Our tender transplants have to be toughened up some before being introduced to their new outdoor garden home. If not they may die or perform poorly.
To take them from the shelter of their seedling home and just throw them in the garden would be a shock they might not recover from.
Hardening off gradually exposes the tender plants to wind, sun and rain.
This process toughens up our seedlings by thickening the cuticle on the leaves so that the leaves lose less water. This helps prevent transplant shock; seedlings that are not properly cared for become stunted or die from sudden changes in temperature.
There are three main methods for accomplishing this hardening off plants process.
First, you can use a cold frame which I have never done.
Second, you can reduce watering. I do not do this either.
The method I use is to gradually introduce my plants to the outdoors by increasing the periods of time they stay outside.
I begin about a week before actual transplanting based on lunar planting. If the Lunar calendar says that August 3rd is the day to plant transplants, then I will back up 7 days from that day to start hardening off plants.
Below is the suggested practice. However, if, like me, you have to work then this process will not work for you.
Place plants in a sheltered, shady spot outdoors. Under a tree or even on your back porch is fine. Leave them for 3-4 hours and gradually increase the time spent outside by one to two hours per day.
Bring plants back indoors each night.
More on Hardening Off Plants
After two to three days, move the plants from their shady spot into morning sun, return them to the shade in the afternoon.
I modify this a bit by putting my plants on a table on the patio that gets about three to four hours of morning sun. At first I will leave a protective covering on them.
Then after two or three days I will take this cover off. I will let them get the morning sun for a two more days. Then I will expose them to the morning and afternoon sun for two more days.
I have found that this practice works very well for the person who works and is not at home to take the plants in and out.
After seven days, the plants should be able to handle sun all day and stay out at night, if temperatures stay around 50 degrees F. Keep an eye out that the soil doesn't dry and bake the plants, if the weather is warm.
After seven to ten days your plants are ready to transplant. Try to do so on a cloudy day and be sure to water well after planting.
You will have to develop your own system but the final objective is to be able to put our new plants in the ground without them dying on us. It may take several tries for you to get your own system down.