Hardening Off Seedlings: Toughen Up Your Babies for the Real World

Starting seeds

The first time you go outside in the bright sunshine after being cooped up inside during winter, what happens? The clothing you wear inside isn’t the same for spending a day outdoors, so you'll probably be dressed differently. You might feel too cold or too hot after being used to room temperature for so long. Maybe the sun will dazzle your eyes, the wind might chap your lips and skin, and you might even get a sunburn if you stay out too long. Now, think about how much harder that process might be for a plant.

seedlings in growing tray

Indoors, gentle lights go on and off on a regular schedule, and even the brightest lights never have the harsh glare of full sunshine. Water comes in the form of a gentle drizzle or mist, never from a rainstorm. Inside air is static, with almost no breeze or wind at all.

What does it mean to harden off plants

Hardening off is the process of gradually exposing your plants to the outside to get them ready for their new home in your garden.

It helps prevent your plants from being damaged or even killed by sunburn, wind damage, or shock. It's vital that you take the time to prepare your plants for their new home outside before transplanting them. Luckily for you, hardening off is a simple process.

How to Harden Off Seedlings

When to start the hardening off process

One or two weeks before setting your new plants outside, and once the seedlings have at least 3 or 4 true leaves, start the hardening off process. Pay attention to the needs of each plant, taking care to transplant only when conditions are right for growing that plant outdoors. Moving your plants out too early can stunt their growth. Penn State’s Extension has a handy reference for growing temperatures for many popular vegetables. 

Bringing your plants outdoors gradually

Don't put tender seedlings in full sun right away. Choose a shady, sheltered spot with indirect light and leave your plants out for two to three hours the first day. Each day, gradually increase the amount of time your plants spend outside by an hour or two. Gradually expose them to direct sunlight as well. 

If the weather is unusually cold, windy, or harsh, bring the plants inside that day. By the last couple of days, your plants should be accustomed to spending time outside, and they can even spend the night outside. 


Adjust plant care for a stress-free transition

During this time, you’ll also be backing off on watering and fertilizing. You want your plants to thrive, so make this a gradual transition as well. Your plants will slow their growth during this period, which will help them to survive outside. 

Do your best to make this a gentle, stress-free switch so your plants get the benefit of an easy start on life while becoming tough enough to thrive outside in the “real world.”

Once your seedlings have been hardened off through this gradual process, they will be ready to be transplanted.

Hardening off takes a little time and care for the gardener, but it prepares your plants for the move from their pampered nursery to their new home outdoors. This process helps to promote firmer growth in the cell walls, encourage root growth and carbohydrate stores, and even reduce the amount of water stored in the plant in case of a cold snap. Your plants will be well-adapted to the fluctuations of outdoor weather and temperatures and will be healthier and happier as a result.

References and More Info: 

Vegetable Planting and Transplanting Guide

Written by Teresa Chandler