Hardening Off: Preparing Indoor Seedlings For Outdoor Transplanting

You tenderly raise your seedlings from scratch, but they wither and die. They wither and die every time you try to plant them outside. You planted them in fertile soil and watered them regularly, but they still died. You might ask yourself what you did wrong, and the answer is you never hardened off your tender seedlings.

Hardening Off

Hardening off is the process of preparing indoor seedlings for outdoor transplanting and the external weather conditions. Hardening off habituates the tender seedlings to the extremes of heat, cold, and the elements and promotes growth and hardiness. Hardening off also prevents shock and damage. 

Seedlings are not used to the outside world when raised indoors. However, there are simple steps to transition from a coddled indoor seedling to a robust outdoor plant that you can follow. Please read on to discover what hardening off is and how to prepare your seedlings for their new outdoor life. 

What Exactly is Hardening Off of Seedlings?

Seedlings grown in a greenhouse or indoors live in a relatively sheltered environment, which makes them unprepared for the more extreme conditions of outdoor growth. The art of hardening off your seedlings is a process whereby you gradually introduce your seedlings to the environment outdoors to ready them to flourish outside.

Hardening off is the process of slowly exposing your plants to outdoor conditions to strengthen the plant. You harden off by gradually increasing your seedling’s exposure to the sun, night temperature drops, and outdoor conditions such as wind and rain. Thus they become used to external conditions. 

Introducing your tender seedlings to their new environment by degrees prevents your seedlings from potentially going into shock or withering and dying under the new climate extremes. 

Why Hardening Off Is Important To Seedlings

Seedlings newly introduced to the outdoors are extremely vulnerable and are not yet fully equipped to deal with their new growing conditions. Like lying on a sunbed after a long winter may burn your skin, young seedlings can be burned by direct sunlight. 

In much the same way, temperature extremes between daylight and nighttime may also shock your seedlings and even kill them. Likewise, wind or heavy rainfall may drown or uproot your tender shoots. 

The physical benefits of the hardening off process include the following: 

  • Hardening off your seedlings reduces their vertical growth rate and thickens the cuticles and waxy layers of the outer tissue. This thickening provides your seedlings with a more robust framework from which to grow.
  • Hardening off stimulates the seedling to produce roots. Root production is one of the most essential processes for your plants to anchor and thrive in their new environment firmly. 
  • Hardening off increases carbohydrate reserves in plant tissues. These carbohydrates are the essential food reserves in your plants that they may draw upon in order to grow.
  • Hardening off reduces water retention in cells. Excess water in your plant’s cells can make them vulnerable to frost damage in colder weather conditions.
  • This process stimulates the production of lignin in the cell walls. Lignin is a class of organic polymer that provides structural support for upward growth and water transportation. Lignin also helps resist rot and lend rigidity. 

How Do You Harden off Seedlings For Outdoor Planting? 

Although the hardening-off process takes some time, you will reap the benefits of stronger and healthy plants. Directly transplanting your seedlings may save you the effort, but you will lose many of your seedlings in their early stages of development at the end of the day. To ensure your seedlings are ready to face the outdoors, you should follow these steps.

Seedling Trays Outdoors

Determine the Hardiness of Your Chosen Seedlings 

The hardening process is different depending on the particular species of seedlings you wish to transplant. Your seedlings packaging usually has information regarding your specific plants’ favored growing conditions and seasons. 

For example, cool-season crops like broccoli, cabbage, and cauliflower make them hardier and easier to transplant outdoors. Tomatoes, peppers, and zucchini prefer warm weather conditions and should not be planted until after the last frost if you live in a colder climate.

You May refer to this handy Vegetable Planting and Transplanting Guide for Penn State Extension if you are unsure of your plant’s requirements. 

Start Hardening Off Two Weeks Before Transplanting

Once you have established the hardiness and the conditions favorable to your particular seedlings, you should wait until two weeks before the conditions favor your specific plant’s growth. Keeping in mind that air temperature is often warmer than soil temperature, you should put off hardening off until temperatures reach at least 45-50ᵒF.

Choose Your Hardening Off Position Carefully

When you begin to expose your seedlings to external elements, you should wisely choose your position to shelter your seedlings from harsh sunlight or wind. Perfect areas are in dappled shade, beneath a table, or under a cold frame. This protection will ensure that your tender seedling’s first introduction to the outdoors won’t send them into shock or harm them. 

Start Hardening Off Your Seedlings

Place your seedlings in a sheltered and shady spot in the warmer part of the day between 12 PM and 5 PM for an hour or two, and then place them back indoors. Ensure that the conditions are not too windy and protect your seedlings from pets and pests. Remember that the initial stages are the most vulnerable time for your seedlings. 

Gradually Increase Your Hardening Off Time

Over the two weeks of hardening off, gradually increase your seedling’s time outdoors by an hour, and take them back inside or shut them inside your cold frame. As your seedlings begin to acclimatize, closer to the two-week mark, you can gradually move them from their sheltered position into a more sunny position. 

The table below displays what an optimal schedule might look like for the most delicate plants, although more hardy plants could be transplanted sooner with a more aggressive hardening off schedule.

Day 1Partial Shade. Wind Protection1 Hour
Day 2Partial Shade. Wind Protection2 Hours
Day 3Partial Shade. Wind Protection3 Hours
Day 4Partial Shade. Wind Protection4 Hours
Day 5Partial Shade. Wind Protection5 Hours
Day 6Partial Shade. Wind Protection6 Hours
Day 7Partial Shade. Wind Protection7 Hours
Day 8Full Sun7 Hours
Day 9Full Sun8 Hours
Day 10Full Sun9 Hours
Day 11Full Sun10 Hours
Day 12Full Sun11 Hours
Day 13Full Sun12 Hours
Day 14Full SunAll Day and Overnight If No Frost

Pay Attention To the Weather Forecast

It is essential to keep up to date with weather changes during this hardening off period. Sudden cold spells or fierce winds may damage your seedlings. Be ready to move your seedlings indoors until inclement weather patterns have played themselves out, even if it means extending your two weeks for a couple of days. 

The Ending of the Two Weeks Hardening off

Once you reach the twelve-hour outside mark, you can consider leaving the plants outside for a day or two towards the end of the two weeks. You may consider reducing the amount of watering as the two weeks progress while keeping an eye out for wilting.

Choosing The Right Day to Transplant Your Seedlings

Cloudy days are great for your transplanting or late afternoon when the sun is no longer at full strength. The transplanting process is stressful for your seedlings, and harsh sunlight can stress your seedling even more.

planting seedlings

Ensure that you tamp down your seedlings after transplanting and water them thoroughly to ensure their roots are firmly embedded in the soil. 

Initially, your seedlings may show wilt, but they should generally perk up in a day or two. Monitor your seedlings closely after transplant and ensure they receive adequate water to grow. 

What Happens If You Don’t Harden off seedlings?

If you don’t harden off seedlings before planting outside, you may not only harm the tender seedlings, but they may wilt and die from shock and exposure. Young plants can suffer from sunscald or freeze damage if the water is too cold. Winds may also bend or snap the stems. 

Transplant shock and heat stress are the silent killers of seedlings. The sun can scorch the tender leaves because they have not grown accustomed to the intensity of unfiltered light. 

As I mentioned earlier, the hardening off process strengthens your seedlings by promoting root growth, slowing down upward growth, and thickening their outer cells. 

Seedlings planted directly outside may become rangy or leggy, making them more prone to wind and rain damage. The lack of proper growth may impact the future health of your plant in terms of fruit or bloom production. 

When Do You Know Seedlings Are Ready For Hardening Off?

Seedlings are ready for hardening off when they are about 2-3inches or have about 3-4 true leaves. These true leaves should not be confused with the first cotyledon leaves, which are rounded and visibly different from the true leaves that emerge later. 

Once your seedlings show 3-4 true leaves, they can feed themselves through photosynthesis and are ready for the hardening-off process to begin. It is important not to start the hardening process before this stage of development as the seedling will not be able to withstand the external extremes. 

Closing Thoughts

Like little green children, your plants need a guiding hand when introduced to the outside world. Although hardening off takes a bit of effort, you will reap the rewards with healthy and robust adult plants. Follow the hardening off steps above and prepare for a happy harvest or a garden full of blooms.