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All Things Garden,  Beginner Basics

Hardening Off Seedlings to Prevent Transplant Shock

So you have a bunch of seedlings that need to be planted outside soon, huh? Well congrats on successfully starting seeds and raising new babies! But hold tight! Do not take your seedlings and plop them down outside without a proper adjustment period to prepare them first. The next step in your little plants journey is to harden them off. This is a very important step, especially if you started your seeds indoors or in otherwise protected conditions. Hardening off seedlings helps to ensure they make a smooth, successful transition to their new home – your garden!

This post will discuss what hardening off is, why it is important, and how to do it.

At the end, you’ll find a video of our greenhouse seedlings that are currently going through the hardening off process, for all you visual learners!


What is hardening off?

Hardening off is the process of preparing seedlings to be planted outside. It is taking slow, methodical steps to enable your small plants to become stronger and more resilient before hand. This helps reduce stress and encourage success once they go outside.


Why harden off?

If your seedlings haven’t been adequately exposed to the types of conditions they’ll encounter in the “big bad world” of the garden, they may flounder. They could bend, snap, wilt, fry, or otherwise die. This is particularly true if you start seeds indoors. Up until this point, indoor seedlings have been very protected; babied even. They likely haven’t been exposed to extreme temperature swings, strong winds, rain, or even direct sun. If you take them from stable, temperate conditions and quickly plant them out in your garden, they could experience transplant shock.

The act of hardening off seedlings to help prevent transplant shock is very easy to do! It just takes a little planning in advance. It is a simple, free bit of insurance to provide your plants. Even if it isn’t done “perfectly”, every little bit of hardening off helps tremendously! There is really no reason to rush and skip it.



Transplant Shock

Transplant shock is a state of shock or trauma that a plant could experience after being transplanted. It may not or may not kill them, but it can set them back. They could be permanently stunted and therefore won’t reach their true potential during maturity.

According to Purdue University:

“Transplant shock is a term that refers to a number of stresses occurring in recently transplanted seedlings, trees, and shrubs. It involves failure of the plant to root well, consequently the plant becomes poorly established in the landscape. New transplants do not have extensive root systems, and they are frequently stressed by lack of sufficient water. Plants suffering from water stress may be more susceptible to injury from other causes such as the weather, insects, or disease. When several stresses are being experienced, the plant may no longer be able to function properly.”

Paul C. Pecknold, Purdue University

An image of a sad, wilted, discolored looking tomato plant, about 1 foot tall. It is experiencing transplant shock.
A tomato seedling in a state of transplant shock. Photo courtesy of Houzz


Another tip to prevent transplant shock is to not allow your plants to become too root bound before planting out. If they are looking overgrown for their current containers, roots sticking out the bottom and all, but aren’t yet ready to go outside – pot them up into larger containers as needed. When planting, don’t ruffle the roots too much on the seedlings.

Also, don’t over-fertilize the soil they’re going in to, which puts them at risk of “burning” and shocking too. We amend our beds with rich aged compost, worm casting, and just a light dusting of mild kelp meal and alfalfa meal – less than the package calls for, just to be safe. After planting, watering with dilute seaweed extract can also help reduce transplant shock.


How to Harden Off Seedlings

We start the hardening off process early, encouraging our seedlings to become their strongest, healthiest selves from a very young age. Just as any good parent would do, right? You take the more deliberate hardening off “steps” the week prior to planting out in the garden, which we’ll discuss in a moment. Yet there are other measures you can do much earlier on to give them the best start possible.

It should be noted that everything I explain next is general best practice and suggestions. It doesn’t need to be an exact science, and you don’t necessarily need to follow a super precise schedule. As long as you do your best to cause some gentle, beneficial stress to prepare and strengthen your plants like I am describing, your best is all you can do! Schedules get busy, days get missed…. I get it.


After germination


Starting about two weeks after germination (that is, after the plants have sprouted), we start introducing a light wind to the seedlings, using an oscillating fan set on low. Don’t put it too close to them, and preferably not only in one spot from one direction, hence the suggestion for oscillation. The movement created by the wind makes the seedling wiggle and bend a little. That action causes them to rapidly strengthen their stems in response to the movement. This helps prepare them for the wind they will experience outdoors.

Using a fan on seedlings is good practice in general, since the increased air flow also reduces chances of disease and damping off. You don’t want the fan blasting on them constantly however. That will make the soil dry out really fast. It can also make the plants develop a lean, away from the wind. To prevent that, try to rotate your trays around on occasion so everyone gets a chance to experience the wind at a different strength and direction. We will generally put the fan on the seedlings for a few hours at a time, repeated a few days a week.

Our seedlings getting their windy wiggle on! Excuse the crusty old fan.


Several weeks before planting outside


The following steps can be taken several weeks before you are planning to transplant seedlings out to the garden.

Not sure when you should be transplanting seedlings outside? Reference your planting calendar! If you don’t have one, you can subscribe below and receive a free garden planning toolkit via email immediately when you do so! It includes garden planting calendars for every USDA growing zone, including when to start seeds indoors, direct sow outside, and when to transplant out.


Heat

Are your seedlings indoors, sitting on heat mats? While heat mats are a great tool to encourage seeds to germinate, and they also help heat-loving seedlings like peppers and tomatoes grow more quickly, they are also another enabler to your seedlings being overly-babied. A few weeks prior to your target plant-out date, you should start to wean your indoor seedlings off of their heat mat. If your seedlings are being kept somewhere that can get very cold or has risk of frost, like in a garage or uninsulated shed, practice wise judgement here. Do not leave your seedlings without a heat source if there is a risk of freezing!

If you are using a thermostat-controlled heat mat like we do, the weaning off process could look like turning down the temperature by 5 degrees every week for several weeks. If you don’t use a temperature-controlled mat, start unplugging the mat overnight and turning it back on during the day, but for shorter durations. A week prior to starting the next step (so about two weeks before planting outside), they should be completely weaned off the heat mat.


A note about using greenhouses:

We start our seedlings in a greenhouse. However, please note that we do not heat our greenhouse! While a portion of our seedlings are sitting on top of heat mats to stay nice and warm overnight, the rest of the seedlings and greenhouse get almost as cold as it is outside. An unheated greenhouse will only stay about 5 degrees warmer than outside at night, though it does get significantly warmer than outside temperatures during the day.

Where we live, we don’t get freezing temperatures, so our greenhouse plants are not at risk. Moving our seedlings off of the heat mats onto an unheated bench in the greenhouse is part of our hardening off process. There, they seedlings are exposed to nearly as cold of temperatures as they will outside!

Looking inside a small 6x8 foot urban greenhouse. The wood shelves are lined with plants all around. On the left, some of the seedlings are still on top of heat mats and under grow lights. On the right, the seedlings are moved away from the artificial heat and light in preparation to be planted outside, as part of the hardening off process.
The seedlings on the right and middle benches, mostly greens and flowers, are going to be transplanted outside soon. They were removed from the heat mats and lights a couple weeks prior, to help ease in their transition. The tomatoes, peppers, and cucumbers will stay in the greenhouse a bit longer, so they’re still on the heat and light. Note that the ones staying in longer are also potted in slightly larger containers, these 4″ pots, enabling them to live in there longer without becoming root bound. We will pot them up soon too. The greens and flowers are mostly in smaller 6-packs, so they really need to out soon, or be potted up!


Light

If your seedlings are currently indoors under a grow light, that is great! Keep it that way. Unlike heat, you don’t want to remove their light source before transplanting. They’ll freak out on you.  However, how long are you keeping your lights on?

It is best practice to keep lights on seedlings for about 14 to 16 hours per day. If you are keeping them on longer than that, for example overnight, definitely start reducing the time they’re on as you prepare them to go outside, to more closely mimic the sunlight hours outside. Veggie seedlings need about 8 hours of darkness.  In our greenhouse, we move seedlings that are destined to be planted out soon away from the grow lights, but they still get a lot of natural light.


The week before planting seedlings outside

It is now about a week before you intend to plant your babies outside. This is when the more methodical hardening off process really begins. By following the steps below, your plants will be well-adjusted and ready to go! That is, follow as closely as you can. Perfection isn’t the goal. The goal is a gradual, steady, increased exposure to sun, wind, and temperature swings.

Day One:  

Check the weather. Try to start the outdoor hardening off process on a day when things are pretty calm out – no extreme wind, rain, frost or things of that nature predicted. Take your seedling trays outside. The morning is preferable, but if all you can do is after work, that is okay too. Start in a shady, protected location. Keep the seedlings out for only a couple of hours this first day. Then bring them back inside.

Day Two:

Again, make sure the weather isn’t going to be crazy. A light rain during the hardening off process is fine, especially after the 2nd or 3rd day. We just want to avoid torrential downpours that could break their stems. Bring the seedlings outside again. Keep them in a shady location away from direct sunlight again if possible. Keep them out an hour or two longer than the first day. Bring them back inside.

6 large trays full of small green seedlings are sitting outside on a gravel pathway in a garden. They are in the shade, just starting out their hardening off process.
Trays of seedlings out on their second day of hardening off. They’re still hanging out in a shady side of the yard.


Day Three:

This time, go ahead and let them be in partial sun to partial shade. Extend the length they are out by another couple of hours. Okay, it’s time to come back in babies. (This is when I start feeling very grateful for our heavy-duty seedling trays that don’t bend and crack. It makes all this back and forth much easier!)

Day Four:

Put the seedlings out in partial sun again. As you start to ease them into direct sun, it’s usually best to give them morning sun over afternoon sun at first, since it tends to be less intense. Now they should be able to stay outside for 7 or 8 hours.

Day Five:

Repeat day four. Maybe a little more sun, and an hour or two more time. Start keeping them out a little later into the evening so they can experience cooler temperatures.

The temperatures they’ll feel while they’re still in their containers will actually be even cooler than that of the soil once they’re planted. A large mass of soil, like that of a raised bed or in the ground itself, holds radiant heat and overall warmth longer and better than the small exposed masses of seedling pots. But don’t forget to bring them back in overnight! I suggest setting an alarm.
 

Day Six:

Go ahead and give them sun all day! This includes hotter afternoon sun. If it isn’t all that sunny? Oh well! They’re still getting use to the outdoor conditions they will need to contend with. Let me them stay out past their bedtime again.

Trays of small seedlings are sitting on top of the soil in a raised garden bed. They aren't planted in the bed yet, but are sitting in the sun to get accustomed to it.
Towards the end of the hardening off process, we put the seedling trays out in full sun, often times on or near the bed they’re going to be planted in. In this photo, we had just harvested all the remaining kohlrabi, radishes, and greens from the bed that needed to be cleared for the next round of plants.


Day Seven:

By now you, should be able to put them out as early as possible in a fully exposed area of your yard. Leave them out as late as possible, but again, bring them in overnight this last time.


Planting Day!

Plant seedlings out in the morning, on a calm and temperate day. Beforehand, check the forecast! Are you good to go? Or has something changed and there is now frost or other harsh weather in the immediate forecast? It is okay to hold off on your plant-out date until you feel comfortable with the forecast. Just continue hardening them off outdoors during the days you can.  

The image is of a weather forecast. The weekend shows clouds with moderate temperatures.  Then on the coming Tuesday and Wednesday the forecast shows almost 2 inches of rain in one day, with winds up to 25 miles per hour. This would not be a good time to plant out baby seedlings. Wait until the weather forecast is more calm.
Check that forecast! We had planned to transplant out a bunch of greens and flowers on March 2nd or 3rd, but looking at the week ahead, we decided to wait a week. The forecast changed even more, and we’re expecting over 2 inches of rain in one day plus 30 mph winds on Tuesday now! That kind of weather has the potential to break and damage seedlings, even if they’re hardened off. So we’ll just continue to harden them off for an additional week as the weather allows.


In regards to actually planting these babes in your garden, I will add more posts about how we prepare beds and transplant seedlings soon!


Notes during the hardening off process:

Remember to keep an eye on your seedlings soil during this process, and give them water as needed just as if they were inside! You may find that they need more or less water, depending on the conditions outside. In our experience, seedlings on heat mats dry out faster than those not on heat mats. But if it is really sunny and warm during your hardening off week, they may need more water than ever!

Keep an eye out for pests too, particularly birds or other wildlife. Tender little greens can be tempting to wild birds. You may need to put a layer of fencing or mesh around your seedling trays. If you have chickens, harden off your seedings in a location protected from them too! One of my friends had a major brain fart here. She put the seedlings she’d raised for months out in a spot the chickens had access to, went inside just for a few minutes, came back, and the chickens had ate them all. Ugh! How devastating. Please don’t make that mistake.

Here’s a little video, walking you through the process we just discussed!



Check out our YouTube channel for more videos by clicking here!




That’s all there is to it folks.

Congratulations! Under your love and care, your fragile baby seedlings have now blossomed into well-adjusted, strong teenagers, ready to take on the world! They thank you for the effort you made to get them this far.

DeannaCat stands in the back yard garden, in front of tall redwood raised beds. She is holding two large tomato seedlings that have been hardened off and are ready to plant in the beds. The tomato plants are about 2 feet tall and in large 8" plastic nursery pots. Several more large tomato plants are on the ground around her feet. A sign that reads "garden" hangs on the wall behind her.
A proud plant parent on transplanting day, showing off her strong, hardened-off tomato teenagers. You can see that these tomatoes were planted up from 4″ pots into 8″ pots, allowing them to get large without getting root bound before transplanting.


I hope you found this post helpful. If so, please share it! Let me know if you have any questions. Thanks for reading. Happy planting out there!



2 Comments

  • Kymie May

    Hi I’m new to the seed planting gardening world and this was all super helpful. Last year after my small greenhouse took off in the wind like dorothys house in the wizard of oz one sunning session I lost most of my seedlings and what I planted after that got leggy and died I was devastated, so all of your mistake guide and how to fix it plus the hardening process was so incredibly helpful and I look forward to more articles and advice! Maybe this year I’ll get it right!

  • Lacey Daniels

    I was JUST telling my husband we need to get our seedlings hardened off so we can transplant them out! (Zone 9B). I wondered to myself if you had tips on making that happen — lo and behold, I opened Instagram shortly after that and saw your new post! Lol!

    Thanks for the great advice and fantastically-written posts! Excited to get my plants out soon once this crazy rain we’re getting passes!

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