That is an excellent question and one that scientists, farmers, and people who love growing plants in their gardens have been considering for hundreds of years! The answer to your question depends on a lot of things and perhaps most importantly, the time of the year when plants are growing. For example, in much of the world, it snows during the winter and plants grow much faster when they are inside and provided with some fertilizer. Outdoor plants in snowy places often die back and survive as underground bulbs. This is something that onions and tulips do in the winter. In places like Santa Barbara, where it doesn't snow (at least not very often!), it's still really cold outside in the winter. So plants may grow better inside, where warm conditions make it easier for their cells to reproduce quickly. Plus, young plants may die if their leaves, stems, or roots freeze during the winter outdoors.

During the spring, it may not matter as much whether a plant is inside or outside, as long as it has access to enough light, water, and nutrients to grow.

I'm glad that you are interested in plants and hope that this answered your question!