Seed-Starting & Transplanting

Hardening Off Seedlings With Niki Jabbour

Hi, I'm Niki Jabbour. And today I want to talk about hardening off seedlings.

Why Should You "Harden Off" Your Seedlings?

Hardening off is a process that acclimatizes indoor-grown vegetable, herb, and flower seedlings to outdoor growing conditions. It's a step that can't be skipped. If you don't harden off your seedlings properly, they can be prone to moisture loss, sun scald and be set back or perhaps even die. So let's learn more about when and how to harden off your seedlings.

When to Harden Off Seedlings

Let's start with the “when.” The hardening off process takes around 5 to 7 days. So I start about a week before I wish to plant them outdoors. This gives the plants time to build up the cuticle and waxy layers on the leaves that reduces water loss and protects the plants from UV light. For tender crops like tomatoes and peppers, I start hardening the seedlings off around the last average frost date. For cool season vegetables and herbs like cabbage, broccoli or parsley, that will be earlier in the season with the timing, depending on whether they'll be transplanted into open garden beds or beneath the mini hoop tunnel. Either way, seedlings need about a week of hardening off.

Where to Harden Off Seedlings

Now let's look at where to harden off seedlings. You need to find a space that offers some shading. This could be beside your house, beside a garden shed, on a front or back porch, beneath a patio table, or you can create your own shade by setting up some hoops—wire hoops, PVC or metal conduit hoops—and covering them with a piece of shade cloth or a couple of layers of row cover.

How to Harden Off Seedlings

Once you've found a good spot for hardening off, it's time to start the process.

  • Day 1

    Begin by watering the seedlings. You want to make sure they're well-hydrated and not water-stressed. You can then move the trays or pots outside, placing them in a sheltered site with full or dappled shade. This is best done on a mild day when temperatures are above 50 degrees Fahrenheit (10ºC). Bring the seedlings back indoors that night.

  • Day 2

    On day two, set the plants outdoors again, this time in a spot where they'll get about 2 hours of morning or late afternoon sun, but not the hot midday sun. If using a mini hoop tunnel to harden off your seedlings, it's easy to remove the cover for a few hours. Bring the plants back indoors for the night.

  • Day 3

    On day three, continue to increase the amount of light the plants receive, giving them 3 to 4 hours of direct sunlight. Again, morning or late afternoon light is best as it's less intense. Bring the plants indoors again that night.

  • Day 4

    On the fourth day, plan to give the seedlings 5 to 6 hours of sunlight. I often set them out early in the morning and cover them up or bring them back inside by 2 p.m.. If the nighttime temperatures are predicted to be mild in the 55 to 60 degree Fahrenheit range (12.7ºC to 15.5ºC), you can cover the plants with a row cover and leave them outside all night.

  • Days 5 & 6

    By the fifth day, the seedlings should be ready for a full day of sunshine. If possible, check on them during the day and if they seem wilted, move them to shade. Leave them out again that night. Repeat on day six.

  • Day 7: Transplant Out

    Day seven is moving day as the seedlings are ready to be transplanted into their garden beds or containers.

Watch the Weather

One more pro tip for hardening off is to watch the weather. Unpredictable spring weather can slow and set back the hardening off process. If you've got a week of clouds, rainy weather or cold temperatures, it will take more than seven days to harden off your seedlings.

I hope you understand more now about hardening off and when to do it. It's an important step that you just can't skip when starting seeds indoors.

Happy growing!