Parsley Turning Yellow. However its scope is much wider. Both overwatering and underwatering can cause parsley herb plants to yellow.
The excess water will cause the roots to drown leading to fungal problems like root rot. But you need to properly dry prepare it for the winter so as not to disappear all the useful properties and should not. The parsley plant is turning yellow because you are overwatering it.
If your parsley does not get enough water and oxygen to its root system.
Parsley can turn yellow for several reasons. The roots will not be able to absorb the available nutrients. Yellow spots show up on the foliage and darken to brown with a golden halo. Usually Parsley leaves turning yellow indicate that the plant is under some sort of stress but there also other different reasons for this like hot weather too much or too little watering stress on the root system wrong amount of fertilizing dense soil or potting mix lack not enough light pests blight or some sort of a disease.