Is hardening off necessary everytime you uppot the plant? Otherwise, how do you let the plant recover from transplant shock? Or won't the high temperature kill the plant whose roots are not established? Here temperatures reach 42 degrees celsius.
You don't need to worry about hardening off when it comes to repotting. What you need to worry about is basically transplant shock (which is generally caused by sun exposure to a plant with damaged roots). It causes plants to wilt and sometimes die. Here are some things you can do to reduce the odds of sun damage after repotting, for most plants:
- Don't repot between the hours of 9 AM and 6 PM. The sun can be too strong during those hours. It isn't always, but if you want to be sure, this is a good idea. Also, check the UV index; ideally, it should be 0 (or as close to it as you can get). You can ask Alexa 'What's the UV index?' and she should tell you a number most of the time (every once in a while she'll just look up what UV index means, though; if that happens, just ask again). How strong the sun looks and feels to a human in the moment isn't a good measure of how it will affect a plant; so, that's why it can be good to transplant during specific hours, and when the UV index is 0. The temperature doesn't really matter a terrible lot (a cold sunny day can sometimes be worse than a hot sunny day, and vice versa). It's the specific kinds of light that the sun produces (and the difference between kinds of light isn't always obvious.)
- Give the plant some potassium sulfate or monopotassium phosphate. (This helps plants absorb more water, and it strengthens the roots. While it's not required, it can prevent shock, and help shocked plants to recover.)
- Water the plant generously (don't waterlog it, though).
- Don't give the plant nitrogen. Nitrogen makes transplant shock worse.
"Hardening off" in most cases refers to taking a plant with soft, weak tissues generated by growing in a protected environment and gradually exposing those tissues to outside effects such as lower temperature and wind. A sudden exposure to these effects can break or disturb a plant even if it is not moved to a larger pot.
The main reason to move a plant into a larger pot is because the existing pot is too small to accommodate the expanding root ball. If there is still room for roots to expand then there is no need to move to a larger pot, and indeed there are good reasons not to pot "up". Far better to wait until the existing pot of soil is fully explored by the roots; that way when you do move to a larger pot the existing root ball comes out in one piece and is easily placed in the new pot. Otherwise a very loose root ball can easily fall apart with consequent physical damage to the fine roots, a very different kind of damage compared to wind or low temperatures.
When moving a plant to a higher temperature, might be better to gradually expose it in its existing pot for a week or two and let the pot fill with roots. If it is already fully expanded into the pot then it can be transplanted immediately without concern for shock since it should come out in one piece with minimal damage.