How to Harden Off Your Seedlings with Ease

If you've been a growing your plants from seed for long, you are likely familiar with the term, "hardening off", which in the simplest terms refers to the necessary practice of preparing your baby seedlings for the wild and wooly outdoors.

After being in their cozy, temperate apartment cell in your home for days, and likely months, your seedlings would be quite shocked if you plucked them out of their current home and plopped them in the soil outdoors without any sort of warning or notice!

We would feel the same, no?

Moving is hard, even for plants; but moving from one environment to a totally different environment without a gradual warmup is brutal.

And for a little baby seedling, no matter how long it's been growing, this change could kill them.

Now, you may be thinking, "I thought the whole point of growing them in the house with grow lights and water and the perfect temperature was to mimic the great outdoors," and you'd be right.

However, generally speaking, our seed-starting setups are a far cry from what Mother Nature has to offer.

You can have the best grow lights available, but you'll never be able to mimic the intensity of the sun.

You can water "as nature would," but you'll never really quite match the benefits or harshness of a natural rain storm.

You can provide your seedlings a fan, to mimic the breeze outside and strengthen their stems, but this isn't fully representative of the sporadic and wild nature of the wind. Even a gentle breeze can feel like trickery at times!

My point here is, your seedlings need to be gently introduced to the inconsistent offerings of Mother Nature, especially the intensity of the sun, before being transplanted into the ground.

The exception here might be plants that were grown in an outdoor greenhouse and had sunlight exposure through the windows, or whose doors were open to allow airflow.

But even then, I'd be a bit apprehensive to plant them in the ground without at least some sort of warm up to their new environment.

WHAT HAPPENS WHEN YOU DON'T HARDEN OFF YOUR PLANTS???

➡️ Well, the least that can happen is damage to some of their leaves, severe wilting, and overall stress on the plant. 

➡️ The worst can happen is death. The shock of the sun, in particular, but also the wind can be too much for the tender seedling and they'll wither away before your eyes!

Wilting and damage to their leaves may not seem like a big deal, but it can stunt their growth, canceling out the head start you gave them in your house.

With rare exception (ex. growing in a hoop house or maybe an outdoor greenhouse), I would never advise a client or friend to skip hardening off process in some degree!

My Method of Hardening Off

I think most gardeners agree that hardening off seedlings is an annoying process because of the time and "work" involved.

Unfortunately, it's not a "one-and-done", "set it and forget it" process. We have to pay attention, constantly observe, watch a number of factors, and be ready at a moment's notice to move your seedlings.

So HOW exactly do we do this?

As with anything, there are different methods and theories, most of which I've tried and experimented with over the years. For the time being, I've settled into a method that works well with my plants AND is less labor intensive.

Now, "labor intensive" is a bit tongue in cheek because it likely won't cause you to sweat, but regardless, it can seem like a lot of work. Weeding is an obvious contender, but at least you're in your garden for that practice!

If you're not familiar with the process of hardening off, it generally involves removing your plants from their grow space indoors and setting them outside for small increments of time. Then, after that time has passed, taking them back inside to their grow shelf for the remainder of the day.

The next day, you repeat the same process, leaving them outside a bit longer, then taking them back inside again. You repeat this "in and out" process for, on average, 7-8 days.

So, you can probably see why this is a somewhat exhausting task, especially if you're like me and have 20+ seed trays you're having to take in and out of your house two times a day!

Not only do you likely have more pressing matters to attend to, you just want to get those babies in the ground and start gardening already!!

But alas, as we already discussed, you don't want to risk killing your seedlings that you've spent weeks and months nurturing inside your home. That is SO disheartening!!

✅ The method I've settled on involves choosing the right spot with the right timing, and leaving plants out overnight.

These all kind of overlap, but I see them as different entities as well. So, let's break it down...

1. Pick the Right Spot

There are a couple of things you want to consider here, including the side of your house, sun exposure, and access to your house.

Everyone's house and property are different, so it's not enough to just say "pick a shady spot on the east side of your house". You have to consider what that spot is going to do at different points throughout the day.

In general, find the shadiest side of your house (or wherever your seedlings are located) and start there. Reason being, placing your seedlings outside in the shade is the gentlest way to acclimate them to the outdoors.

They'll be exposed to the temperate, outdoor air; they'll experience the breeze and slight changes in temperature, but they're NOT being exposed to direct sunlight yet.

If you can find a spot that is shaded all day, with the exception of some early morning sun, that is generally what I'd recommend BECAUSE you won't have to worry about setting a timer reminding you to bring plants inside before the sun hits them! Early morning sunshine is generally much less intense, and your seedlings should be able to tolerate it. 

However, by 10 or 11 am, they're still exposed to the sun, the heat (depending on your specific locale) can be start to get intense, and you may need to bring them in or shade them.

If your seedlings are in this type of setting, SET A TIMER to remind you to check in on them, as well as the temperature and the intensity of the sun. You don't want to forget they're out there and come back to find withered and baked plants!

IDEALLY, I'll place my seedlings in a spot that receives some morning or maybe some late evening sun, but is shaded throughout the day. 

This allows me to leave my plants out ALL DAY, and possibly through the evening (as long as temperatures don't drop below their liking), which means LESS WORK FOR ME and more time for my seedlings to acclimate!

THIS IS A WIN-WIN!

2. Pick the Right Timing

You can see how this overlaps the last topic, but there are some things to note here as well. 

For example, sometimes late evening sun exposure is still too intense for weak seedlings. It's really a judgment call, but if it feels hot to you, it likely feels hot to your seedlings.

Think about early mornings once the sun has come up....It's still cool out and sometimes breezy. You might even need a jacket. And as long as the temperatures are at or above what your seedlings prefer, early morning is a great time to gently acclimate them to some sun.

HOWEVER, as mentioned above, keep your eyes on the temperature! Even in early summer, the heat can rise quickly once the sun is up, and you'll want to shade your plants or bring them in.

Same thing goes for evening sunshine. Depending on your locale, evening sun exposure may still be too much for young seedlings; but in other locales they may be just fine hanging out in the evening sun. 

And if that's the case, you might be able to leave them there overnight into the next morning.

So, on that note, let's talk more about timing.

How do you determine how long to leave your plants outside? Well, it depends (don't you just love that answer?!)

1. It depends on the temperatures outside
2. It depends on if you're seedlings are exposed to the sun or not
3. It depends on the weather (rain, wind, etc.)
4. It depends on how long you've been hardening them off

I don't want to put time limits on how long to leave your plants outside, because it really depends on the above factors.

Some say to start them outside for an hour, then steadily increase their time outdoors over the next few days; BUT an hour outside with sweltering temperatures (even if they're in the shade) might be too much. In that case, maybe only 30 minutes will do.

In other cases, being in the shade for 1-5 hours initially might be just fine. And if they've been hardening off for several days, they might be able to handle the higher, more intense temps for an hour or more.

This is where it gets really nuanced and, if you're not an experienced seed starter, maddening!

This is why I HIGHLY recommend finding a spot that has shade for most, if not all the day, and placing your seedlings there. 

As long as it's not too hot outside, they will likely be just fine there until it's time to bring them in. You'll just want to keep an eye on them to see if they start looking "wilty".

3. Overnight Your Seedlings

One of the biggest time-saving practices I use is picking a spot where I can leave my seedlings out overnight, if possible.

BE SURE & CONSIDER THESE FACTORS.

1. What are the overnight temperatures? 

If they are above your seedlings' preferred temperatures, you could leave them out overnight and possibly into the next morning. If not, you'll want to cover them with a sheet or frost cloth, depending on the evening lows or bring them in for the night.

Obviously, depending on the plant varieties you're hardening off, you'll be looking for different temperatures. But in general:

• Cool Season Plants can handle lows between 30°F and 40°F (I talk more about the nuances of cool season plants in this blog post.)
  
• Warm Season Plants (ex: tomatoes, corn, some herbs) can handle lows down to 40°F but not lower.
• Hot Season Plants (ex: beans, eggplants, peppers) can handle lows down to 50•F but NOT lower!

CLEARLY you will want to check the specifics of each variety you're hardening off to know what temperatures your seedlings can handle! The above are just generalities.

But if the overnight temperatures are within their desired temperature range, you'd be good to leave them out. If you're a bit nervous, bring them in or cover them up. Depending on your choice of cover, you can raise the temperatures underneath a degree or so.

2. Will they be exposed to early or late morning sun? 

If you leave them out overnight, will they be exposed to early or midday sun that might be too warm for them? If not, then you've just saved yourself some more time. If so, set a timer/alarm to remind you to bring them in or shade them.

I have my seedlings on an east porch that only gets early morning sun, which means I don't ever have to bring my plants in unless the temperatures get too low or there's a rain storm. 

If there's not room on that porch, my second choice is my north-facing porch that will only have the early evening sun. At this point, letting my seedlings hang out in some early evening sun is not a problem.

THIS SAVES ME SO MUCH TIME because I rarely have to move or adjust my plants while hardening them off! Of course, after a few days of being outside in the shade with some sun exposure, I am more confident to move them to a sunnier spot where they can start taking in more sun.

So, let's talk about that.

When to Give Seedlings More Sun

So, the question is, how do you acclimate seedlings to 14-16 hours of sun if they've only had shade or some early morning or evening sun?

On about Day 3 of hardening them off, I'll start adjusting my seedlings slightly - pushing them further into the sun for a couple of hours and them pulling them back if I feel it's getting too intense.

Generally, the late spring/early summer temperatures here in SW Michigan are not extreme, so after a couple days of being outdoors, the seedlings can handle more sun exposure.

By Day 4, I'm pushing them out in the sun a little longer, Day 5 a little longer, and by Day 6 or 7, I'm pretty much letting them hang out in the sun full time.

AS ALWAYS....I'm keeping a watch on them to make sure they're not looking wilted - this is a tell-tale sign of stress. So, if you're able, keep watch over them as the temps warm up and adjust as necessary.

** IF YOU WORK AWAY FROM HOME and are not able to keep watch over your seedlings, you probably want to keep them in a shaded spot for a day or two longer, pushing them a bit further into the sun with every day. Keeping them outside as long as you can will help them acclimate more quickly WITHOUT the risk of them dying while you're not on watch. The other thing to consider is putting them in a spot where they will not be in the sun during the hottest part of the day until you're home to watch them.

You'll be able to tell when they're ready to stay outside full time when the slight wilting stops.

Top Considerations When Hardening Off

1. Never put your pampered baby seedlings in DIRECT SUNLIGHT (mid-day to afternoon) if they've only been inside your house for the breadth of their life.
2. For the first couple of days, pick a place where you can keep them out most of the day and possibly through the night THAT'S 80-90% SHADED,
3. SLOWLY ACCLIMATE them to more and more sun over a week's time.
4. Bring them in at the FIRST SIGN OF WILTING! But don't necessarily wait for this sign because it might be too late for some seedlings.
5. WATER THEM BEFORE you put them in the sun (unless their soil is fairly moist). This will keep them from drying out and may help with the stress.
6. BRING THEM IN BEFORE IT RAINS, unless it is a light rain that won't flood their cells and trays OR if they're in a tray that drains. That being said, small seedlings are more likely to wash out of their individual cells if more rain is coming in than is draining out.
7. PROTECT THEM FROM HEAVY WINDS. Heavier winds can also stress them out if they're not used to movement. A light breeze should be fine and will help strengthen their stems while hardening off, but anything more than that the first couple of days might be too much. THE EXCEPTION HERE is if you've had fan on them for most of their time indoors or if you've been "brushing" them with your hands. Both of these help strengthen their stems and roots, preparing them for life outdoors.
8. WATCH YOUR FORECAST! Don't forget to keep an eye on your hourly/daily/weekly forecast. As you know, weather can change in a moment and you don't want your seedlings to be caught unawares. 
9. KNOW YOUR SEEDLINGS' LOWS. Whether you decide to leave them out overnight or throughuout the day, just make sure your plants can handle the lower end of the temperature spectrum. In most cases, you'll likely be fine if you're past your last frost date, but don't assume. Know the varieties you're planting and be sure to bring in a frost-sensitive seedlings if there's even a hint of temps dropping below their liking.

Closing Thoughts

Unfortunately or fortunately, however you choose to look at it, hardening off just is what it is... a process. And in most cases, it's just something you have to do to make sure your precious, pampered seedlings survive The Great Outdoors.

I like to think of it as a one-time thing (maybe two if I'm hardening off my cool season plants in the spring) - for the most part, it only happens once a year, and at the end of it, I get to start gardening! Which is our ultimate goal and joy, right????

So, yeah, it's not my favorite gardening chore, but it's the "great buildup" to one of my most FAVORITE TIMES OF YEAR!!! So with that, I can find great joy & satisfaction! 

So, off with you, fellow gardener, go harden off your babies....