Transplanting is an integral part of growing. Transplanting gives your plant's root system more space to spread out, allowing your plant to grow healthy and robust and flourish! If you don't transplant, your roots can become rootbound. (This is when the plant won't be able to stretch out; they can get tangled and run out of room to grow), this can prevent the plant from developing correctly and stunt its growth, causing it to look sickly and even killing it! A healthy root system will lead to a healthy plant, AKA big roots = big fruits.
When transplanting, start it in its First container, usually a (1-gallon pot), then transfer it to its second container, a 3-gallon pot. I do every transplant in a 2-4 week window from the last container development. The subsequent transplant depends entirely on your garden set, timing, training method, and more. Then transplant it from the Second container (3-7 gallon pot) to its third and final container (7-10 gallon pot). I do the last transplant about two weeks before I start flowering. Some growers may only transplant once. For example, from a 1-gallon to a five or 7-gallon container, skipping the two or 3-gallon. The size of the pots I use depends on how big I want my plants to be. I recently got a breeder's cut clone at Homegrown Supplies in St. Charles, IL., and I did a 1 gallon to a 3 gallon to a 7 gallon.
When transplanting your plants, I always recommend using mycorrhiza. I spread the mycorrhiza on the soil where I'm about to place the clone in the new pot, and I spread it around the root base of the clone. My favorite is Great White, but many other brands, such as Bigfoot, Mykos, and Cultured Biologix Dr. Root, work well. Mycorrhiza does form a symbiotic association between a plant and a fungus. The plant makes organic molecules such as sugars by photosynthesis. It supplies them to the fungus, giving the plant water and mineral nutrients, such as phosphorus, taken from the soil.
I like to put my Breeder's cut clones in highly amended soil like Sohum, Purple Cow Indicanja, Build a Soil 3.0, or any other soil that can be water-only. The nice thing about using grounds like these is adding more of the amended soil every time you transplant, which helps feed the plants! Each plant is different, and you must consider what they tell you. You may need to add some nutrients and microbes to keep the plant happy, especially during flower! I always use a bud booster to give them a push in bloom!
Check out our transplanting video: https://youtu.be/zXiRMEEYHMo