Gopher mounds will be found in a fan shape radiating outward from a focal point. Gophers are voracious herbivores and make plants disappear. Their teeth can do damage to roots of plants, as well as plastic pipes and electrical wire that are buried in its path. Moles cause plant damage when foraging for insects and grubs near the plants roots but do not eat the plants. Voles will stop to "snack" on vegetation found in the creation of their burrow, but foraging for food underground is not their typical practice. Gophers can be found from sea level up to 12,000 ft, residing in a wide range of soil conditions varying from rocky to loose conglomerate, but will avoid wet, saturated, or heavy clay settings. They prefer sandy soils.
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Moles and gophers can be controlled to some degree with hot pepper and castor oil products or home mixtures of the ingredients. Injecting the materials into the ground in the problem areas is more effective than spraying the surface. Planting castor beans around the perimeter of the garden can help. Devices that rattle or vibrate can also be part of the solution. One listener called in with the advice of connecting the mower exhaust to a hose and gasing the little beasts. Might work, not sure about soil contamination.
Another listener has had repeated success applying cracked corn, both from a seed drill and by broadcasting. What it is about corn that repels gophers is a mystry.
The Deer Scam company is experimenting with a new repellent and the preliminary results are strongly positive.