Tree Cuttings drop from Goobbue Farmers in Rolanberry Fields.
The average price for them is 2k to 4k each. They allow you to add two crystals when gardening instead of just one. The equation the game uses to determine what item you receive gets another variable put into it when using the cuttings compared to normal seeds. Typical the plant yields more valuable items, or in more of a quantity. As in gardening with other seeds, Fruit/Vegetable/Grain, it is dependant on many factors including the moon, time, and day.
I myself have grown ingots, crystals, and food ingredients. There are so many combinations, and I have yet to see and website/book which describes the equation or solid method used.
For the most part, all people have been doing is logging the intricate details involved in gardening. Power insomnia gamers like Pikko here on the board have journals you can review. There is probably a japanese site out there somewhere with more information.
Hope I answered your questions. :)