How to Harden Off Your Seedlings to Plant Outdoors
So, your seeds are now seedlings, your last frost date has come and gone, and you’re wondering… what do I do next? The next step for any seedling that has been grown indoors is hardening them off. This method is applicable to any and all seedlings that you have started indoors. If you direct sow your plants after your last frost date, you of course, don’t need to do this step as your seeds will have “grown up” in outdoor conditions already, and aren’t susceptible to transplant shock like your indoor-grown seeds will be.
After all of this, you may be thinking “Why am I starting these seeds indoors if I have to do this much work?” – the answer is really quite simple (and completely up to you to decide if it’s worth it), starting your seeds indoors means your plant will bloom a lot sooner, versus if you direct sow after your last frost date. You’re essentially getting all of the waiting period of growth out of the way weeks in advance so that you can enjoy blooming flowers for a longer period of time during the growing months. Though hardening off may seem tedious, and it is, it’s really not that complicated of a process – let me show you!
Note: It is always important when starting your seeds that you check to ensure that they transplant well in the first place. Poppies are known to not be successful transplant seedlings, so it’s best to direct sow them after your last frost date.
What is Hardening Off & Why is it Important?
Let’s start by discussing what hardening off is, and why it’s integral to the transplanting of your seedlings. Hardening off is the process of slowly taking your seedlings that have been grown indoors, and acclimatizing them to outdoor conditions. There is a vast difference between artificial grow lights versus real sunshine & heat, and no wind at all versus varying strengths of wind for your seedlings, so it’s important to slowly get them used to these new conditions. Similarly to when you’d bring home a new goldfish from the pet store, and you had to put the bag inside the tank for a couple of hours, and poke small holes so the fish would get acclimatized to the new tank water, seedlings need this slow process so they don’t go into shock when they’re transplanted out. If you don’t harden them off, they’re likely to experience transplant shock, which results in them dying.
The hardening off process should always be done after your last frost date unless your plant is cold weather tolerant (ex. ranunculus, anemones, stock, pansies), and can handle a light frost. If the plant is cold weather tolerant, you can still follow this exact process, just sooner in the growing season!
My 1-Week Hardening Off Schedule
Over the last few years, I’ve followed this same hardening off schedule for all of my seedlings. Whether they be snapdragons, cosmos, ranunculus or even tomatoes, this has been a very successful schedule for me:
DAY 01: Put seedlings go outside for 2-3 hours in partial sun, ensuring they’re protected from the wind (in a greenhouse is a good way to block the wind), then bring them back inside.
DAY 02: Put seedlings go outside for 2-3 hours in partial-to-full sun with access to a light breeze, and then bring them back inside.
DAY 03: Put seedlings go outside for 4 hours in full sun with access to a soft breeze, and then back inside.
DAY 04: Put seedlings go outside 5-6 hours in full sun, unprotected from the wind, then back inside.
DAY 05: Put seedlings go outside all day in full, and bring them back inside at night.
DAY 06: Put seedlings outside all day and all night.
DAY 07: It’s time to transplant your seedlings!
And that is how you harden off your seedlings! If any questions about how to harden off your plants arise during the process, or even before, drop them in the comments below & I’ll be sure to answer what I can!
