Shiny Tangela in Pokemon GO will be appearing more frequently during the first week of Mega September, the event celebrating the launch of Mega Evolution. The event will be occurring the entire month of September and will feature challenges and newly released shiny Pokemon.
Tangela will spawn more often between Sept. 1-7 along with the newly released shiny Lotad. Because of this, now is the perfect time to try and net yourself a shiny.
Shiny Tangela Pokemon GO: How to Catch
Shiny Tangela isn't too different from its standard form, its vines are a brighter shade of green but its boots are the same color as regular Tangela's.
Tangrowth is the same, with neon green vines and red accents. Even with the increased spawns you can maximize your chances further by going out in sunny weather. Grass types spawn at a higher rate during sun with the weather boost mechanic. This method should hopefully drive up the numbers of spawns a get the RNG in your favor.
Tangela can also be found in tier three raids. If you really want a shiny finding it in raid battles is the easiest method, since raid Pokemon have a higher than normal chance of being shiny.