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

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 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.

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.