How to Harden off Seedlings Before Transplanting Outside

Hardening off young tender plants so they can withstand the heat, wind, rain and cold swings of Spring is the most important thing you can do before transplanting seedlings in your garden, particularly if you started your plants indoors.

Jump to: Step by Step for Transplanting Outdoors | Pro Tips | Troubleshooting | Video

For me, hardening off young plants is a difficult task because I grow so many of them for sale at the farmers’ markets, and it is therefore difficult to give them the individual attention they would love.  For the home gardener with fewer seedlings, the step by step process would go like this:

Step-by-Step for Preparation for Transplanting Seedlings

When to start hardening off young plants:

Depending on the weather, start exposing your seedlings to the outdoors around 3-4 weeks after they have germinated and have at least one set of leaves beyond their seed leaves (seed leaves are those skinny smooth leaves that come up first at germination and generally fall off after the plant has matured a bit);

Prior to hardening off seedlings they should be potted up from tiny cells into larger pots (I like 4” pots). For a post on how to do this, click here.

Gradual Exposure to Sunlight and Wind

1. on their first day outdoors, go slow and cool.  Do NOT put them out in direct sunlight on their first day out and do NOT put them out on a windy day.  Direct sun or wind (warm or cold wind) will do a lot of damage and really set them back.  Ideally you would have a shady spot with dappled sunlight and you would put them out for a few hours in this spot (morning or afternoon is fine as long as the spot is semi-shady and the weather is still;
2. Bring them back in after a few hours.  There is no set time here, as it depends entirely on what the weather is like that day, how far along your seedlings are, and what their inside growing conditions have been.  Just check them every hour or so and if they are looking sad or wilted, bring them back in;
3. Repeat these steps for a few days, gradually giving them more sun and more time outside;
4. It usually takes about 7-10 days to get the tomato and pepper seedlings fully hardened off.  If you do it gradually and add more sun to the hardening off area each day, they should be able to take full sun, rain and gentle winds after this.  Hail and strong cold winds however, will probably damage your plants, no matter how careful you have been in hardening them off.

Pro Tips:

• make sure they are watered before you put them out.  The weather is harsh on these young seedlings and they need to be babied for a few days;
• it helps if you open a door or window and let cool air and breeze into the growing room before you take them out (not absolutely necessary, but it helps);
• Don’t keep your seedlings on the ground if they are out for any length of time.  Rabbits, voles and mice love the tender growth and will be quick to eat them while you’re away.

Trouble Shooting Problems:

• Vegetable plants are often stronger than you think, so don’t completely despair if something goes wrong.  I have had my tomato seedlings decimated in hailstorms, put them aside to compost them,  and then be amazed to see them come back stronger and greener than before (no guarantees, but don’t be too quick to toss them);
• If your plants are exposed to sun too early or for too long they may get sunburned and the leaves will look more of a pale yellow color rather than a dark lush green.  They are probably OK.  Just look at the new growth and if it looks good, your sunburned leaves will not affect your plant or its fruit as it matures.;
• If the weather is bad for a good part of the spring and you can’t get your seedlings into the ground to start growing, you can fertilize them with some kind of nitrogen-rich foliar spray (seaweed, fish emulsion or something like that). Just make sure they have been potted up to larger pots before fertilizing.

Transplanting Seedlings Outdoors

If the steps listed above are followed, it should take about 5-7 days before seedling can be safely planted outside. This is going to depend on the weather during those 5-7 days and how strong the seedlings were prior to hardening off.

Many vegetables are vining plants or large plants (pole beans, tomatoes & peppers, etc.) and will need some sort of support. It is a good idea to get your support system in the ground (or pot) shortly after transplanting outside while they are still small plants. Here’s a detailed post on cages vs stakes vs trellis system.

Guides for successfully growing tomatoes

• For germinating seed indoors
• Growing tomatoes in pots
• Best seed catalogs for tomatoes
• Best trellis systems for tomatoes
• Season extension for tomatoes
• Taste of heirloom tomatoes based on color

Video Tutorial on How to Harden Off Seedlings

Hardening off tomato & tender vegetable plants for Spring

Watch this video on YouTube.
Watch the Step by Step Video for More Details