Hardening Off Seedlings For Beginners

1 comment by Halley -Author at MIGardener

Hardening off seedlings is one of the most vital steps to starting seeds indoors. Seedlings need time to adjust to the outside garden environment before they can grow up as healthy plants. Wind, rain, UV intensity from the sun, and temperature fluctuations can potentially harm seedlings that are not properly adjusted. The phrase "hardening off" is used to describe each seedling's strength growing over a period of time. Once you understand this process, your seedlings are guaranteed to thrive year after year.

Let's get started!

Know The Hardening Off Time Frame 

Start the hardening off process 7-10 days before your plants go out into the garden. A slow introduction allows plants to adapt to their new environment in stages. This time frame lets them quickly bounce back from the typical transplant shock associated with transplanting them into their new home. Over this time period, you want to increase the hours spent outside in both shade and sun.

Day 1: Start in the shade. Set plants in a shady spot outside for 2-3 hours before bringing them back in. 

Day 2: Have them spend 3-4 hours in a shady spot outside. 

Day 3: 3-4 Hours in the shade into an hour of the evening sun. Your seedlings must first adapt to the evening sun before they can be exposed to the bright mid-day sunshine. (even on overcast days)

Day 4: 3 Hours in a shady spot and 2-3 hours in the evening sun. 

5: 3-4 Hours in the evening sun - bring them in after sundown. 

Day 6: Repeat day 5 (after checking your weather forecast), leave them out overnight, and bring them in an hour or two after sunrise. 

Days 6: Increase the morning hours and bring in before the heat of the day. 

Day 7: Continue to start them during evening sun, begin introducing them to an hour at a time of midday sun for 8+9

Day 10: At this point, you should be able to leave your plants out all hours of the day and night. 

transplanting seedlings

Important note: if you see a low temperature or a storm of some sort on your forecast during your hardening out process. Skip that day. You do not want to introduce them to that intensity during this time. Come back to your schedule after it passes. 

Tips to start indoors 

In previous blog posts, we've discussed tips for starting seeds indoors. Adding a fan to the room where your seedlings sit is a great way to strengthen them as they grow. A fan makes the hardening off process easier for them to adapt to outdoor growing conditions. A few tips: Don't place the fan in a location where the air is blowing directly on the seedlings. Place it in the corner of the room to circulate the air and add a gentle breeze to your space. Your seed starts will strengthen every day as they grow with this method.

hardening off fan


Simulate Day & Night With Your Grow Lights

seedlings with grow lights

It's very tempting to leave grow lights on as long as possible in the name of "jump-starting" your seedlings. However, lights should not be left on full time. We recommend leaving grow light on for no longer than twelve hours a day. This time frame simulates day and night for your seed to start and gives them time to rest at the end of the day. This technique strengthens each plant and adapts them perfectly for life in the great outdoors. The hardening-off process will be much simpler if endless rays don't spoil your seedlings at the beginning of their lives. It's also important to note that sunlight is still filtered through them in some capacity, no matter how clear your windows are. Light from windows is definitely not a replacement for hardening off your seedlings outside.

What Happen If You Skip Hardening Off

Hardening off is truly the most crucial step in the seed starting process. All of your hard work, time, and love can go to waste after a couple of hours in direct sunlight without the proper precautions. Seedlings that are not hardened-off can be at risk for sunscald/sunburn. You'll notice your plants are sunburned if you see discoloration or thinning foliage. Sunburn can be reversed if you catch it early enough! Don't prune away burnt foliage but wait until the plant has the opportunity to produce new growth. If you've already transplanted your seedlings too early, cover them to shield them from the sunshine until properly adapted.

Some plants can recover from sun scald, but only if they have enough foliage to photosynthesize properly. Even if they do survive, they can be held up for weeks in their maturity.  Another issue that may arise in your seedlings is transplant shock. Transplant shock causes your plants to wilt, brown, or leaves to curl and fall away. During the waiting period for your plant to bounce back, care for it as your normally would. Here is the MIgardener how-to video on how to harden off seedlings easily! 

House Plants Need Hardening Off Too!

hardening off house plants

Not only do our garden plants need to be hardened off at the beginning of the season, but we can also do the same for house plants. Many house plants love to spend the summers outside. Putting these warm-weather-loving house plants outside can be a great way to break them out of a stagnant growth pattern. Harden them off the same way you would harden off garden seedlings. Do your research into each plant's preference for sunlight. Enjoy increased growth and even flowering on a variety of house plant types. 


Hardening off needs to happen at the end of the growing season as well. Remember, hardening off is acclimating a plant to a new environment. After spending a long period of time outside in the elements, it will need to be reintroduced to the less potent aspects of their indoor home.  Hopefully, these tips will equip you for the entire growing season. There is nothing more empowering than looking out an entire garden you started from seed! 


– Kaitlynn from MIgardener

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1 comment

  • Stacy

    I have tried copying and pasting into a word document and also an email to myself and each time it freezes my computer. I also cannot print it and want to put this info into my gardening binder for future reference. Do you happen to have a text only version that I can print out without large blank areas where photos should be but are not showing up (and no photos anyway because of the amount of ink it uses up)? Thank you for your help :)

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