Salinas, California. Intercontinental Rubber Producers. Approximately four-year-old guayule plant. The seedlings remain in the nursery for eight or nine months, the age of the shrub being determined by the length of time it is in the field. Guayule reaches maturity in from four to six years. At maturity the plant is about three feet in height. The rubber content is from eighteen to twenty percent of the dry weight