In AE, "clustered along the branches like bittersweet berries" might work. Or "mountain ash berries" or "hawthorn berries," though none of those are quite as dense as sea buckthorn (which shrubbery catalogs offer here as an exotic, but the name is not recognizable for most readers). Not sure about UK fruit trees and shrubs. How tightly spaced do bullaces grow? For growth habit, "bittersweet" would work in both AE and BE because "European" or "Oriental" bittersweet, which has this along-the-branch growth habit, dominates in both places. On the other hand, it might be a problem to evoke a visual image of berries which are, unlike those of the sea buckthorn, inedible. ... This approach also might not work if the context is strictly Russian-literary; i.e., the speaker's invoking a local image. In that case, you could always write "clustered along the branches like sea buckthorn berries" and let the reader look up images of this striking plant:
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