Ask a Scientist: Rachel the Botanist
What is the oldest living thing? What is the largest living thing? Find out on our first episode of Ask a Scientist a Question! Heather interviews Rachel Jones, a botanist who works at CSU.
The oldest individual living organism is a Great Basin bristlecone pine (Pinus longaeva) in the White mountains of Nevada/California. The exact location is kept secret to protect the tree, but the tree’s name is Methuselah and it’s about 4850 years old .
 source: http://www.rmtrr.org/oldlist.htm
Hmm, you may be thinking of a large colony of fungi (specifically Armillaria ostoyae) in Oregon. Whether you consider this the largest organism depends on a couple definitions: this “humongous fungus” is a colony of genetically identical mushrooms which are all connected. It is certainly reasonable to call them a “single organism”, and by that measure this is most likely the largest living thing by biomass. This colony probably weighs somewhere between 7600 and 35000 tons ! However, fungi are not plants!
 source: https://web.archive.org/web/20190803005438/https://www.fs.usda.gov/Internet/FSE_DOCUMENTS/fsbdev3_033146.pdf
The tree with the highest volume of wood in its trunk is a giant sequoia (Sequoiadendron giganteum) located in Sequoia National Park, California. You can actually go visit this tree! I have, and it was literally awesome. The tree’s name is General Sherman, and its trunk contains about 52,500 cu ft of wood (~1500 m^3) .
 source: https://www.nps.gov/seki/learn/nature/sherman.htm
Indeed! And the tallest tree is actually located nearby, in Redwood National Park, California, the precise location is kept secret (again, for the tree’s safety). This tree, a coast redwood (Sequoia sempervirens) named Hyperion, and it is about 380 feet (~116 m) tall  source: https://www.conifers.org/cu/Sequoia.php