black cohosh declining

lauren02 02

I've been having some problems with my new black cohosh plants. I have about a dozen of them planted a month or so ago and have already had to replace a few. There seem to be three symptoms:

1. Leaves curling up (I circled an example curling leaf with blur):


2. New growth dying (there was a third leaf between the two new ones that is now black and dead, again circled the area in question with blur, and in the two days since I took this picture the whole thing has gone):


3. Lighter/blotchy leaves that slowly decline into death:

These signs seem to be the precursor to plant death. Does anyone have any ideas? Moisture-wise, things seem to be good -- it's not sopping there but is pretty consistently moist. Any help is appreciated!


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WoodsTea 6a MO(6a)

I can't really say what's wrong here -- it's possible that some of these signs aren't serious. My black cohosh plants often have leaves that look like the ones in the last photo, but they're reasonably healthy otherwise and bloom well.

I would suggest pulling the mulch back from the plant at least a couple of inches, that could be one source of trouble. How much sun do they get?

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lauren02 02

OK, good to know about the leaf blotchiness, thanks.

I will definitely pull the mulch back, right now it's right up to the stems.

They are between a few trees, so they get very dappled sun (but mostly shade) for the large part of the day, and then get an hour or so of late afternoon sun.

I also have clay soil, which typically has a slightly alkaline pH. Could this be a pH issue? The internet doesn't seem to agree on whether black cohosh is fussy about pH/clay or not.

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WoodsTea 6a MO(6a)

I would think that being a forest plant of the eastern US that the plant would prefer soil on the acidic side. You could try posting to the Garden Clinic forum -- they might not know this species well but they have plenty of experience in diagnosing foliage problems generally.

My bet is the mulch -- when it's too deep or up against the plant stems there can be a problem getting oxygen to the roots, especially in a heavy soil.

Mostly I don't fertilize native plants, but in some shady mulched areas where I have young plants -- especially near trees -- I do sprinkle a little Osmocote Indoor/Outdoor around early in the spring. The better and more natural alternative would be to top dress the soil around them each year with compost or leaf mold, again keeping it away from the stems. Over time that will make your soil more like what black cohosh has in its native environment.

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