How to tell what's going on
Look at the plant's age and shape first. A tall central stalk with fine, feathery leaves and flat clusters of small flowers means the plant is bolting. Cilantro can do this within weeks in warm weather; parsley usually does it in its second season. Next, pull up one yellowing plant and look at the roots. Firm, pale roots point to age, food, or pests. Brown or black roots that slip apart when you handle them point to rot. Finally, check new shoots and leaf undersides for clusters of small, soft-bodied aphids, and notice whether the oldest leaves yellowed first or the whole plant yellowed at once.
Why it happens
Age and bolting are the most common causes. Parsley is a . University of Illinois Extension notes that in its second season it sends up a seed stalk and its leaves turn tough and bitter, which is why most gardeners start new plants each year. Cilantro is a that switches from making leaves to making seed as days get warm and long, and the plant winds down once seed sets. Yellowing lower leaves on a plant that is already flowering are often part of that natural decline rather than a disease.
Wet soil is the next suspect. Both herbs like steady moisture, but not standing water. Utah State University Extension warns against overwatering cilantro, which does poorly in damp conditions, and University of Connecticut notes that crown and root rots make herb leaves yellow and wilt, stunt the plants, and turn roots dark brown or black. Pale, yellow-green older leaves on a plant in well-drained soil can mean it needs nitrogen; Utah State suggests a nitrogen fertilizer once or twice a season for cilantro and notes that too much can dull the flavor. Aphids cluster on tender new growth, leaving sticky honeydew and white cast skins behind.
What to do now
Harvest what's usable from bolting plants, then pull them, or let a few cilantro plants finish so you can collect coriander seed. For soggy soil, hold off watering until the top inch dries, make sure pots drain freely, and pull plants that have collapsed with rotted roots rather than them. For pale plants in good soil, lightly with a nitrogen fertilizer at the label rate. Knock aphids off with a strong spray of water and check again in a few days. On parsley, cut the outer, older stalks at the base so the center keeps producing.
Next season
Sow cilantro in small batches every few weeks in cool weather, starting about 2 weeks before your last spring frost and again in late summer for a fall crop. Utah State recommends planting early while it's cool and keeping the soil below 75°F with and steady watering. Start new parsley each year instead of counting on second-year plants. Give both herbs well-drained soil, water when the top inch dries, and move carrot-family crops such as carrots, parsley, cilantro, dill, and celery to a new spot each year so soilborne problems have less chance to build up.
Growing it again? When to plant parsley for your ZIP code
Growing it again? When to plant cilantro for your ZIP code
- Root RotPlants wilting despite wet soil, with brown or slimy roots — wet-soil pathogens destroying the root system.
- AphidSoft, clustered insects on new growth causing curled leaves and sticky honeydew.
- Nitrogen DeficiencyUniform yellowing starting on the oldest, lowest leaves while new growth stays green — nitrogen is being pulled from old tissue to feed new growth.
- Crown RotThe base of the plant turns brown and soft at the soil line, and the plant collapses — caused by wet-soil pathogens attacking the crown.