This is the European Snowball Bush Viburnum (Viburnum opulus 'Roseum', a.k.a. 'Sterile').

Walnut "toxicity," long thought to be a problem for juglone-sensitive plants, seems to not be a problematic issue after all. While some plants may struggle under Walnut trees, there are other potential reasons outside of juglone exposure. If you are curious, the link to the recent review of the evidence by Washington State University is available on this page:
https://extension.umd.edu/hgic/topics/walnut-toxicity-juglone
If it matters, this species of Viburnum is purportedly sensitive, but in actuality this plant is already within the juglone "contaminated" zone by being a distance of less than twice (or more) of the tree's dripline - tree roots extend well beyond their dripline. The larger plant in the photo seems to be doing well.

Koreanspice Viburnums are more vulnerable than some Viburnums to root-rotting infections and possibly Botryosphaeria stem infections when under stress. Their decline and death due to juglone is unlikely, and would probably not occur nearly that rapidly. (Instead you would likely see signs of stunted growth the following spring/summer or nutrient deficiencies as root function is suppressed.)

Barberry is very adaptable to soil conditions (one reason it is very invasive in wild areas), as are their cousins Nandina (also invasive in some areas). Hosta enjoy consistent soil moisture and probably aren't the best indicator for the very good drainage the Koreanspice would require. Similarly, Viburnum obovatum (of which 'Mrs. Schiller's Delight' is a cultivar) is well-adapted to wet soils where it occurs in the wild. The fact that all of these species are prospering suggests the area was too damp for the Koreanspice, or that perhaps it was over-watered at some point.

In more shade, the Viburnum may not flower as profusely. If possible, it might be best to leave it where it is thriving and to plant an alternative shrub in the location closer to the Walnut. Juglone aside, other shade-tolerant (or shade-adapted) shrub species include the following, all fragrant (since the Koreanspice would have been fragrant):

• Summersweet (Clethra alnifolia) - very good in wet conditions
• Carolina Allspice (Calycanthus floridus) - a distinctive fruity scent
• Witchhazel (Hamamelis x intermedia, Hamamelis pallida)
• Paperbush (Edgeworthia chrysantha or papyrifera) - does need good drainage though
• Winterhazel (Corylopsis - several species all very similar to each other)
  
• Sweetbox (Sarcococca hookeriana humilis, for a groundcover, and Sarcococca confusa for a shrub)
• Banana Shrub (Michelia figo, a.k.a. Magnolia figo)
  
• Swamp Azalea (Rhododendron viscosum), Pinxterbloom Azalea (Rhododendron periclymenoides), Alabama Azalea (Rhododendron alabamense), Roseshell Azalea (Rhododendron prinophyllum), Sweet Azalea (Rhododendron arborescens), Coast Azalea (Rhododendron atlanticum), and Piedmont Azalea (Rhododendron canescens)
• Camellia (several species, dozens of cultivars) - while not known for notable fragrances as a whole, several cultivars have detectable scent as they age and flower production increases
  

Miri

Dear Miri,

Thank you for your quick and insightful response. I think you are absolutely right because that plant bed is in a pretty wet area. I'm glad to know the black walnut wasn't the cause after all. Are all of the items in the list you sent black walnut compatible? I looked up each one and love the shape of the witch hazel for that particular spot (woody on bottom and spreading out at the top) for the purposes of providing privacy but not making the area feel totally closed off by a dense and wide plant. But ideally we wouldn't want anything over 8 feet since it is under the low canopy of the black walnut. I'd also be happy with something evergreen or semi-evergreen so we'd still get some privacy if sitting on our patio in winter. Flowers and fragrance are great, but less of a priority. Given that, is there anything specific you would recommend? Maybe I'm asking too much of a plant that also has to survive under a black walnut! I am attaching a picture of the spot (with the Korean spice when it was still healthy) so you can see the space we are working with.Thanks again!

We did not cross-check with juglone tolerance/resistance as it is likely not an issue. If you wish to check their ratings, there are a number of juglone plant lists from universities, arboreta, and so forth, though surprisingly they don't always agree with each other. Sometimes they are also not specific enough; for instance some list "Viburnum" as tolerant but don't expand on that to state which species are tolerant, as not all are (according to other lists).

Some cultivars of Witchhazel are shorter-growing than others, though as a group they are large-statured shrubs. Light trimming after flowering in spring may help keep their size restrained a bit without sacrificing too many flowers. Of the prior suggestions, only Sweetbox, Banana Shrub (which may suffer from a harsh winter), and Camellia will be evergreen. Options setting aside fragrance for shade-adaptable plants in that general height range include the following, but few if any will have that V-shape that Witchhazel does; soil wetness tolerances (which is a more of an issue than juglone) will vary among these:

• False-holly (Osmanthus heterophyllus, though there are a couple other species which will probably remain undamaged in winter - their hardiness is more marginal) - this happens to be fragrant and very late-blooming; you could trim the lower branchlets or foliage off if you wanted that vase-shaped look, though it will be unlikely to grow back without harsher pruning if you ever wanted to change its shape
• Daphniphyllum (Daphniphyllum macropodum) - very Rhododendron-like in general appearance, sans flowers; here too, lower stems can be trimmed off as it ages to make it more visually "top-heavy"; tends to grow a little taller than you'd prefer; this is uncommon and hard to find, and may suffer light damage in a harsh winter; there's at least one planted in the Smithsonian gardens in D.C. that appears to be doing well enough that it looks at least a decade old based on its size
• Anise-shrub (Illicium anisatum, Illicium floridanum, cultivars of Illicium floridanum x mexicanum, Illicium parviflorum, and Illicium henryi) - this too is uncommon as shrubs are occasionally damaged by harsh winter weather; of all the species, parviflorum may be the most reliable in terms of overall performance, though it is one of the largest-growing, but can be trimmed
• Chinese Fringe-flower (Loropetalum chinense) - a relative of Witchhazel with very similar flowers (minimal to no detectable fragrance, though); again, sensitive to winter cold extremes and drying winds which will make it more deciduous than evergreen, though it can be either; does benefit from good drainage
• Japanese Plum Yew (Cephalotaxus harringtonia) - largely broad low-growers but cultivar 'Fastigiata' is upright, though fairly rigidly so

Miri

Thanks very much!