The only silver plant we have in our garden is Astelia chathamica Silver Spear. They do flower every few years, but it's not a very attractive flower. However we think they are handsome architectural plants, which require minimal maintenance. They can be grown in full sun, but prefer shade. They are slow growers, so just as suitable for a pot as in the ground.
Depends on the climate/degree of shelter in the recipient’s garden, but a potted olive tree might work if they have a warm, sheltered spot. Peter Beales has two white roses called Silver Anniversary and Silver Wedding which would be an obvious choice and should work in any situation, but at this time of year it’s coming up to bare root season, so not especially spectacular looking at the moment, even if potted up for them. Eryngium Silver Ghost or anything with silver foliage, but again, subject to climate, soil and their preferred garden style, really..
Mountainous Northern Catalunya, Spain. Hot summers, cold winters.
Cornus controversa variegata - known as the wedding cake tree. Will need to go in the ground, eventually, and it is deciduous, but you may avoid duplicating what others buy.
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It has no connection with silver but it’s damn stylish
https://www.birchamgallery.co.uk/catalogue/artist/Alice:Palser/DZ00611/?category=sculpture
Argentea = silver.
Leaves smaller than Cornus controversa variegata
https://www.google.com/search?q=cornus+alternifolia+argentea&client=firefox-b-d&source=lnms&tbm=isch&sa=X&ved=2ahUKEwjNl8aCronsAhVNecAKHc7aBxcQ_AUoAXoECA0QAw&biw=1280&bih=579
https://www.rhs.org.uk/Plants/90328/Camellia-japonica-Silver-Anniversary/Details#:~:text=Details 'Silver Anniversary' is an,producedfrommid- winter to early spring