Without seeing the overall planting conditions from more of a distance (where the whole trees are visible), it's hard to say what may be stressing the trees. Are they growing along a property line, where roots spread into a neighbor's yard and/or the browning branches face their yard? We're not suggesting they are deliberately interfering with the plants, but their actions in terms of how they might be irrigating parts of their own landscape, applying any herbicides or pesticides to the lawn or plantings near that area, or other factors could be having an unintended impact on the Leylands.

Although the browning appears to be abiotic in nature (that is, not having been caused by a pest or disease) based on how it's manifested, there are some pests and pathogens that can contribute to foliage browning or branch dieback in Leylands that we can't rule out based on the photos. Two different fungal cankers can cause damage, especially when trees are under stress, such as from drought. Mites can also feed on foliage, especially in hot and dry weather, and cause some browning if populations are high enough. While it would be uncommon for mites to be limited to only one side of a tree, if that side was hotter due to poor air circulation (such as being near a building wall or solid fence) or reflected heat (wall, fence, or pavement), then it might make them favor that one side.

Drought alone might be a major factor if the plants weren't being monitored for watering needs this spring and summer. Along with much of central Maryland, Frederick County has been experiencing drought for the bulk of this growing season, and plant stress is accumulating for those not irrigated periodically.

For now, all you can do is to water the root zones of the trees if or when the soil feels somewhat dry to the touch about six inches deep next to the roots. (If damp when checked, they should have enough moisture.) This might entail watering with a sprinkler or soaker hose about once a week during warm weather, and a bit less often during cooler weather, while we aren't getting much rain. The forecasted rainy weekend might help with this if it doesn't rain so hard that most of the water just runs off before soaking in.

Most conifers, including Leyland Cypress, cannot replace lost foliage, so browned needles will eventually shed and not regrow. Branch tips with live growth can still fill out over time since new leaf buds can still form there, and this may help to gradually hide a bare interior. It's also normal for some degree of interior needle shedding as plants age, since that foliage becomes more and more shaded as the evergreen matures, so these two causes can overlap. Don't trim back any branches with mostly-brown foliage and green still-alive tips unless you want to permanently remove that branch, because that will similarly not be replaced by the plant once removed. We're not sure if we see a green metal plant stake in one of the photos or not; if so, take it out if it's tied to the tree and make sure any wire, plastic, or twine ties around the trunk are removed, since staking should not be needed for more than 6 to 12 months after planting, and ties left around trunks can "strangle" the wood after a while if they become snug and embedded in bark.

Miri

Hello Miri,

Thank you for this information. The trees back are next to woods and have room for circulation.  I don't very often view the trees from the woods as I only go there, every few weeks,  to retrieve my dog's tennis balls. The branches have not been this brown in the past. There are no stakes around the trees and I do not usually put pesticides on the trees. I put spray in May to prevent bag worms. I'm hoping that the weekend rain will give the plants some relief. 

Thanks again. 

You're welcome.