Depends where you live. here in California's Central Valley, there's a plant that produces zillions of needle sharp thorns with a knob (think of organic carpet tacks) that lie in wait for unwary bicycle tires. The thorns are 6-8 mm long. After a recent 15 minute ride, EACH of my wife's tires had 4-6 of these little darlin's in it, EACH ONE causing a tube puncture. You couldn't patch fast enough to keep up! The only viable option is some sort of barrier, combined with self sealing tires.