The Army Corps of Engineers said yesterday it has postponed plans to use the controversial herbicide Diquat in an effort to eliminate hydrilla from parts of the Potomac River and will instead start today to remove the weed mechanically.

"It's really gotten too late to use chemicals," corps spokesman Gary Holloway said, explaining the decision. "As the weather gets cooler and the days get shorter, hydrilla dies back on its own, so we can'tget any meaningful data from the tests of Diquat's efficacy now."

Holloway added that Virginia, responding to the corps' request for approval of proposed experiments in hydrilla control, gave the go-ahead for tests of mechanical means of weed control, but requested further time to evaluate the use of Diquat.

Four groups have argued that the corps' proposed use of Diquat would harm the environment and violate the National Enviromental Policy Act.

Jay Feldman, a spokesman for one of thegroups, said yesterday, "the decision not to use chemicals because it's the end of the season does not undo the unacceptable behavior we've seen up to this point. . . .

"Hydrilla may be dying, but the issues surrounding it are not," Feldman said. "And just as the hydrilla will revive in the spring, so will the issues surrounding it."

The corps will inaugurate its mechanical experiments at 1 this afternoon in the Belle Haven Marina south of Alexandria when an aquatic mower is scheduled to begin the attack on the submerged weed.