Eldergleam Sapling is a quest item found in the The Elder Scrolls V: Skyrim.
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It can be obtained from the Eldergleam Tree in Eldergleam Sanctuary, and serves as a replacement for the dying Gildergreen in Whiterun during the quest "The Blessings of Nature."
The alternative solution is to take a sample of Eldergleam Sap from the tree; however, this is considered a hostile action and will cause the summoning of several Spriggans.