Most of the arborvitae losses we've seen in the past year or two were due, directly or indirectly, to drought. Many counties in Maryland experienced drought or abnormally dry conditions for much of the past two growing seasons, and if that didn't damage roots directly, it can make plants more vulnerable to attack by certain pests (like wood-boring beetles) and diseases (like Botryosphaeria canker). Secondary issues like those are not uncommon, and no new pest or disease has been found to be responsible for the deaths of arborvitae or other established evergreens.

We are unable to determine from the photos what factored into the loss of the first plant, but it likely stemmed from stressful environmental conditions, such as over-watering or under-watering. (Over-watering is less likely given the droughts, but still possible if plants were watered too heavily or too often.) Monitoring plants for watering needs during periods of little to no rain can help reduce their stress so they're not easy targets for the above pest/disease issues.

It's hard to tell from the low angle of the sun (casting a yellower color on the foliage) if the shorter arborvitae in the second photo is off-color or not. Stunted, perhaps, if it was installed at the same time and has not caught up in growth. This might be caused by similar factors -- too little or too much water (or poor drainage) stressing roots -- or soil compaction, or a lack of loosening a root-bound root ball when it was first installed. The minor amount of foliage browning on the interior of the branches is normal in the autumn, as some degree of old leaf shed is typical for evergreens like these, even though on denser plants it's less noticeable.

We recommend that, for the remaining plants, any mulch used around their base (if it isn't, we suggest that it is) be expanded outwards to cover at least out to the drip line (the edge of the branch spread) to minimize the risk to the trunk of injury from mowers or string trimmers, since such injuries can be serious or fatal. Expanding a mulched zone as the arborvitae mature will also help to reduce root competition for water and nutrients with the surrounding lawn by keeping the grass further away. Keep the mulch layer only around 2-3 inches thick, and not piled against the trunk base itself. If any stakes and ties are left on the trunk (we don't see any), make sure they're removed so no "strangling" of the trunk occurs that restricts growth.

Evergreens like arborvitae do not recover if the foliage turns entirely brown, so for the future, pesticide use will not provide any benefit in that situation, nor would it help once discoloration symptoms manifest too extensively (such as the foliage becoming a sickly olive-green color instead of the normal richer green). While some degree of "bronzing" (blushing a bit brownish or caramel-colored) during winter can be normal for a variety of evergreens, it's minimal on 'Green Giant', and any branches that might turn noticeably brown out to their tips at some point will have died and should be pruned off. Pesticides also cannot treat/cure any existing borer issues or fungal canker infections, which is another reason to confirm the culprit before trying to treat, especially since some pesticide ingredients can harm beneficial insects.

For now, other than removing/replacing the dead individual arborvitae (try not to replant too late into the fall), just monitor the rest of the plants for watering needs if our autumn or winter skews dry again. The national drought monitor map can help you judge when we're behind on rainfall compared to an average year (the map is updated once a week), though the best monitoring method is to feel the soil near the root zone about six inches deep; if somewhat dry to the touch at that depth, watering is probably needed. (If still damp at that depth, watering can probably wait.) No other intervention is needed at this point.

Miri