What does 'hardening off' refer to in gardening?

Hardening off refers to the process of acclimatizing seedlings to outdoor conditions before transplanting them into the garden. This is a crucial step for young plants that have been started indoors or in a controlled environment. When seedlings are grown indoors, they are not exposed to the elements such as wind, direct sunlight, and temperature fluctuations that they will face once planted outside.

By gradually introducing them to these outdoor conditions over a period of days to weeks, gardeners can help the seedlings adjust and reduce stress, which minimizes the risk of transplant shock. This process typically involves placing the seedlings outside for a few hours each day, gradually increasing their exposure to sunlight and outdoor temperatures.

The other options do not accurately describe this process. Planting seeds in the ground, pruning plants, and increasing the frequency of watering are all important practices in gardening, but they do not pertain to the hardening off method, which specifically focuses on preparing seedlings for the transition to an outdoor environment.