Clethra Dying - Ask Extension
I have about a dozen clethra alnifolia planted in the buffer zone near the Bay, which is subject to occassional flooding. The get
mostly full sun, ...
Knowledgebase
Clethra Dying #876797
Asked July 11, 2024, 10:03 AM EDT
I have about a dozen clethra alnifolia planted in the buffer zone near the Bay, which is subject to occassional flooding. The get
mostly full sun, though they do get late afternoon shade. They were planted last fall, and budded nicely this Spring. The deer were browsing them, so a fishing line fence was put up, which seems to have solved that problem.
The clethra have continued to look unhealthy. We're in a drought, but they have been watered once a week or so. Recently I gave them a deep watering (30 secs of hose), and then over the next few days, several of the clethra uickly lost all their leaves. Currently, the amended, clay soil is moist when I dig a few inches down. I'm sure if these are overwatered or underwatered, or if there is some other problem. Thanks!
Talbot County Maryland
Expert Response
While Clethra do have some degree of salt tolerance in their root zone, it's on the lower end for salt-tolerant native species. Do you happen to have laboratory soil test results from this planting area that has assessed salt levels (including sodium)? If not, that might be worth pursuing given that you mention the occasional flooding. Learning the soil acidity level (pH) from the test may also help, if it happens to be close to the edge of Clethra's pH preferences, as this impacts how easily roots can absorb the nutrients they need, and acidity can also impact the vulnerability of plants adapted to wet habitats to root-rotting pathogens.
It's hard to tell from the symptoms pictured what is going on, but soil moisture issues are likely a core factor. The leaf "burn" areas could just be scorch, though if a fungal infection instead, it's usually minor. (And probably unrelated to the branch dieback.) A 30-second hose watering might not be enough to soak the entire root zone (at least to a 6-inch depth or so), so you may want to feel the soil at that depth before and after watering to check on its effectiveness. (If the soil has become somewhat dry to the touch at that depth, then watering is probably needed, especially for species like Clethra that want to stay moist. If damp when checked, though, watering is probably not needed yet.)
Branch dieback, if not directly moisture-related, might be caused by indirectly by drought stress, potentially from weather patterns months ago. Botryosphaeria is a fungal canker disease that can affect a wide variety of shrubs after they first become stressed by an environmental factor (often high heat or drought). (The linked page focuses on Rhododendron, as they are a common victim, but generally applies to other shrubs as well.) Not curable or treatable, any dead branches that result would need to be pruned off. If too much dieback occurred, the plant should be replaced instead, though if you are confident the roots are in decent shape, the plant can be kept in place (after pruning) and it may regrow via suckers and replace those lost branches over time.
If over-watering or an over-saturated root zone is responsible instead, then unfortunately nothing can be done to intervene since there is no curative treatment. Either the damage will halt after conditions supporting the infection resolve, and the plant will have to regrow roots in order to recover, or the plant will succumb. Usually, advanced root rot that leads to prominent branch dieback or foliage loss does not reverse itself.
If you remove any plants and want to inspect the roots, you might be able to tell which issue was contributing. Roots dead from rot tend to be mushy and darker, pulling apart fairly easily into an outer layer and stringy inner core. Roots dead from other causes (or which are still alive) will be firm, take more force to break, and tend to snap in two.
Miri
It's hard to tell from the symptoms pictured what is going on, but soil moisture issues are likely a core factor. The leaf "burn" areas could just be scorch, though if a fungal infection instead, it's usually minor. (And probably unrelated to the branch dieback.) A 30-second hose watering might not be enough to soak the entire root zone (at least to a 6-inch depth or so), so you may want to feel the soil at that depth before and after watering to check on its effectiveness. (If the soil has become somewhat dry to the touch at that depth, then watering is probably needed, especially for species like Clethra that want to stay moist. If damp when checked, though, watering is probably not needed yet.)
Branch dieback, if not directly moisture-related, might be caused by indirectly by drought stress, potentially from weather patterns months ago. Botryosphaeria is a fungal canker disease that can affect a wide variety of shrubs after they first become stressed by an environmental factor (often high heat or drought). (The linked page focuses on Rhododendron, as they are a common victim, but generally applies to other shrubs as well.) Not curable or treatable, any dead branches that result would need to be pruned off. If too much dieback occurred, the plant should be replaced instead, though if you are confident the roots are in decent shape, the plant can be kept in place (after pruning) and it may regrow via suckers and replace those lost branches over time.
If over-watering or an over-saturated root zone is responsible instead, then unfortunately nothing can be done to intervene since there is no curative treatment. Either the damage will halt after conditions supporting the infection resolve, and the plant will have to regrow roots in order to recover, or the plant will succumb. Usually, advanced root rot that leads to prominent branch dieback or foliage loss does not reverse itself.
If you remove any plants and want to inspect the roots, you might be able to tell which issue was contributing. Roots dead from rot tend to be mushy and darker, pulling apart fairly easily into an outer layer and stringy inner core. Roots dead from other causes (or which are still alive) will be firm, take more force to break, and tend to snap in two.
Miri
