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How to Harden Off Seedlings in 7 Days

Start the gardening season off right by learning how to harden off seedlings successfully in just 7 days’ time. You’ll enjoy healthier plants, yield larger harvests, and get the job done all within a week. Come see how I do it!

How to harden off seedlings starts by placing them outside in a safe place as shown here on a stoop.

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It’s that time of year again. I personally love gardening season. Even if it does require a good amount of planning and preparation. 

This year I’ve condensed my garden and will be growing less of a variety… but I’m convinced I’m going to have higher yields due to changing my gardening methods and amending the soil really well.

In particular, though, I want to discuss how to harden off seedlings. This ensures a successful start to growing outdoors and allows plants to thrive all season long. You’ve put in the work to grow these over the last few weeks–you wouldn’t want them to give up now. 

The best part is I’m going to show you how to harden off seedlings all in a week. How easy!

What does hardening off mean?

Hardening off seedlings means to gradually acclimate a plant to the outdoors. This is done before transplanting them into raised garden beds or directly into the soil.

They’ve likely been started indoors, in a climate controlled environment with lots of added protection. If you introduce them to the outdoors too quickly, extreme changes in the environment could really have detrimental effects. Doing this over a course of days allows the plant to adapt so it will survive once transplanted.

Furthermore, all plants have a cuticle, a layer of protective coating that helps regulate moisture levels and UV light exposure. This cuticle does not fully develop when started indoors. Gradually introducing them to the outdoors helps this fully develop before the next big step: transplantation. 

Why is hardening off seedlings so important?

Hardening off plants before transplanting is important for several reasons. 

Benefits to hardening off seedlings:

  • Allows plants to adapt to a wide range of weather (temperature, wind, light rain, unfiltered sunlight)
  • Saves energy for plant to focus on growth instead of surviving its new environment
  • Prevents damage like sunburn, shock, etc.

What happens when you don’t harden them off first:

  • May experience transplant shock if not “weaned off” from the indoor environment first
  • Could experience heat stress that can dehydrate the plant and kill it
  • Winds or rains could damage seedlings if not properly hardened off first

Bottom line: If you fail to properly harden off seedlings, you take a pretty huge risk in undoing all you’ve grown over the last few weeks. 

However, I do want to note that hardening off plants is not necessary in larger plants purchased from nurseries and other stores. These should have already been hardened off, as most of them are already kept outdoors. They are also not quite as delicate since they’re larger, have developed deeper root systems, and can take a little more of a hit without completely being devastated by factors like strong winds, heavy rains, and temperature changes.


Seed Starter Tray Kits

Grow Lights for Indoor Plants

Pepper seedlings are being hardened off over 7 days time.

How to Harden Off Seedlings

Prep them indoors beforehand | how to harden off seedlings

In the days leading up to hardening off, start by either brushing the seedlings with your hand a few times a day or placing a fan on low near them to help strengthen their roots. This helps them acclimate to part of their new environment outdoors. 

Also, be sure to plan the correct date to begin hardening them off outdoors. Use this calendar HERE to check your planting zone to see when they can survive outdoors. Then start hardening them off about a week before you plan on transplanting them.

Day 1: Outdoors in partial sun for 4-5 hours

This is their first exposure outdoors. I keep this time minimal since they’re not used to unfiltered UV rays. Try to place them in dappled light, but if full sun cannot be avoided keep them outside for less time, keeping an eye on them. Check soil when you are ready to bring them in. They’ll likely need watering at this time. After bringing them back inside, place them in their usual spot with the same artificial lighting you’ve used up until this point. 

Day 2: Outdoors in partial sun for 5-6 hours | how to harden off seedlings

You can increase the time outside by a couple of hours today. Again, proceed with caution in full, direct sun. If leaves begin curling and drying, you’ll know it’s time to bring them in or move them to a shadier spot.

Marigolds start to grow inside cells of a seedlings starting tray.

Day 3: Outdoors in full sun for 6-8 hours

Increase outdoor time again. Full sun is recommended by days 3 and 4 to get them fully acclimated in time. Be mindful and check them once or twice during the day for signs of stress and dehydration. If needed, you can move them to a shadier spot and water them midday.

Day 4: Outdoors in full sun for 8-10 hours | how to harden off seedlings

Repeat that same step from Day 3, except add on a couple more hours. They’re almost ready to be outdoors all day in the sunshine!

Do be sure to check for squirrels and rabbits getting into them over these next few days. They are notorious for showing up at the worst possible times. They will dig plants up and get into anything. Be on the lookout.

Day 5: Outdoors for 12 hours (all day)

Your seedlings can now be outside for the duration of the daytime hours. I recommend watering both before and after their outdoor break today since they’ll be in full sun for a full 12 hours straight. They are getting so strong and almost there.

Basil begins to grow inside cells of a seedling tray in an effort to learn how to harden off seedlings.

Day 6: Outdoors all day and night | how to harden off seedlings

Finally, they are ready to be outside for a full 24 hours. Tonight they’ll begin adapting to the change in hot day temperatures to the cool nights. This can be a lot, so do not transplant them just yet. Keep them in their cells or pots and they can become acquainted with their new home tomorrow.

Day 7: Transplant

By this time, your seedlings can now be moved to their permanent garden bed! They should be used to the elements enough to properly survive outdoors now. However, some experts recommend transplanting on a cloudy day. Although they should have adapted to the sunlight by now, transplanting to a new home in new soil with a lot more space can shock a plant. By planting on a cloudy day, this helps the plants focus on getting accustomed to one thing at a time. 

To transplant, space them according to seed package directions. Intersperse vegetables with flowers and herbs to help prevent pests and disease. Apply a top dressing of mushroom compost or rabbit manure (unless you’ve prepped this a couple weeks ahead of time) to fertilize the soil. When dealing with leggy seedlings, bury them in soil until it hits their first leaves and this should allow them to grow full and strong to produce a lot later.

Be sure to keep them watered properly during these first few days/weeks until they get bigger. Now is not the time to neglect them. If there is a risk of frost after transplanting, cover plants with fabric coverings in the evening and remove the next day.

Leggy tomato seedlings get some sunshine on a porch before they get transplanted.

Tips for Success | how to harden off seedlings

When hardening off seedlings, always start slow. Slow introduction to direct, unfiltered sunlight. A slow introduction to the elements: windy days, rain showers, and extreme temperatures (i.e. hot days and cool nights). Use your best judgment. For example, if thunderstorms are occurring in those first few days of hardening off outdoors, it’s better to bring them in during storms. Light showers may be fine for a very short time, but strong winds and heavy rain over the course of hours can kill your seedlings. 

Set an alarm each day so you don’t forget when to place them outside and when to bring them in. Timing is everything during the first few days when plants are adjusting to the outdoors.

Don’t forget to check your seedlings for moisture once you’ve brought them in for the day. Direct sunlight can really dry up the soil quickly. 

How to harden off seedlings includes setting them outside for a few hours each day, shown here.

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