It shall be unlawful for any person, individual, copartnership, association, firm or corporation or agent or employee thereof to permit to grow within a neutral zone or within a prescribed zone any variety or strain or species of plant forbidden by the regulations of the said regents to that stage of maturity in which buds are produced, whether the said plants are planted or have been allowed to grow as volunteer plants or plants escaped from cultivation. It shall be the duty of such persons, individuals, copartnerships, association, firm or corporation or agent or employee thereof to destroy any plants whether planted or volunteer the growing of which is forbidden within any zone or neutral zone before the said plants have grown to that stage of maturity at which buds may be produced.