The redwood (Sequoia sempervirens) is also known as the coast redwood. It is native to the coasts of northern and central California and can grow to between 70 and 90 feet tall. These iconic conifers can be grown individually, in groves or even as a tall hedge. From insects and disease to improper care, there are a few things that can turn redwood needles brown.
Caused by fungal infection, cankers are dead, sunken spots found on trunks, limbs and branches of infected trees. These infected areas can cause needles or entire limbs to turn yellow or brown and die. Cankers can girdle and kill limbs and entire trees. To prevent cankers, avoid planting redwoods in areas where growing conditions aren't suitable. Redwoods prefer moist, cool areas. Prune out infected limbs immediately and provide proper irrigation, especially during drought conditions.
Spider mites are not insects but are closely related to spiders. They use piercing mouthparts to suck the sap from leaves and needles. As they feed they leave behind webbing that can affect your redwood's appearance. Large infestations can cause needles to turn yellow or bronze. Avoid dusty conditions around your redwoods as they can cause spider mites to proliferate. Provide your redwood with adequate irrigation. Washing mites off the tree with a strong jet of water can provide control and prevent further infection by washing dust off the tree.
Cypress Tip Miner
The cypress tip miner is the larval form of a small, silvery tan moth commonly found on redwoods, arborvitae, cypress and juniper. Caterpillars spin small white cocoons between twiglets when they pupate that may aid in identification. Feeding by the cypress leaf miner causes needles to turn yellow in early winter and then brown by late winter. By spring needles are generally green again. Prune out severely infected branches and apply a broad spectrum insecticide like acephate between March and May.
Cultural and Environmenal
In their native environments redwoods grow in cool, coastal regions with large amounts of winter rainfall. During summers they are cooled and irrigated by fog and their roots are shaded by leaf litter and a dense canopy. In warmer regions redwoods may not get the moisture and shade they need to maintain green foliage. Over pruning can also cause brown foliage by exposing the trunk to sunlight and heat. Lack of mulch to conserve water and protect roots can also cause issues. Avoid unnecessary pruning, keep trees mulched and irrigate adequately to avoid browning needles.