For centuries, oil derivatives have been applied to plants for pest control. The novel horticultural oil called Civitas(TM) has been developed for use in the turfgrass industry. Incorporation of Civitas with low rates of fungicides provides excellent disease control reduced environmental impact. Civitas, like other horticultural spray oils, can be phytotoxic. Chlorosis, membrane disruption, and reduced photosynthesis, transpiration, and growth can result from horticultural oil phytotoxicity. Chlorosis is masked with a green pigment added to Civitas. Unsubstantiated claims by turfgrass managers suggest Civitas increases clipping yield, reduces ball roll, can cause stand decline. The primarily goals of this work were to understand how Civitas affects turfgrass in the field and then understand how physiology is affected in the growth chamber. The objectives of this dissertation were to document how Civitas affects turfgrass growth, performance, and quality in the field, investigate factors known to induce spray oil phytotoxicity, measure turfgrass transpiration and photosynthesis, and finally monitor turfgrass carbon partitioning and nonstructural carbohydrate status. Putting green clipping yield increased and golf ball roll distance declined for 10 days after application of Civitas. Ball roll was not correlated with clipping yield which indicates some other factor besides increased leaf elongation affected ball roll distance. Civitas application to a mixed annual bluegrass (Poa annua L.) creeping bentgrass (Agrostis stolonifera Hud.) putting surface caused chlorosis, which was masked with the green pigment, and decline in stand density. Civitas also increased turfgrass canopy temperature 0 to 2°C depending on solar intensity. The phytotoxicity induced by Civitas is consistent with chronic phytotoxicity described in the literature. While Civitas did not directly damage or alter the cuticle quantity or composition, oil deposition on the leaf reduced gas exchange. The decline in gas exchange reduced transpiration rate and radiation use efficiency (RUE) for several days after application. The decline in RUE was particularly dramatic at higher photosynthetic photon flux densities. Carboxylation efficiency was more sensitive to Civitas than transpiration, most likely due to a reduction in Rubisco activity. Increased clipping yield and reduced RUE reduced the content of storage carbohydrates, starch and fructans.