Yes, You Should Always Harden Off Seedlings Before Transplanting–Here's How (2024)

To plant your garden in spring, you likely start seeds indoors or purchase greenhouse grown seedlings at garden centers. But before you move these tender young plants out into the elements, you need to know how to harden off seedlings to avoid transplant shock. Hardening off just means gradually letting seedlings acclimate to the outdoors before transplanting them so they are better able to withstand your garden's conditions. This guide explains everything you need to know to successfully harden off seedlings.

Why Seedlings Need to Be Hardened Off

Indoor seedlings are cultivated in a sheltered environment where they’re protected from harsh winds, bright sunlight, and cool spring breezes. As a result, the seeds germinate and grow faster than they would in outdoor gardens. However, indoor seedlings can develop transplant shock or sun scorch if they’re moved outdoors or into full sun too quickly.

Transplant shock is relatively common, and it can cause plants to have stunted growth, drop leaves, or in severe cases, wither and die. Sun scorch, on the other hand, typically causes plant leaves to wilt, discolor, or turn crispy.

Hardening off seedling before moving them outside helps tender seedlings transition to outdoor living and makes the transplanting process easier. By slowly exposing seedlings to increasing light and wind levels and lower temperatures, you encourage the seedlings to adapt and develop a thicker leaf cuticle that slows water loss and prevents the plants from drying out. Seedlings started outdoors or in unheated greenhouses may not need to be hardened off, but seeds started indoors or in warm greenhouses usually benefit from this process.

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When to Harden Off Plants

To determine when to begin hardening off indoor-grown seedlings, you need to know the date of your last spring frost. The hardening off process generally begins two weeks before your last frost date; however, cold-hardy seedlings can be hardened off even earlier—roughly four weeks before the last frost of spring. A good rule of thumb is that most plants can be hardened off when daytime temperatures are between 45°F and 50°F.

How to Harden Off Plants

To begin hardening off plants, move the seedling pots and trays outdoors to a sheltered section of your yard that receives dappled light. It’s best to do this on a warm day when the sky is partially overcast. Allow the seedlings to rest outside for one hour before moving them back indoors under grow lights.

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The following day, place the seedlings outdoors again, but this time allow them to stay in dappled light for two hours. Repeat this process over one to two weeks while gradually increasing how long the seedlings are outside and the light levels they are exposed to.

The hardening off process is complete when the seedlings are outside for most of the day and exposed to the light levels that are appropriate for their growing needs (shade to full sun, depending on the plant).

At this point, seedlings should be ready to transplant as long as nighttime temperatures are above 50°F (or above 60°F for tomatoes and other nightshades). For best results, transplant the seedlings on an overcast day, handle their roots with care, and water deeply after planting.

If a cold snap occurs after transplanting, shelter tender seedlings with frost blankets or DIY cloches made from overturned milk jugs until the temperature improves.

Yes, You Should Always Harden Off Seedlings Before Transplanting–Here's How (1)

Tips for Hardening Off Plants

Keep these simple tips in mind when hardening off plants, and you’re sure to have success:

  • If you have a lot of plants that need to be hardened off, keep your seedling pots and trays in a wheelbarrow and wheel them outdoors during the day and into a sheltered garage at night. This will save you a lot of time.
  • Water your plants regularly during the hardening off process, but slowly decrease how much water your plants receive. Gardens are typically drier than grow rooms, and gradually reducing water levels can help plants adapt to gardens more readily.
  • Place seedling pots and trays on a sturdy table when you’re hardening off your plants outdoors to keep slugs and snails getting to the seedlings.
  • Placing seedlings inside a cardboard box with an open top can prevent pots from toppling over if there’s a strong breeze.
  • Cold frames and greenhouses are not required for acclimating plants to outdoor life, but these structures make hardening off plants easier. Plants can be hardened off in greenhouses and cold frames by simply opening the doors of these structures during the day and closing them again at night.
  • Always keep an eye on the forecast. Spring temperatures are unpredictable; if frost or extreme cold is expected, keep your plants indoors for a few days until the weather warms up.

Frequently Asked Questions

  • What is the lowest temperature for leaving plants outside?

    That depends. Cold-hardy plants, like radishes and broccoli, grow well when temperatures are between 40°F and 70°F. However, heat-loving plants, like tomatoes and most nightshades, are sensitive to cold and should only be planted outdoors when temperatures are at least 55°F to 60°F.

  • What is the best thing to protect plants from frost?

    Frost blankets and cloches can protect plants from frost, but if you are in a pinch or prefer DIY options, you can shield plants with sheets, lightweight blankets, cardboard boxes, or overturned milk jugs.

  • How do I transplant hardened off seedings when they are ready?

    The best time to transplant seedlings is on an overcast spring day when temperatures are above 50°F. To transplant, dig a hole as deep as the plant’s root ball and position the plant in the hole so it is at the same depth that it was growing in its original pot. Mix a bit of compost or worm castings into the soil, backfill the planting hole with soil, and water deeply to help the plant settle in.

Yes, You Should Always Harden Off Seedlings Before Transplanting–Here's How (2024)


Yes, You Should Always Harden Off Seedlings Before Transplanting–Here's How? ›

Hardening is the process of exposing transplants (seedlings) gradually to outdoor conditions. It enables your transplants to withstand the changes in environmental conditions they will face when planted outside in the garden. It encourages a change from soft, succulent growth to a firmer, harder growth.

Why is it important to harden off seedlings when transplanting? ›

Hardening is the process of exposing transplants (seedlings) gradually to outdoor conditions. It enables your transplants to withstand the changes in environmental conditions they will face when planted outside in the garden. It encourages a change from soft, succulent growth to a firmer, harder growth.

What happens if you don't harden off seedlings? ›

If you don't harden your plants, the tender plants will get burned by the sun, the shock of cold, or the wind. Some plants may recover from burn (even fully), but their growth will be set back a few weeks while they recover.

How fast can you harden off seedlings? ›

We recommend hardening off seedlings over the course of at least several days to one week. Day One: Choose a warm, sunny day, but place the pots, flats, or trays out in full shade, bringing them back inside at night.

What is the general rule to use to transplant seedlings is after? ›

The general rule of thumb is that when a seedling has 3 – 4 true leaves, it is large enough to plant out in the garden (after it has been hardened off).

Do you really need to harden off seedlings? ›

If you don't harden off your seedlings, their growth can be seriously stunted, setting back your harvest possibly for weeks.

How do you harden seedlings for transplanting? ›

Lengthen outdoor time for your plants bit by bit. Each day, leave your seedlings outside for an hour more than you did the previous one, gradually exposing them to more sunlight and wind until they are able to spend an entire night outside. Keep the soil moist at all times during the hardening-off period.

Can you harden off seedlings too early? ›

Although not a difficult process, hardening off involves foresight and planning. You don't want to start the process too early, or you run the risk of cold temperatures damaging your plants. Wait too late, and your plants might still suffer from being in pots too long.

Will plants survive without hardening off? ›

"You Must Harden Off Your Seedlings" If you don't harden off your indoor seedlings before planting them outside, they will suffer shock from the brutal sun. Without the opportunity to adjust first, they'll get sunburn and might even die. In some situations this is 100% true.

Can you harden off seedlings in the rain? ›

If it's raining or overcast when you harden your plants, keep in mind that your plants haven't adjusted yet to sunny weather. This is when you might want to prolong your hardening off period to two weeks. You'll want your plants to get some sun exposure before you transplant.

Can you transplant seedlings too soon? ›

It's important to avoid transplanting seedlings too early, as this can lead to shock and stunted growth. Similarly, waiting too long to transplant can result in the seedlings becoming root-bound, which can limit their growth and health.

Can I leave seedlings out overnight? ›

Only leave them outdoors through the night for a single night initially and then bring them in first thing the following day to see how they handle the adjustment. If they appear well adjusted and happy, try leaving them out overnight for another two or three nights before fully transplanting them.

How long does it take for a plant to recover from transplant shock? ›

The last step in a successful transplant process is patience! Some trees take two or more years to get rid of all their stress symptoms. Occasionally, it can even take up to 5 years for trees to fully recover. In most cases, it takes a year or so for trees to shake off transplant shock.

What is the best time to transplant seedlings? ›

Try to transplant when it's cloudy so plants don't face a full day of direct sun right away. For each plant, use a hand trowel to dig a hole just deep enough for the root ball to fit.

At what stage should seedlings be transplanted? ›

How big should seedlings be before transplanting? They should have 3–4 sets of true leaves. You can wait to plant them out until they are much larger, however—but you may need to pot them up into larger pots if they start to get potbound.

What does hardening off seedlings before planting outdoors help them become accustomed to? ›

That means slowly introducing the plants to environmental conditions like sunlight, wind, and changes in temperature that they have not yet experienced. If plants are not hardened off enough, they could get burned by the sun or go into shock from more wind exposure or lower temperatures than they are used to.

How can you reduce root damage and transplant shock when transplanting seedlings? ›

Water the plant thoroughly when transplanting – An important transplant shock preventative is to make sure that your plant receives plenty of water after it is moved. This is a good way to avoid transplant shock and will help the plant settle in to its new location.

What should you add to the soil before transplanting the seedlings Why? ›

Before planting the seedlings (or any direct seed planting), mix about 2/3 organic soil with 1/3 compost in the container or garden bed. You can also add organic fertilizer appropriate for what you are planting. This can be done a few days before transplanting.

Can I put my seedlings outside during the day? ›

Initially place seedlings outdoors in a sheltered spot – protected from wind and direct sun. Each day following, expose plants to another 30-60 minutes of filtered sunlight. Work your way up to giving plants direct morning sun, followed last by noon-day sun.


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