Hardening Off Seedlings Before Transplanting

My baby broccoli and cabbage seedlings have been loved and nurtured like a newborn baby for the past month.  Their lives have consisted of lazy, peaceful days indoors being watered regularly, feed when needed, kept at a sweet 70 degrees, brutally thinned, and being given  the bright, delicious light they crave.  They look big, green, strong and ready to transplant into the garden.  WAIT!  Hold on there, Sparky.  Would you put a baby out in the hot sun all day, only to let it freeze for the next 12 hours in the cold, damp ground without proper preparation?  Heavens to Betsy, NO!   That baby would be sunburned to a crisp and suffer hypothermia, or worse, and so will your baby plants. First, you have to harden those babies off.  (The baby plants, not an actual human baby.)

Hardening off plants is a process that involves gradually getting your plants used to being outside, where, unlike the cozy house, the plants will experience the harsh sun, the cutting wind, and the biting rain storms of spring.  Slowly introducing your plants to these conditions helps them develop a harder, thicker skin, much like a callous from running, so they can take the sun’s heat and the brunt of spring’s fickle weather.  If you skip this step in the seed starting process, your plants will more than likely suffer shock once transplanted into the garden.  Transplant shock can cause weak, diseased, unproductive plants and can straight up kill them.  It will only take a week or so to harden off your seedlings, and will be well worth the extra effort of slugging those seed trays inside and outside each day.

Day 1 – Begin on a mild day, cloudy if possible, and place your seed trays in a protected, shady area for 2-3 hours.  In other words, make sure the trays are not in direct sunlight and are kept out of the wind and rain.   After a few hours, bring them back inside to their warm cozy spot under the grow lights.

Day 2 – Place your seedlings outside in a protected area for 4-6 hours, then bring back in.

Day 3 – Find a spot for the seedlings that receives 1-2 hours of sun in the morning, and then move them to a shady spot the rest of the day.  Bring trays in after 6-8 hours.

Day 4 – Place trays in a spot that receives morning sun for 2-4 hours, and afternoon shade.  Leave outside a total of 10-12 hours.

Day 5 – Continue to increase the amount of time the plants spend outside and in the sunlight.  Work around your daily schedule, this doesn’t have to be precise.  Just make sure you are steadily increasing your plants exposure each day.  If you forget to put them out one day, no biggie, just continue where you left off the next day.  This is not an exact science, only a guide.  At the end of day 5, if you have stayed on schedule, they should have spent 12-14 hours outside.

Day 6 – As the plants reach this point, they will now be experiencing the warmth of the morning sun, the partial heat of the afternoon sun, and the evening cold.  If you are lucky enough to have a cold frame, you can leave your plants outside over night in the cold frame where they will be kept warm.  If you do not have a cold frame, leave them outside until right before you go to bed so they can spend as much time outside as possible.

Day 7 – Repeat day 6.

Day 8 – The plants will be spending their first full 24 hours outside tonight, but before that happens, you need to make sure the evening temperatures will not reach freezing.  If you are in the clear, bid your seed trays sweet dreams and tuck them in for their first night outside in the big, cold world.  If the temps are predicted to freeze, hold off on their first full night outside until temps increase to above freezing.

Day 9 – Repeat day 8.  Plants are now ready for transplanting.

A couple of last notes on hardening off.  First, keep your seedlings moist during this time, but do not fertilize during the hardening off process.  As soon as you have transplanted them into the garden, you can give them a nice dollop of fertilizer, like compost tea, to give them a healthy jump-start.

Secondly, even after your plants have been transplanted, it is important to take note of possible freezing temperatures.   Some plants, like broccoli and cabbage are considered ‘hardy’ and can take a light frost.  However, tomato and pepper plants are considered ‘tender’ and will not survive a frost.  If your new transplant is tender, and a frost is immanent, protect it at night with a floating row cover, an upside down milk jug, or anything you can rig up to keep those babies cozy and frost-free.

As I stated earlier, this is not a scientific process, it is just my personal way of hardening off plants and should only be used as a guide.  Feel free to lengthen, or shorten this process based on your own growing conditions, weather, and plant’s needs.  Hopefully, this post gives your transplants a less shocking and much healthier, happier start in their new garden home.  Happy hardening!



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