This often occurs with herb and vegetable seedlings if they’re subjected to several stresses simultaneously with no hardening off.
Examples of Sensitive Plants
While all seedlings prefer a gradual hardening off, some types are far more sensitive than others. Here are a few categories of plants that often struggle or die without proper hardening:
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Herbs – Basil, cilantro, dill, parsley and other tender herbs often deteriorate. Some hardy perennial herbs like thyme can adapt better.
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Lettuces and Greens – Spinach, kale, chard, lettuces, arugula, and other leafy greens usually burn or bolt rapidly with sudden exposure.
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Impatiens – One of the most vulnerable annuals, impatiens quickly succumb to sunburn. All varieties struggle.
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Begonias – Both fibrous and tuberous begonias perform poorly without hardening off, especially in intense sun.
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Petunias – Prone to leaf burn and wilting. Surviving plants are stunted and unattractive.
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Peppers – Can suffer sunscald on fruit in addition to leaf burn and slow establishment.
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Tomatoes – May experience sunburn and blossom drop if transplanted directly to a hot, sunny garden.
So in general, leafy greens, herbs, flowering annuals, and vegetables require a hardening off period for best results. On the other hand, some “bullet proof” plants handle direct transplant just fine:
- Coleus
- Marigolds
- Zinnias
- Most perennials (hostas, daylilies, sedum, etc)
- Tomatoes transplanted on a mild, cloudy day
Use your best judgement based on your climate and the plant’s disposition. Observe how test plants respond when you intentionally skip hardening off.
Can Plants Recover If Not Hardened Off?
Yes, it is possible for sensitive plants to recover after transplanting without hardening off. However, it takes very diligent care and ideal conditions to nurse them through the shock.
To help plants recover, you must provide:
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Filtered, partial sun – Use shade cloth, lattices, etc to reduce sun exposure by 50% or more.
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Additional wind protection – Create shelters to protect from drying winds.
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Consistent moisture – Water more frequently to prevent severe wilting.
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Ideal temperatures – Move pots to warmer microclimates at night if needed.
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Nutrients – Apply a weak fertilizer solution to aid recovery.
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Pest prevention – Inspect for pests and treat promptly before they compound damage.
With TLC and protection for 7-14 days, less hardy plants may adapt to outdoor life despite no hardening off. It’s extra work, but possible.
Alternatives to Traditional Hardening Off
I know many of you are thinking “This hardening off process sounds like a pain, but I want my plants to thrive too!” No worries – there are some shortcuts and alternatives you can try:
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Transplant on an overcast day – Less intense sunlight helps reduce shock.
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Provide shade – Use shade cloth, netting, lattices, etc to filter sun after transplant.
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Transplant in late afternoon – Avoid the peak sun of mid day.
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Use a cold frame or cloche – These devices transition seedlings gradually.
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Bring outdoors on warm days – Let seedlings experience partial outdoor time.
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Use row cover – Fabric row cover protects from wind and cools soil.
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Transplant into containers first – Containers can be moved as needed before garden planting.
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Use high power grow lights – Strengthens plants to withstand more stress.
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Select resilient varieties – Some cultivars tolerate transplant better.
Get creative with shading and sheltering seedlings for the first week or so. Keep a close eye on their performance and make adjustments as needed. With attentive care and coddling, you can often avoid the lengthy traditional hardening off process.
To wrap up, not hardening off can mean anything from superficial sunburn to complete death in struggling seedlings. Tender herbs, greens, annuals, and vegetables are most at risk. However, alternatives like shading and container planting can minimize negative effects. Get to know your microclimate and plant’s needs. Adapt your transplant practices accordingly for the healthiest, most productive garden possible!
When to harden off seedlings
Start seeds indoors and let them become healthy, strong seedlings first, without starting with hardening them off too early. A schedule of when to start seeds and when to plant the seedlings out can be very helpful so you don’t start too early or too late.
Once the seedling is ready and the weather conditions are right, it can be transferred to the outside, aka hardened off. We talk more about the weather below.
If you follow our indoor seed starting schedule, you will notice that most plants have a different planting date. It all depends on the weather of course. But this way we do not have to harden off all the seedlings in one go. It is always only one plant category.
Once the seedling is ready and the weather cooperates, you can start hardening off plants the easy way.
Where to Harden off seedlings
To make your job easier, prepare a location for your plants to harden off. The worst place is an open, sunny, and often windy deck, which will be hard on the seedlings.
In our case, we have a greenhouse and a raised window-protected garden. In spring, we use the indoor garden and greenhouse to grow seedlings; in summer, we grow heat-loving plants.
We started with just a few recycled window frames held together by cinder blocks as a temporary little greenhouse. It worked so well that it turned into a permanently raised bed, but the temporary structure worked perfectly for hardening off plants.
The idea is to give the plants some wind protection and the ability to provide shade.
Plastic will do just as well if you don’t have recycled windows. Railings on a deck or balcony can be converted; any sheltered location can work.
We do not recommend the mini greenhouses, they overheat during the day, and give you a false sense of security at night.
Note that plastic little greenhouses do not protect plants from frost. If you have a frost-sensitive plant, keep it indoors till all danger of frost is over.
Damage From Not Hardening Off Explained Garden Quickie Episode 134
FAQ
Do you really need to harden off plants?
Young plants bought from nurseries or grown from seed or cuttings at home for summer display outdoors when the weather improves often need to be hardened off. Hardening off allows plants to adapt from being in a protected, stable environment to changeable, harsher outdoor conditions.
What happens to plants that are not hardened off?
If you don’t harden your plants, the tender plants will get burned by the sun, the shock of cold, or the wind. Some plants may recover from burn (even fully), but their growth will be set back a few weeks while they recover.
How many days does it take to harden plants?
When Are Plants Ready to Be Hardened Off? Plants should get at least 25 to 30 days to grow indoors before you begin transitioning them outdoors.Jan 2, 2024
Can you skip a day when hardening off plants?
Sure, it’s possible to shortcut the 10-days, but there’s risk involved. I may use cloudy days around Day 8 as my green light to transplant, but I won’t ever risk it will full sun days and just throw them out into the wild before getting a really slow ramp up to the scorching full sun garden light.