How To Harden Off Indoor-Sown Plants

By Harry RamosLast update: 2024-07-11

What will happen if you don’t harden off your indoor-sawn plants?

They might become weaker when growing outdoors due to the sudden change of the growing environment. Your seedlings are susceptible to heat stress and transport shock if you don’t harden them in the right way. So, if you haven’t known how to do it properly, check my guide below:

How To Harden Off Your Seedlings In A Week?

The process can last from one to two weeks, depending on your indoor-sown plants. Most plants just require about one week, so I will teach you how to harden off your seedlings in 7 days.

Before starting, make sure that you just harden off seedlings that are a few inches in height with the first set of leaves.

After germinating, you can create a slight breeze with your hand across your seedlings to test the strength of the stems. This action shows if it's the right time for your plants to face a stronger wind in the garden.

During the 5 first days of the hardening-off process, bring your seedlings back inside your home at night.

How To Harden Off Indoor-Sown Plants

Day 1: Put your seedlings in an unheated greenhouse, or a cold frame, because your plants haven’t withstood direct sunlight on the first day. The best time is 2 hours of light in the afternoon (the warmest part of a day) per day.

If you don’t have an unheated greenhouse or a cold frame, simply put your seedlings under a sheltered spot, a shady tree, or any covered area where you can protect your plants from wind.

How To Harden Off Indoor-Sown Plants

Day 2: Allow your seedlings to expose 3 hours of afternoon sunlight outside in the old position. They can put up with a light breeze but dramatic changes of weather at this stage. So, you can open the windows a little wide to help them accommodate themselves to the outdoor elements like slight winds under a cover protection.


Don’t open the window entirely because the wind can speed up the evaporation, making your seedlings wilt quickly.

To avoid the wind better, you can cluster your pots of indoor-sown plants into crates, tubs, or buckets which function as a windbreak.

How To Harden Off Indoor-Sown Plants

Day 3: Now, your seedlings can withstand 4 hours of full warm sun and a soft breeze.

How To Harden Off Indoor-Sown Plants

Day 4: Your seedlings are stronger, so they happily welcome 5-6 hours of direct sun. You should put them outside earlier in the day to allow them to experience cooler temperatures.

How To Harden Off Indoor-Sown Plants

Day 5: Place your plants under the full sun all day, even if it's okay to have the breeze.

How To Harden Off Indoor-Sown Plants

Day 6: Put your seedlings and plants outside all day and night, but be sure that the temperature is above freezing without any frost’s danger.

How To Harden Off Indoor-Sown Plants

Day 7: Your seedlings have been hardened off on this day. You just need to pick up permanent locations for them.


In the first 3 days, keep your seedlings from the bright sunshine because their delicate bodies and leaves can’t put up with this thing.

Pro Tips

  • The shade will move, requiring you to cover your seedlings using shade cloth, fleece, or row covers if you can’t check on them regularly.

  • Check the moisture levels regularly because seedlings tend to dry out quickly when growing outside.

  • Don’t put your plants in an area where birds can knock over and slugs can ribble.

  • Bought-in plants also need to harden off because they are grown in a sheltered condition, but these seedlings need less time to acclimate to outdoor elements. Whatever, both of them require a hardening-off process.

How To Shorten The Hardening-off Time?

If you don’t have much time for a hardening-off process, you can shorten this time by applying the following ways:

Transplant your seedlings with the first set of true leaves in your garden when the weather is cloudy. This way is risky due to the unusual change of weather. You can try it when living in a place with a mild climate.

Protect your seedlings outside by using a portable greenhouse that can provide your plants with heat from the sun and protect them from winds.

Do You Need To Check The Frost Dates Of Your Seedlings?

Yes. The different seedlings have different levels of ability to withstand cold temperatures.

Some can withstand freezing temperatures and hard frosts in a short time. It’s okay to plant them 4-6 weeks before the frost-free date. They include:

  • Turnip

  • Spinach

  • Salsify

  • Rutabaga

  • Rhubarb

  • Potato

  • Pea

  • Onion

  • Mustard

  • Lettuce

  • Kohlrabi

  • Kale

  • Endive

  • Collards

  • Asparagus

Others just can put up with light frosts, so you need to plant 2-3 weeks before the frost-free date. They include:

  • Radish

  • Parsnip

  • Onion

  • Jerusalem artichoke

  • Chinese cabbage

  • Chard

  • Celery

  • Celeriac

  • Cauliflower

  • Carrot

  • Cabbage

  • Brussels sprout

  • Broccoli

  • Beet

Meanwhile, snap peas, tomatoes, and sweet corn can be injured and killed by frost; you had better plant them on or after the frost-free date.

Some can’t withstand cold temperatures, even if they need warm soil for germination. Thus, you should plant them 1-2 weeks before the frost-free date. They include:

  • Lima beans

  • Watermelon

  • Sweet potato

  • Winter squash

  • Summer squash

  • Pumpkin pepper

  • Okra

  • Muskmelon

  • Eggplant

  • Cucumber

Any indoor-sown plant needs a hardening-off time to help it familiarize itself with a new growing environment, exposing to stronger elements, such as winds and bright sunlight.

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