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

On 10/18/2024 1:21 PM EDT Ask Extension wrote:

 

 

Unfortunately we can't make out any diagnostic detail in the pictured fungal material (which looks mostly like bark or mulch...hard to tell). Fungal growth on the soil/mulch surface is usually not related to tree dieback or rarely the cause of it; rather, fungi producing mushrooms directly out of wood (trunk or surface roots, if you can confirm they are attached to actual wood when they appear and not just growing out of mulch or soil) might be pathogens, but they too could be harmless and only decaying wood that was already dying or dead. We usually do not see active wood-decay fungal pathogens killing arborvitae or other conifers, compared to, say, oaks and maples. Even in that situation, removing visible fungal growths emerging out of wood will not remedy the situation, because it doesn't do anything to the "body" of the fungus itself inside the tree's tissues.

Fertilizer will not benefit a stressed or dying plant (sometimes it can make stress or secondary pest/disease damage worse, actually), so for the future, we do not recommend using any on a plant that is ailing. Fertilizer isn't a source of food for plants (that would be mainly sunlight); if anything, it's more akin to a multivitamin, supplementing situations where the soil might be deficient in certain nutrients. In soil that has no significant deficiencies, it has limited ability to spur growth, because the roots already have everything they need. (And if the roots are unhealthy and not functioning properly, as can happen with drought damage or over-watering damage, then fertilizer nutrients can't be absorbed either.)

The majority of plants generally do not absorb nutrients through their foliage, only through roots, so a foliar application of fertilizer won't benefit them much. Iron can indeed help to green-up chlorotic plants (the type of nutrient deficiency resulting from insufficient iron, manganese, or magnesium), but it's a temporary fix, and doesn't address the core cause of soil deficiency or, more likely, a soil acidity (pH) outside of the plant's preferred range, which impacts how well roots can access the nutrients they need. Or, roots are damaged from root rot or other causes, or don't have enough moisture to move those nutrients up into the sap (as they need to be absorbed in diluted form, as they pull in water). Over-applications of fertilizer can "burn" roots or foliage, causing toxicity symptoms, so always apply them as the directions list on the label, and try to avoid double-dosing with more than one fertilizer type, at least if they overlap in nutrient content. (If both contained iron, for example.)

Overall, fertilizer can sometimes support healthy plant growth and blooming (like with potted annuals and houseplants), but they won't fix a declining plant unless a nutrient deficiency is the only issue (and that's rarely the case). A laboratory soil test will reveal nutrient levels and show if there is a deficiency, plus it will measure pH more accurately than home test kits or probes. Thuja plicata, one of the parent species for the 'Green Giant' hybrid, is fairly adaptable when it comes to soil pH, so that's probably not a factor here.

The progression of symptoms described on the now-dead arborvitae sound like either drought damage or Botryosphaeria canker damage. For plants that suffered drought stress but didn't manifest obvious symptoms, they still might contract Botryosphaeria dieback next year or beyond, as it can take time for such secondary issues to appear or progress to the point of killing branches. While Botryosphaeria is a fungus, it does not produce external mushrooms or other overt signs of fungal growth outside of the plant.

We have seen numerous instances this summer/fall where 'Green Giant' arborvitae in a row or grouping have lost some individuals while others remained green (for now, at least). This could be due to differences in soil drainage (even if they appear to be growing on level, uniform ground), soil moisture retention, or even just individual plant health and vigor before the drought worsened. Plants that look the same size, purchased at the same time or from the same nursery, won't always have comparable root system health, and any with stunted or under-developed root systems (or root-bound plants whose roots weren't well-loosened prior to planting) would be more vulnerable to later stresses or could lag behind in growth.

Miri