We just moved into a new home with beautiful hedges surrounding our property. We have lived here for about a month and we noticed dry or dead portions of the hedges. Why are they dying? How can we stop this from spreading to all the hedges?
The plant looks like it may be leyland cypress pruned to maintain a hedge. Most of it looks pretty good. The browning may be due to possible winter damage, reflected heat from the sidewalks, root competition for moisture and nutrients from the other trees when planted too closely together, lack of sunlight, etc. The tree in the right photo looks like it is growing in the shade (lack of sunlight and should be removed).
There is not much you can do other than prune out dead branches and wait to see if new growth will develop. Keep the trees well watered and mulched in times of drought. Avoid pruning when the trees are damp. Also, see if you can improve the drainage in the area if need be.
Also, there is a new invasive pest called the japanese cedar long horned beetle that can cause branch dieback and browning. Look carefully for oval exit holes about 4mm x 2mm on the damaged limbs. Peel back bark and look for tunnels. http://extension.umd.edu/hgic/invasives/japanese-cedar-longhorned-beetle
Here is more information on leyland cypress.
In recent years it has become evident that they look great for the first 15 years or so, and then start to decline. Very often we see them planted too closely together which leads to competition for water and nutrients. As the trees mature, lack of sunlight to the interior of the tree leads to the decline of the inner branches.
Stressful conditions like drought, poor drainage (the root systems will be affected in soils that drain poorly or standing water), or cold winters can make Leyland cypresses susceptible to different insect pests and diseases. Common problems include 'winter burn', which is a browning desiccation injury from drying winter winds. Here is an article from our plant pathologist that addresses some of the common problems with Leylands: https://marylandgrows.umd.edu/2018/04/11/why-is-leyland-cypress-turning-brown-winter-took-its-toll/
In addition, bagworms are an insect pest of Leyland cypress. If not controlled, these can also lead to browning and decline. https://extension.umd.edu/hgic/bagworms-trees-and-shrubs
For the future, The best plan is to diversify the planting in the area when a tree declines. A mixed planting will be more forgiving if you start losing them to poor site conditions, insect , or disease issues. Plan for mature height and width based on the site.