How to Harden Off Your Seedlings Before You Plant Them and Why It's Important

  

Moving your tomato, pepper and other warm weather crops outdoors successfully is really all about easing them into their new environment—a process often called “hardening off.” This gradual transition helps them adjust to stronger sunlight, wind, and temperature fluctuations compared to the controlled indoor environment they are used to.   

 

This applies whether you grew your own tomato, vegetables or flowers from seed or bought them from our greenhouse. Moving your plants from the house or greenhouse directly into the garden without giving them time to adjust will set them back significantly and in some cases, they’ll fail to survive.   

 

This slow transition from indoors to the outside is called “hardening off”. Hardening off plants gradually exposes them to outdoor light, wind, and temperatures. The process is easy, but it takes time. In fact, you should plan for a week or so to get your immature plants ready to transplant into the garden.  

 

On the first day, pick a sheltered, shady spot under a tree or on the patio and leave them there for 5 or 6 hours.  

    

Bring them in that night. The second day, place your plants where they will receive morning sun and afternoon shade. This will help your seedlings adjust to direct sunlight. Leave them in this spot for 6 to 7 hours. On the third day, increase the exposure time to direct sun and leave your transplants out after dark.  

 

On the fourth day, if the nighttime temperatures stay above 50° F, leave your starter plants out all night. Follow the same routine for days five and six but be prepared to bring your plants back in if the nighttime temperatures fall. After this amount of time, your tomato, pepper, and vegetable seedlings should be ready to plant in the garden.  

 

 

If nighttime temperatures continue to drop into the 30’s and 40’s, consider using season extenders like N-Sulate or Insulated Plant Protectors to protect your plants. Insulated Plant Protectors help protect tomatoes and peppers and other crops when temperatures drop. N-Sulate is another way to protect warm weather crops against cold temperatures. This cloth product helps raise the temperature around your plants by 6° to 8° F.    

  

When you do decide to plant, pick a cloudy day. This will help your seedlings make the transition from container to the garden even easier.  Even then, you will want to keep an eye on the forecast. Our average last day of frost is May 13, but this can vary greatly. Have a frost blanket or floating row cover ready to protect your plants. Do not use plastic to cover your plants. If plants are small enough, throw a box over them to keep the frost off.   

  

Make sure you keep your plants watered during this hardening off process. Being outside means they will dry out faster, so keep an eye on the watering.  Moist soil will cope with temperature swings better than dry soil. 

 

 

 

Now’s the time to give your seedlings a boost

  

As we get closer to moving our seedlings out into the garden, there are steps we should take now to ensure they’re ready to go through the acclimation process and establish well once outside. This is especially true if your seedlings are small or growing spindly. This is a sign they need energy. Without enough energy, young seedlings will struggle to grow.   

Once germinated, seedlings require light, water and nutrients to ensure enough energy to achieve optimal growth and development. Here are some key steps you can take to help ensure your seedlings have what they need.  

First, provide a consistent source of light.   Once your seeds break through the soil, they need plenty of light to develop strong, sturdy stems. Typically, seedlings benefit from 12 to 16 hours of light each day, mimicking the natural daylight cycle. Most of the time the light coming through a bright, sunny window isn’t going to be enough for your seedlings. They will tend to stretch and get thin and spindly looking for light.    

 

Adding a plant grow light is the best way to ensure your plants get the light they need. You have several choices when it comes to selecting a grow light including fluorescent lights (CFLs), high-output T5s and LEDs.  Plan to run your grow lights fourteen to sixteen hours per day, mimicking the natural daylight cycle.  It’s also important that your light setup is adjustable, so that you can raise the lights as your plants grow. When your seedlings are small, place your lights five to six inches above the tops of the plants. As the seedlings grow and develop, move the lights up accordingly.  

In addition to providing a dependable light source, maintaining a consistent watering schedule is also important. Check your seedlings every day and keep the soil moist, not soggy. Don't use water that is too cold (or too hot). Fill up your watering can and let it set out overnight. That way the water will be at room temperature when you water your seedlings.    

To develop strong stems and root structures, your seedlings need to be fed. At this stage of their development, a liquid fertilizer works best. This is because the nutrients are dissolved in water, they’re easier for small roots to absorb. 

Begin fertilizing once your seedlings have 3-4 sets of true leaves. The first set of leaves to emerge are the cotyledons, sometimes referred to as ‘seed leaves”. They are not “true leaves”. The next set of leaves and all subsequent leaves are true leaves.    

Use a mild liquid fertilizer with a formula such as 6-4-4 or 3-2-2. Mix the fertilizer to about one-quarter of recommended strength to prevent damage to the roots. Plan to fertilize your seedlings every 10 to 14 days.    

  

If you used a humidity dome for starting your seeds, now’s the time to take it off. Leaving the humidity dome in place can encourage fungus and mold to develop. Same is true of heat mats. Leaving heat mats in place can lead to bolting and weak stems.  

Seedlings, especially tomatoes, need air movement to develop strong stems. Place a small fan nearby to create air flow. The air movement will also help prevent fungal issues. In lieu of a fan, brush your seedlings with your hand a couple of times a day.   

   

One other important task is to give your seedlings plenty of room to grow by thinning out weak plants. This is especially important for tomato and pepper plant seedlings. Cut off the weakest seedling with a pair of sharp trimming scissors. Don’t attempt to pull it out of the pot. This will damage the root system of the seedling you’re trying to keep.   

 

Fresh air helps reduces the risk of fungal diseases and build strong stems. A fan set on low and placed near your seedlings will mimic outdoor conditions, preparing the seedlings for eventual transplanting.   

   

Maintaining the right growing environment and supplying your seedlings with light, water and nutrients will help ensure the best quality plants for transplanting into the garden.