Leyland cypress is fast-growing and easy to maintain. Given good conditions, it quickly grows into a thick screen, providing privacy and shelter from winds. It also makes a pretty specimen tree. As easy as they are to grow, Leylands sometimes hit a bump in the road and turn yellow. Yellowing needles on a Leyland cypress often indicate chlorosis. Close attention to soil conditions will both prevent and cure chlorosis in Leyland cypress.
The leaves of Leyland cypress should be a dark forest green and the needles should have a waxy texture. Affected plants turn yellow-green–inside needles turning first–then completely yellow. If untreated, growth stops, followed by continued decline and death.
Chlorosis is caused by iron deficiency. Iron, present in most garden soils, is not available to plants unless soil pH is correct. Leyland cypress prefers acidic soil, but is tolerant of most alkaline soils. Still, soil pH of 7.5 or greater can block iron absorption in Leylands.
Leyland cypress should be planted in well-drained soil with neutral to acidic pH--5.0 to 6.5 is ideal. Soils above 7.50 should be amended with iron sulphate or aluminum sulphate to increase acidity.
Treat affected plants by scratching iron sulphate or aluminum sulphate into the soil above the roots. Adding in 1 inch of compost to soils at this time will enrich soil, providing moderate alkalinity and a medium for safely scratching supplements into the soil without disturbing roots. To quickly green-up yellowing Leylands, spray with a chelated iron solution according to label directions.
Replenish your Leyland with mulch every year, and let fallen needles decompose in places where they will nourish roots and contribute acidity to soil. Apply a fertilizer in late fall that has micronutrients and is formulated for acid-loving evergreens. Apply iron supplements to soil as soon as the needles begin to turn yellow.
Yellow leaves on Leyland Cypress can sometimes be caused by too much water. Soil should never be waterlogged and should be left to dry before irrigation. Leylands planted in wet soils will continue to fail and die, so transplant them to a more suitable location.