From the Ground Up Blog


What "Hardening Off" Seedlings Means and How to Do It

If you've ever tried starting vegetable seeds indoors, you've probably heard someone say, "Don't forget to harden them off!" a time or two. But what does that mean, and why is it so important? 

What is Hardening Off 

Hardening off is the practice of slowly acclimating young plants to a new environment to prevent them from going into shock. 

When we compare the conditions of your home (a seedling's temporary home) to the conditions of your garden (a seedling's permanent home), they are vastly different. One has a controlled temperature and humidity, no wind, virtually no pests, filtered sunlight or artificial light that never gets too hot, and predictable moisture. At the same time, the other is entirely unpredictable regarding temperature, humidity, wind, sunlight, and many other factors.

It should be no surprise to us that if we move an adolescent tomato plant from its perch on our sunny kitchen window sill to an outdoor vegetable garden at the end of spring when the days are warm, the nights are chilly, and spring rains make moisture control unpredictable, the seedling at the very least is going to go into shock.

In most cases, however, tender seedlings go from transplant shock to death so quickly that it's not often something you can fix. This is why proper hardening methods are so vital.

More: How to Care for Young Seedlings> 

Tomato Seedlings

How to Harden Off Seedlings

So how do you go about hardening your new plant babies?

When you first start your seeds indoors and they begin to germinate, you will notice that their leaves look nothing like that of a mature plant that you may have seen before at your local greenhouse or u-pick farm.

That is because most seedlings don't get their "true leaves," or cotyledons, for two to four weeks after germination. Seedlings will need several weeks to grow into these new leaves and set good strong roots before you should begin the hardening process. The stronger the plant, the easier it will be for them to adapt to their new habitat.

When to Start Hardening Off

In general, begin the hardening process 7-10 days before the date you intend to plant them out in your garden and once the danger of frost has passed. If your growing zone generally recommends that starts don't go outdoors until the last week of May, begin hardening seedlings a little more than a week before. 

Select a Shady Outdoor Space

Once it is time to start the process (and it is a process that will take a decent amount of time and attention on your part), you need to select a spot in your yard that gets dappled or indirect sunlight and is fairly protected from the wind.

We're just going to give your new little starts a taste of what the great outdoors are like without overwhelming them entirely. Remember, transplant shock can happen easily and quickly, so we want to be careful about where we put them out for the first time. 

Slowly Acclimate Seedlings to Outdoor Space

Once you've found a good spot, leave starts out for an hour or two max and then bring them in and return them to their original site indoors. The next day, return them to the same spot outdoors during warm daylight hours and increase the time by another hour or two depending on how they did the day before. 

Continue this same pattern, increasing outdoor daylight hours by small increments each time. This process may seem tedious a few days in, but it is essential from strong, healthy starts. Avoid putting seedlings out on days that are particularly windy, chilly, or rainy. 

Move Seedlings to Direct Sunlight

After three or four days of testing out the protected shady spot, you can now move your seedlings to a location with more direct sunlight and exposure to the elements. 

But again, be mindful of the time you expose them to these conditions.

If the day previous to moving them from their shady spot, you had them outdoors for a total of five hours. Starting at five hours in their new full sun location should be sufficient and begin to increase their exposure time an hour or two at a time again after that point. Each day the seedlings are growing stronger and stronger and slowly adapting to the conditions of their new home.

Transplants Seedlings in Sun


When to Leave Seedlings Outdoors Overnight           

Once outdoor temperatures remain around 50 degrees Fahrenheit through the night, and if the seedlings are tolerating the hardening process well, you can start to leave them outdoors overnight. This should take place 7-10 days into the hardening process. 

Only leave them outdoors through the night for a single night initially and then bring them in first thing the following day to see how they handle the adjustment. If they appear well adjusted and happy, try leaving them out overnight for another two or three nights before fully transplanting them.  

Keep in mind that outdoor conditions will dry their soil faster than indoor conditions, so they should be nice and moist before being placed outdoors for such an extended period. 

If you notice that the seedlings struggle a little after their first overnight stay outdoors, go back to a daytime hardening schedule for two or three more days and try again. It won't be long before they're thriving in their new home! 

After Seedlings Have Been Hardened

After the starts have been fully hardened and are ready to be planted in their new homes, here’s how to make their new surroundings as comfortable as possible. 

First, water seedlings in their containers before you transfer them and then water them again after you've planted them. Continue to keep them consistently moist but not soaked for the first couple of days, and then begin to taper back slowly to a regular watering schedule. 

Second, when you are planting (with the only exception to this being tomato plants), be sure that the soil level of the hole in your garden meets with the top of the soil level of the seedlings. Burying the start deeper than it is already can cause the plant to suffocate, shock, and die. 

Lastly, don't forget to feed your new starts. An all-purpose vegetable fertilizer is just great. Be sure to read any usage labels before applying to avoid burning or overfeeding plants. 

So what are you waiting for? Get growing! Just don't forget to harden those babies off once you do. Following all these steps with care will result in healthy, well-adjusted, strong starts for the coming garden season. And happy starts make for happy gardeners!