Can't see how grit could be poisonous
Most likely to be some non native plant from a packet of summer annuals. Most common weeds can be found in my garden and none of them look like that
something like bullace I should think.
Have you looked inside one to see what it looks and smells like?
Alliaria petiolata, Jack by the Hedge
Food plant for the larvae of Orange Tip butterflies
I'd say Walnut
there is always a choice re using chemicals
Some kind of bramble, Rubus laciniatus maybe
i would plant. There's no time in a bulbs natural life when it's stored.