Strange to see an 18" spear that hasn't branched and ferned. If they're intact as you say, cut them off at the soil line (as ALL asparagus is cut) and eat the top, tender part, and discard the toughter bottom part. If by some miracle it's all tender, cut it in manageable lengths and eat it all.

If the bottom part is tough don't throw it away!

Peel the fibrous skin and eat the tender, juicy insides. Too tender to cook but pefect for salads if you can resist eating the juicy cores as you peel.

The tallest spears were a bit stringy, but overall, it was delicious!!!