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You were certainly on the right track, Silver surfer. After a very convoluted search, I finally found the name of the plant my son was...
Yes to D. congestum.
It looks like an Allium sp...an onion.
I do not know which ones you have there but have found a link ...so you can check each one listed...
Reminds me of Persicaria virginiana 'Filiformis'
Persicaria virginiana 'Filiformis' - Google Search
Many named cultivars.
helianthemum - Google Search
It looks like Aralia elata.
Square stems, opposite leaves.
Might it be Stachys?