One of the basic steps to successful transplanting is hardening off the seedlings. Sometimes they may look like they're ready to plant, but a seedling can't simply go into the garden without being prepared for the extreme conditions that exist outdoors. They must be acclimated. If you omit this step, many of your transplants probably won't survive.
Moving is stressful for plants
Moving a plant from one location to another is always stressful for the plant. Seedlings usually start life in a sheltered environment. Then they're moved to the harsh outdoor climate where it can be extremely cold at night as well as very hot during the day. The transplants move from a protected indoor climate or greenhouse to the garden where they're subjected to all kinds of conditions.
For your transplants to succeed and flourish, you can't skip the hardening off process. Young plants may not survive if planted directly into your garden without this important transitional step.
Seedlings are generally transplanted when the first pair of true leaves appear. Shade should be provided if the area is hot or dry. A commercial vitamin hormone concentrate can be used to help avoid transplant shock.
Acclimate transplants gradually
Seedlings grown indoors in a controlled climate don’t have the opportunity to develop the strength or structure to survive out in the elements without first becoming acclimated to their new home.
In order to harden off your seedlings, gradually expose them to the outdoors. At this point, it's helpful to keep the seedlings in trays in order to make carrying them easier.
It's a process
Move the seedlings to a protected location outside for one hour the first day. Do this each day for a week, adding one hour each day. At the end of the week, the plants will have 7 hours of outdoor exposure and are ready to transplant.
While inside, seedling stems aren't exposed to winds. Plants need to gradually increase their resistance to wind in order to become strong.
If you want to help the plant stems strengthen, add a fan to the area where you keep your seedlings. Use it to gently move the air. Too much direct breeze from a fan can dry out the seedlings and do the same damage as wind.
Early in the hardening off process, keep seedlings sheltered. Don’t put them in direct sun or in a windy location.
Relax, there's no one right way
Gardeners have different approaches to watering during the hardening off process. Most like to keep seedlings evenly moist from the time they start them until they're moved to the garden.
Make sure the seedlings have been watered and fed before planting them outside. Always provide consistent moisture. Until they become established, seedlings are susceptible to any extremes.
The process of hardening off doesn’t have to be perfect or entirely uniform in order to succeed. If you forget to move your plants outside one morning, just start again the next day. If the area gets too sunny as the day goes on and they wilt, don't give up. Plants can be very forgiving and will usually survive as long as you give them at least a little attention.
There is some effort required when moving the plants outdoors and back in again each day for a week. But after babying your plants for weeks, the added effort provides good insurance that they will do well once they're moved to your garden.
When is the best time to transplant?
Generally, when a seedling has three to four true leaves, it's large enough to plant out in the garden after being hardened off. When you plant a seed, the first leaves to emerge are called cotyledons. These leaves will look different from the leaves that grow later. They provide nutrients to the seedling until its true leaves open and begin the process of photosynthesis.
To keep transplant shock to a minimum, choose the coolest part of the day to transplant, either early morning or late afternoon. Summer is the most difficult time for transplanting.
Read more at Gardening Know How: When And How To Transplant Seedlings Into The Garden https://www.gardeningknowhow.com/garden-how-to/propagation/seeds/when-to-transplant-a-seedling-plant-into-the-garden.htm