In Pokemon Diamond and Pearl you will have to go to route 228 and talk to an old man, I think you might have to do the same in Platinum.
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you cant get leech seed in Pokemon platinum, Pokemon can learn leech seed by lvl, like torterra and Roselia
on route 228 there is a house talk to a man he let torterra learn frenzy plant ,same 4 infernape but he learn blast burn again with empoleon i think he learn hydro pump
You can't teach a Roselia Solarbeam without using a TM and only finally evolved starter grass Pokemon can learn Frenzy Plant.
yes it learns it at about lv. 23 and you dont need a TM.