DALLAS — Drug Enforcement Administration officials discovered a large number of marijuana plants growing in a wooded area near the Trinity River in Dallas Thursday afternoon. The secret pot farm was practically right under their noses. In fact, if you could walk in a straight line from the riverbank through the marshy underbrush and tangled wilderness, you’d walk just 200 yards before you hit the DEA’s office building. Right now, agents have no idea who planted and carefully tended to the elaborate farm, it appears whoever it was spent a lot of time and money on the project. From the air, four agents could be seen pulling up huge plants — some up to four or five feet tall — and confiscated them. Officials say they found more than 300 plants, worth at least $300,000 on the street. The raid comes just two days after Richardson police discovered more than 1,100 marijuana plants growing in a house on La Mesa Drive. They arrested 25-year-old Brett Lachance, who is listed as the house’s owner.