Why are the leaves of my Japanese Spiraea turning yellow?

Yellow leaves on Japanese Spiraea can indicate several issues. The most common cause is overwatering, which leads to root rot and prevents the plant from absorbing nutrients properly. Check if the soil is consistently wet or has poor drainage. Other causes include: insufficient light (move to a brighter location), nutrient deficiency (especially nitrogen - consider fertilizing), natural aging of lower leaves, or pest infestations. Temperature stress, either too hot or too cold, can also cause yellowing. Examine your plant carefully, check the growing conditions, and adjust accordingly. If only the lower leaves are yellowing occasionally, this is often normal as the plant sheds older foliage.