Greatwood Trees spawn as large trees, roughly as large as a large oak with an 2-by-2 tree trunk. The logs are used in the majority of Thaumcraft recipes involving wood. These should be saved whenever possible; in a lot of cases, normal wood can be used in these recipes, but Greatwood Logs will make them more efficient. For instance, using 4 Greatwood Logs instead of 9 normal wood logs on an Arcane Worktable will yield 4 Arcane Wood Blocks instead of 2.
The Greatwood trees are grown from one, and only one, sapling. The tree's growth is accelerated by the Hoe of Growth, but bonemeal does not work on the trees.
"These large and majestic trees can be found dotted all over the countryside.
Their wood is able to absorb impressive amounts of magic, making it a popular component in the crafting of magical constructs and devices."
Some naturally generated greatwood trees may conceal a secret compartment beneath their roots. This room contains a spider spawner and a treasure chest. You can identify these special trees by the webbing that hangs from their leaves.
Greatwood logs only have uses in magic and cannot be crafted into planks. They are a good source of the Praecantatio aspect of magic. If you are also using the Forestry mod, you can use a carpenter to create planks from Greatwood logs. Greatwood logs can be smelted into regular logs.