Hardening Off Seedlings — Why and How it's Done

Give Those Pampered Starts a Fighting Chance

🌱 Plant Talk | by Guy Saldiveri | August 17, 2026

You've spent over a week waiting for your seeds to germinate, then another three weeks nursing them in a controlled environment, getting them to grow to a decent size.

Now you're ready to drop them in the ground, or in those pots, and watch them explode with growth. 

The only problem — many of them won't make it if you do that, and here's why.

Plants grown in a controlled environment with no real temperature swings, no wind, and especially no sun, will fail more often than not when transplanted into their future homes. 

Seedlings are babies, and these babies need to be weaned before they can handle the harsh environment you're about to plant them in. In order to get them ready, we do what's called hardening off. 

So, what is hardening off?

It's the process of getting those pampered seedlings ready for the harsh growing environment they'll be living in for the rest of their lives. The heat, the temperature swings, the wind, rain, and yes, the SUN.

Have you ever been in the dark and have a bright light turned on all of a sudden? Your eyes snap shut, and your brain wants to explode — I tend to mumble a few things under my breath as well…

And think about those times when you've sat in the sun. After about 30 minutes, your exposed skin usually gets hot, turns red, and you start to sweat. If the sun is intense enough, and you stay there long enough, you can actually blister. 

Would you like to guess what happens to those seedlings that have known nothing but 70-degree temperatures, calm wind, and gentle light? Yeah, many times they'll go into major shock, have the plant equivalent of massive heart attacks, and either melt where they sit or flop over and never stand up again.

Ok, maybe that was a bit dramatic, I probably watched too many silent movies as a kid — but the sentiment is real. 😎

So what do we do?

We transition them, and we do it gently over a period of about a week or so. 

Note: Some plants may require more time, and some less, but this is a basic schedule to give you a general idea.

A simple 7‑day schedule

• Day 1 — 1 to 2 hours outside in bright shade.
• Day 2 — 2 to 3 hours in bright shade plus an additional 30 minutes in dappled sun.
• Day 3 — 3 to 4 hours outside, including 1 hour of gentle morning sun.
• Day 4 — Half‑day outside; morning sun only.
• Day 5 — Full day outside; avoid harsh mid-day sun.
• Day 6 — Full day outside plus 1 hour late‑afternoon sun.
• Day 7 — Leave out all day and overnight unless temps dip below what the seedlings can handle.

My rule of thumb for Day 7 is about 50° F, but it really depends on the particular plant.

It would also be good to have them in an area that doesn't block the wind. The movement caused by the wind actually helps the plants produce nice, thick, healthy stems. Without the wind, seedlings can stay too thin and have problems supporting the weight of the plant as it matures.

Now, I have to say, while I'm providing this information, this is one of the reasons I choose to wait a little longer to get my seeds going each year. I prefer direct sowing when I can because that negates the requirement for seed trays, worrying about bringing them in and out of the weather, working with extra equipment such as grow lights and heating pads, AND, it also bypasses the need to harden off seedlings. 

When you direct sow, they're already sprouting in the environment they'll live in, so there's no need to make the transition. And some things like carrots, celery, and beans don't transplant well anyway — for those plants, direct sowing is the better way to go.

One other thing to note:

Hardening off is something that's usually done for spring plantings. Generally, it's too cold outside for the seeds to germinate and the seedlings to survive. Most folks start their plants indoors and move them out afterwards. 

Getting set up for a fall garden doesn't require this step. I seed my trays and just leave them outside in a shady area until they're large enough to transplant. I know many think it's too hot outside in August, and some seeds won't germinate at those temps. My take on that — if it's too hot to get them to germinate, it'll be too hot to transplant them when they're ready. Wait another 2 weeks and you'll be good to go.

See you around the shed. 😎

Growing Beauty — Sharing Knowledge — Loving Nature

Happy Gardening 🌱