In the fourth generation of Pokemon games, the Sinnoh region introduced Pokemon fans to some new Pokemon. Some of those Pokemon were evolved forms of pocket monsters added earlier. One such Pokemon was Tangrowth, the evolved form of the Tangela Pokemon from the Kanto era. To no shock, Tangela and Tangrowth are back for Pokemon Shining Diamond and Shining Pearl. Here's how you can evolve Tangela into a much bigger and more powerful monster.
To evolve a Tangela into a Tangrowth, you will first need to acquire a Tangela. From here, you'll need to teach Tangela a Rock-type move. But not just a Rock move, as Tangela will only evolve if you teach him the Ancient Power move.
Tangela learns Ancient Power at level 24. If you have a Lv. 24 or higher, you will need to go to the Move Tutor in Pastoria City. Here you can teach Tangela that move to proceed. Keep in mind that in Pokemon Brilliant Diamond and Brilliant Pearl, learning the moves is not free. You will need a heart scale to teach a move via the Move Tutor.
From here, all you'll need to do is level up Tangela by one, once the Grass-type Pokemon knows Ancient Power. Once Tangela has leveled up, he will become involved in Tangrowth.