All five senses are enlisted to stop these invasions. Plants that fail deer-elk taste tests are one defense. Garlic sprays on roses are another. Sprays with ammonia or rotten-egg components, nylons stuffed with motel soap or human hair gathered from barber shops, or predator scat collected at zoos use the sense of smell. Sound shocks include pie tins clanging in the wind, loud static or music triggered by motion detectors, and timed noisemakers and propane cannons.
Most defenses work briefly and unpredictably.
Deer and elk get used to them. Or the next rain dilutes them. Or they work in one place but not another, or at one time but not another. And if alternative browse is scarce, almost nothing will keep Oregon's 120,000 elk and 600,000 deer from the cotton candy you grow. A stout 8-foot fence topped with a strand of electric dissuader will stop most interlopers.