Do construction workers need “harmful plant” training?

Yes. This requirement for training in the recognition of and first aid for exposure to harmful plants such as poison oak or ivy is located in several places:

• 1926.21(b)(2) Safety training and education—The employer shall instruct each employee in the recognition and avoidance of unsafe conditions and the regulations applicable to his work environment to control or eliminate any hazards or other exposure to illness or injury.
• 1926.21(b)(4) Safety training and education— In job site areas where harmful plants or animals are present, employees who may be exposed shall be instructed regarding the potential hazards, and how to avoid injury, and the first aid procedures to be used in the event of injury
• 1926.604(a)(1) Site clearing—Employees engaged in site clearing shall be protected from hazards of irritant and toxic plants and suitably instructed in the first aid treatment available.

The bottom line is that if you work in areas where harmful plants can be a problem, you need to provide training and take measures to protect your workers.