Vapor pressure deficit (VPD) is a measure of the difference between the amount of moisture in the air and the maximum amount of moisture the air can hold at a given temperature. It is often used in agriculture and horticulture to assess the environmental conditions for plant growth. VPD is important because it affects the rate of evapotranspiration, which is the process by which plants lose water and take up nutrients. Monitoring VPD can help optimize irrigation, ventilation, and temperature control in growing environments to ensure optimal plant health and growth.