Hardening Off Seedlings to Prevent Transplant Shock

Last Updated on August 18, 2023

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 seedlings is the process of preparing indoor-raised seedlings to be planted outside. It is taking slow, methodical steps to get them accustomed to the great outdoors. By hardening off seedlings, it helps them to become stronger and more resilient, and thus reduces stress and increases success in the garden.

Why harden off seedlings?

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 or easily get damaged.

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.

About 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

Would you like to save this?

We'll email this post to you, so you can come back to it later!

I agree to receive email updates.

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 hardening off seedlings early, encouraging them to become their strongest, healthiest selves from a very young age. Just as any good parent would do, right? We take the more deliberate hardening off steps the week prior to planting the seedlings outside in the garden, which we’ll discuss in a moment. Yet there are other measures you can do while they’re still inside 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.

Soon after germination

Starting about two weeks after germination (that is, after the plants have sprouted), we start hardening off seedlings by introducing a light wind – using an oscillating fan set on low. Don’t put it too close to them, and preferably not only 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.

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!

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 process of hardening off seedlings 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.

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.

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 weather forecast for the coming week. 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. Then when you’re ready, read our top transplanting tips here, and this guide on how we amend our soil before planting.  

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 insect netting 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!

And that’s how to harden off seedlings!

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.

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!