Certified Professional Horticulturist (CPH) Practice Exam

Horticulture oils may be applied at any time of the year except when the temperature is:

1. Below 0°

2. Below 32°

3. Above 75°

4. Above 100°

The correct answer is: Below 32°

Horticultural oils are effective pest control agents and can also help with plant diseases, but their efficacy and safety can be impacted by temperature conditions. When the temperature is below 32°F (0°C), the oil can become less effective, and there is a risk of it behaving similarly to an ice layer on plant surfaces. This can potentially harm the plant tissue, leading to undesirable effects such as phytotoxicity or decreased plant health. In contrast, while oils can generally be applied at temperatures above freezing, extreme heat can also become problematic as it may cause excessive evaporation of the oil or stress the plants. However, the critical point at which horticultural oils should not be used is specifically when temperatures are at or below the freezing point. Therefore, the restriction on application due to temperatures below 32°F is particularly significant for both the safety of the plants and the effectiveness of the product.