Hay

It costs nothing but a walk. Swing a Sickle across any grassy surface and you collect building material faster than you could mine it, and overgrown tall grass pays out better per tile than a short lawn does.

The better reason is that Corruption, Crimson and Hallow cannot spread through it. A hay skybridge or a hay corridor stays clean permanently with no containment work at all, which is why long distance builds use it instead of stone or wood.

Only the Merchant's Sickle cuts plants into hay. The Ice Sickle and Death Sickle are weapons and give you nothing. Equip Flower Boots and walk while swinging: fresh plants sprout under your feet about as fast as you can clear them, which turns a full stack into a few seconds of work.