"
Arrrwwwrrraaaaughhhhhhhh!"
That's the sound of a torturous explanation. Or possibly Bilbo as he spits out a mouthful of foul acorn coffee outside the new Hobbiton branch of Starbucks and vows never to go there again (good taste...).
Seriously, surely the only consideration is whether something could conceivably be grown in The Shire, considering its latitude and climate. Comparing time frames to our own world doesn't really work, as LotR isn't set in a time we could possibly compare to anyway. If it was set in Medieval times then the Fourth to Sixth ages must have been ruddy short. And anyway, doesn't Tolkien say they could all be measured in thousands of years? So, LotR is set when? Some six thousand years ago minimum? Well if it is then just about
everything (steel swords, armour, horsemanship, etc) would be an anachronism, surely?