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.
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.
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.
TRADITIONALLY: To harden off your seedlings you need to slowly introduce them to outdoor light levels. Put them outside for an hour and then bring them back in. Then the next day, put them out for 2 hours.
Begin hardening transplants 1-2 weeks prior to setting out plants in your garden. The easiest way to harden transplants is to place them outside in a shaded, protected spot on warm days, bringing them in at night. Each day, increase the amount of sunlight the transplants receive.
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.
Day 1: Set your seedlings outside in dappled or partial sun for 2 to 3 hours. Day 2: Set your seedlings outside in partial to full sun for 3 hours. Day 3: Set your seedlings outside in full sun for 4 hours. Day 4: Set your seedlings outside in full sun for 5 to 6 hours.
Keep the plants well-watered. Protect them from strong winds. Plant on a cloudy day or in the evening if possible so plants can recover out of strong sun. Provide a little nutrient solution to the roots, especially one that is high in phosphorus.
How big should seedlings be before transplanting? Seedlings should be at least 2-3 inches tall before transplanting and should have their two "true leaves."
The actual process of hardening off, should occur approximately 7-14 days before transplanting out in the garden. Usually, you want to give them ample time to adjust.
Fanning plants for ten minutes or so several times a day helps them develop stronger stems that can better handle wind and rain after they are set out in the garden. A fan can go a long way toward discouraging fungus gnats, which are very weak fliers.
If temperatures remain warm both day and night (at least 50 degrees Fahrenheit), the seedlings should be able to handle increasing amounts of sunlight and can be left outdoors overnight. Make sure the soil doesn't dry out if temperatures should suddenly turn warmer.
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For peppers, tomatoes, and most tender seedlings, a good rule of thumb is to wait until night time temperatures are steadily at (or above) 10°C 50°F before even contemplating transplants outdoors. It may work earlier with the help of a greenhouse, cloche cover, or cold frame, but that's another subject.
Any drop below 10C (50F)will impact growth. Any stagger in growth has potential to stunt plants and reduce the health of the plant. Seedlings should be protected from large fluctuations in temperature which can lengthen maturity. Hardening off before transplant should accomodate those temperature needs.
And then you face your biggest time commitment yet: hardening your seedlings off. Each tray will need to be transported outside so that those little seedlings can bask in the sun for a bit before being brought back inside. That's every day. For at least a seven-day period.
Cause: If a batch of otherwise healthy seedlings fall over and wither seemingly overnight, they are likely victims of damping-off, a fungal disease that attacks stems at the soil surface and is usually deadly. Excess moisture or nutrients create conditions that promote damping-off.
Before transplanting, turn the soil with a shovel, fork, or a tiller and add humus or compost or organic fertilizer and work it in. I also like adding kelp and/or greensand for added nutrients; both of these come from the sea and add trace minerals. Soil for the vegetable garden should be friable and loamy.
Smooth out the soil after for proper permeation of the water.
Try not to fuss with the roots too much, especially for the ones with a long tap root or that are sensitve to roots being tampered with such as ammi or celosia.
Most leggy seedlings can be be fixed by transplanting them deeper into pots, planting the seedlings so the soil level comes partway up the leggy stems. This is a great way to fix leggy tomato seedlings or plants as tomatoes can form roots up the length of the stem.
Introduction: My name is Saturnina Altenwerth DVM, I am a witty, perfect, combative, beautiful, determined, fancy, determined person who loves writing and wants to share my knowledge and understanding with you.
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