How to harden off your plants
“Noooooo! What happened to my beautiful seedlings that looked so good at the greenhouse or growing indoors?? They look horrible now that they’re in the garden!”
Hopefully you’re here reading this before you’ve ever had an experience like this. I have sadly heard this many times - it is a common mistake, and not intuitive unless you’ve heard about it before. Or you may know about hardening off, but get impatient, and wonder “what’s the worst that could happen?” I’m hoping this article can save you some heartache and your plant's some unnecessary stress (or even death). So if you’d like to know how to harden off your plants, read on!
Plants from the greenhouse or grown indoors aren’t ready to be thrown into the deep end of being exposed to the outdoors elements from their sheltered newborn lives. Hardening off refers to the process of gradually acclimating the plants to the elements to prepare them to be grown outdoors, including full sun, temperature fluctuations, and wind. This is a really critical step. It’s terrible to see a newly planted starts get sunburned and then have to outgrow it. It’s stress on the plant that should be avoided. I will say, hardening off is a pain though. Especially if you have many trays of plants. Moving them in and out, over and over is no one’s idea of a good time, but alas, it is for the best. This is my life right now. Solidarity <3
There is no exact formula: do X for this long, then Y for this long, and viola! Perfectly hardened off plants! I’m going to go over some of the principles to guide you in the thought process so that you can make flexible decisions based on your space available to set them out, and specific daily conditions. I will give a sample schedule for those that are visual and like seeing a step-by-step. If you’re going to use it, you’ll still need to take variable daily conditions into account - whether it’s overcast, windy, hot, cold, etc - and make adjustments.
Which plants need to be hardened off?
Any plants that you have started indoors and plants that you get from somewhere else that have either been grown by seed indoors (ask if they’ve been hardened off), or in a greenhouse. Why do plants grown in a greenhouse need to be hardened off? A lot of people assume they can buy plants from a nursery and go home the same day and plant them. They may not die because of this, but it’s still a good idea to harden them off. Greenhouse-grown plants will have one thing going for them that plants grown under artificial light don’t - since they've been grown with sunlight, they’re unlikely to get sunburned. But conditions in the greenhouse are a lot milder than in the garden. A greenhouse (and inside of a house) is climate controlled. The plants will not have been exposed to winds and hot and cold fluctuations of the garden. Plants sheltered in the greenhouse have a rude awakening if put right into the garden and you could end up with broken or weak, bent over stems and struggling plants that have to suddenly adapt to the dry climate and cool night temperatures. So take the extra time and give your plants the best chance of success. Plants you find in full sun at a big box store won’t need to be hardened off since they’re already fully exposed to the elements.
When to start
When you start this process will depend on what you are growing. A good time to start is 1-2 weeks before you’d like to plant them in the garden. So cold tolerant crops will be earlier than warm season crops. If it is a warm season crop, start early-May if you’re on the Front Range and have a rough date of last frost of mid-May. Then they will be ready to plant the second half of (late) May. This would be for things like tomatoes and peppers.
To figure out hardening off and transplanting times for more than just tomatoes and peppers, take a look at the transplanting information in the chart above by Home Grown Foods for broad guidelines, and this calculator from Johnny’s Seeds. It is a seed starting calculator, but also includes columns for when to plant out based on your last frost-free date. On the Front Range of Northern Colorado, I recommend using around May 15th. This is closer to the last chance of frost rather than the average last date of frost, which means there is a 50/50 chance you could still get frost after that date. So, plug the date into the calculator, and take a look at when it says to plant out, and count back 1-2 weeks for your hardening off window.
Choose a spot
If you have an east-facing, sheltered porch or patio area, or somewhere that offers some wind protection, this is a great choice for a spot to harden off your plants. Morning light should be your first choice, especially early on in the hardening off process. The more gentle the light, the longer you’ll be able to get away with leaving the plants out without risking sunburn. If you don’t have a sheltered area near your house, choose a spot under a tree, but try your best to keep it a bit protected from wind.
East-facing patio, sheltered
Let's get started
To begin, choose a day that is warm (maybe 60s or above for warm season crops) with little to no wind. Check to make sure your plants are watered. Nothing will kill your plants faster than going outside on a particularly warm day, dry, and having their roots bake in black containers. Choose days or times of day that you will be around the house or yard and can check in on your plants. Place them in your sheltered spot, either getting a bit of direct sun, or not, for about an hour.
Tip: Set a timer as a reminder! This is so important. You don’t want to accidentally sunburn your plants or come back to plants that have their roots dried out and have baked (been there!). I tell my Google Home to set a timer so I don’t forget.
Gradually increase the time that they are exposed to direct sun and some wind over the course of a week or two, until eventually they are spending all night outside.
Here is a sample schedule if you’re in a hurry
Day 1 - set the plants out near the house to protect them from wind. If it’s an overcast day, leave them out for an hour or so, if it’s sunny, leave them for 30-45 minutes.
Day 2 - increase the time out by about an hour (e.g. 2 hours max of direct sun)
Day 3 - continue to increase by another hour (e.g. 3 hours max of direct sun). Perhaps day 3 or 4 is when you move them out of their more sheltered area more where they can get some gentle wind. If this is a day that is windy, hold off for a day with low wind speeds.
Day 4 - Continue to increase time of sun exposure. (e.g. 4 hours max of direct sun)
Day 5 - By now, your plants may be getting 4-5 hours of direct sun. As long as they are shaded from harsh afternoon sun, they could spend the night outside if temperatures are in the 50s.
Day 6 - Plants are getting 5-6 hours of sun. Continue as in step 5
Day 7 - plants can be out all day, though continue to check on them in case the pots dry out. I recommend direct sun for no more than 6-8 hours when they’re still in containers. They’ll be used to the full strength of the sun, but won’t be stressed out by the drying qualities of the sun, heat, and wind, which they will be better suited to tolerate once their roots are in the cooler ground.
Day 8 or beyond - can plant in the garden.
When I harden off plants, I do it over a couple of weeks. I don’t like to rush the process and risk sunburning them. Throughout the process, it is important to check on your plants before the end of your timer. You will often be able to see that something is going wrong. If they are wilting, get them into the shade and check if they are drying out. If the leaves begin to get whitish marks on them, it means they are getting sunburned, and it’s time to move them in shade. Obviously you’d like to prevent this, but catching it early is better.
These are not hard and fast rules. Just some general guidelines. Use your best judgement, and when in doubt, err on the side of being more conservative. If you want to leave them out for just one hour, two or three days in a row, go for it. They’ll still be getting used to the full strength of the sun. If you are in a hurry to get them hardened off, you may choose a more aggressive schedule where you will be done in one week. Do what is best for you!
Alright, there you have it. I hope this gives you the knowledge and confidence to harden off your plants. Good luck and happy planting!