Which of the following is a way to judge when plants need water?

Determining when plants need water is essential for their health and growth. Checking the weight of the plant is a practical and effective method to assess its water needs because when soil moisture is adequate, the weight of the pot or container will be noticeably heavier due to the water retained in the soil. As the plant uses up that moisture, the weight will decrease, signaling that it is time to water again.

This method can be particularly effective because it directly reflects the moisture content of the soil and allows gardeners to gauge whether the plant is sufficiently hydrated. Many experienced horticulturists and gardeners often rely on the weight of the pot as a reliable indicator, especially when dealing with potted plants.

The other options, while they may provide some cues about plant health, do not effectively measure soil moisture levels. Smelling the soil may indicate issues such as rot but does not give a clear indication of water needs. Touching the leaves could help assess the plant's overall health but does not provide specific information about soil moisture. Looking at the color of the leaves can indicate stress or nutrient deficiencies, but it is not a reliable method for determining whether or not a plant needs water.