The garden club was taking its annual May tour through the Shasta-Trinity National Forest in northern California to identify flowers and trees when a forest ranger informed the group that a detour was necessary. A hand grenade had been found earlier, apparently a booby trap rigged to keep poachers out of a nearby marijuana patch.

"We were scared. This is something they take us on every year," said Eulah M. Beel, 69, of Junction City, Calif. "It just gave us all a cold chill."

In the Ozark National Forest in Arkansas, a forest ranger examining trees last April accidentally waded into a marijuana patch. A gunman stopped him, searched him, and ordered him to "get off his mountain and not to come back."

More than 100 law enforcement officers came back last month and raided 100 illegal marijuana patches that were planted, tended and harvested by free-lance squatters. Police arrested six growers and destroyed 10,000 plants, an estimated $20 million haul that forest supervisor James R. Crouch says he believes constitutes 10 percent of the illegal crop growing in the Ozarks.

At least some marijuana has been found in all 155 national forests in 43 states -- including Jefferson and George Washington forests in the western part of Virginia, the forests nearest Washington -- but officials are most concerned about reports of growers using threats and intimidation to defend their valuable plants.

"In some cases, the marijuana appears to be an occasional plant established by individuals for their personal use. In others, it is clear that large, sophisticated commercial operations are under way," said Ernie Andersen, law enforcement coordinator for the Forest Service.

"These are elaborate plantations with irrigation systems and armed guards and surveillance," Andersen said. "[They]move into a very remote area and effectively close it off to use by the public."

Although much of the associated violence appears to be directed at people who might steal the crops, forest officials, local and federal authorities have begun to crack down on the illegal annexation of forest land. And some rangers are beginning to warn campers, hikers and forest service employes to stay out of specific areas that may be dangerous.

The plots are hidden in shrubbery, difficult to spot from the air, miles from any road. Fully grown plants stand 12 feet tall, and each plant is capable of producing marijuana with a street value of up to $6,000 after the four-month growing season.

"Growers are beginning to take an attitude of 'shoot first, ask questions later,' " according to California forest service official William Derr.

Joseph P. Russoniello, U.S. attorney for the Northern District of California, said, "We're finding that the growers either are arming themselves, or they're hiring professional guns."

David Wright, a Shasta-Trinity forest ranger, has been fired on 13 times.

"We have a lot of law enforcement people who refuse to go into these areas without an army to back them up," Wright said. "You always hate to admit that you don't administer the land on behalf of the good of the people, but in certain areas -- certainly not all -- we do not have adequate control at this time."

A recent General Accounting Office report, quoting a Forest Service official, said, "It is only a matter of time before a forest visitor or one of our employes is seriously injured or possibly killed because they stumbled onto an illegal marijuana plantation."

American-grown marijuana became more attractive to buyers after the Mexican government began a policy of spraying its patches with the herbicide paraquat.

Last year, 84 tons of marijuana -- valued at $254 million -- were confiscated from California's 18 national forests, and the federal Drug Enforcement Administration considers the problem there serious enough that it recently called a meeting of local authorities to devise a strategy. Among other things, aerial surveillance will be intensified and more growers prosecuted.

But as California cracks down, other growers are setting up shop along the Salmon River in Idaho and in Oregon and Montana, authorities say.

Four gunmen ordered a family picking huckleberries to leave Montana's Kootenai National Forest several weeks ago. The family "grabbed their buckets and left," said forest official Henry Sixkiller. "They had never heard of anything like this before."

There also has been an environmental impact. In Oregon's Siskiyou National Forest, growers cleared 1 1/2 acres, constructed a dam, a watering system and a wooden shed with a kitchen and running water.

"It ruins our wildlife habitat," a local sheriff's deputy said. "They're diverting streams, cutting off water flows, and it creates tremendous erosion problems."

Shasta-Trinity's Wright said, "We're supposed to be managing [this] as a pristine environment . . . . No impact of man is supposed to be visible."

Two weeks ago, authorities confiscated $600,000 in marijuana plants from Georgia's Chattahoochee Forest, ending a long tradition that had its origins in prohibition days.

"A lot of the moonshiners have turned from growing corn to growing marijuana," said forest official Karen Wheeless. "Frankly, [they] have been some of our best protectors for keeping fires out of the forest, because they don't want their product damaged."