I have an arborvitae and a group of pencil holly that each have suddenly...
I have an arborvitae and a group of pencil holly that each have suddenly turned brown in large portions of the plants. Water appears to have been sufficient from rain and hand-watering. Arborvitae: plant is 9 years old (from Meadow Farms), planted on the east side of the house. Morning sun, good soil. No problems until now. No problem with similar plant located on other side of front walk, also with morning sun. One of the main trunks is affected, and a second seems to be following suit. Pencil holly: Four plants clustered. One was cut back severely except for one trunk this spring after much of it had turned brown. The remaining trunk is fine. Two 9-year-old plants are almost entirely brown, though one has a green stem at the bottom. The fourth was planted this spring (from Costco) and is partially brown, partially brown. Plants are close to chocolate vine and gold crown cypress. Receive afternoon son. At corner of townhouse lot. Good soil. Pencil holly in three other locations are fine.
District of Columbia County District of Columbia
The browning on the arborvitae and pencil holly looks like it may be due to a possible root or trunk problem. This can be difficult to diagnose. Arborvitae can remain green for a while before they turn brown. This probably happened fairly recently. Look around the base of the trunks or stems for rabbit (gnawing around the base of trunk) or vole damage. Look for silver dollar sized holes in the soil around the base of the plant. Voles can feed on the roots of trees and shrubs and cause dieback. http://extension.umd.edu/hgic/voles-groundcovers
Look along the trunk for small insect holes on the arborvitae. There is a new invasive pest called the japanese cedar long horned beetle that can cause branch dieback and browning http://extension.umd.edu/hgic/invasives/japanese-cedar-longhorned-beetle
Also, check the drainage in the area and make sure the soil drains well and there are no drainage spouts dumping water. Mulch should be no thicker than two inches and keep away from the base of the stems.
At this point, the arborvitae and hollies need to be removed.