A federal judge here yesterday temporarily blocked the Reagan administration's plans to curb marijuana use by spraying the toxic herbicide paraquat on marijuana plants in national forests.

The administration, citing evidence of increased marijuana cultivation nationwide, began spraying last month in Georgia and Kentucky, prompting widespread protests.

After an hour-long hearing yesterday, U.S. District Court Judge June L. Green said the Drug Enforcement Administration failed to conduct an environmental impact assessment before spraying, and ordered the program halted pending a hearing on Sept. 26.

DEA officials, who had argued that a statement was not needed because spraying would have no significant environmental impact, said after the ruling that spraying would cease until the issue was resolved.

DEA spokesman Robert H. Feldkamp said the administration could litigate that issue or produce the impact statement in time for the marijuana harvest season next summer. He said this year's marijuana harvest season ends in a few weeks.

Green's ruling came in response to lawsuits filed Sept. 2 by three environmental groups and the National Organization for the Reform of Marijuana Laws (NORML), which sought to end the spraying program.

Justice Department attorney James Druade argued that paraquat had been used safely for 16 years in agriculture and that there was no need to issue an environmental impact statement.

Druade said opponents of spraying "conjure up the image of blanket spraying of forest lands, but that is not so." He said the DEA had "exercised a great deal of control" in the spraying program.

Frederick S. Middleton III, an attorney for the Sierra Club, Friends of the Earth and the National Coalition Against the Misuse of Pesticides, argued that paraquat's harmful effects cannot be controlled in wildlife areas.