Why are my tree's leaves turning yellow in the middle of summer?

Yellowing leaves in summer (not fall) — called chlorosis — means the leaves aren't producing enough chlorophyll. The pattern of yellowing is the clue to the cause.

Read the pattern:

• Yellow leaves, green veins (interveinal chlorosis): Classic iron or manganese deficiency, very common on pin oak, maple, and river birch in high-pH (alkaline) soils where iron is present but locked up and unavailable. Common across the West and in limestone soils.
• Uniform yellowing, oldest/inner leaves first: Possible nitrogen deficiency or general stress.
• Yellowing with wilting: Often a root or water problem — both drought and overwatering/poor drainage cause this.
• Sudden one-side or whole-canopy yellowing/wilting: Could be a vascular disease (e.g., verticillium wilt) or root damage — investigate further.

Common underlying causes:

1. Soil pH too high for the species (locks up iron) — most common cause of interveinal chlorosis
2. Poor drainage / overwatering suffocating roots
3. Compacted soil or girdling roots restricting uptake
4. Drought stress

What to do:

1. Check watering and drainage first — correct extremes before reaching for products.
2. Test the soil pH. If it's high and the species prefers acidic soil, you can apply chelated iron (quick) and work on lowering pH with elemental sulfur (slow) — or accept that some species are simply mismatched to alkaline soil.
3. Rule out girdling roots / deep planting at the trunk flare.

Use our Diagnose tool to narrow it down, or consult a certified arborist if yellowing is rapid or one-sided.