Obviously I'd not expanded on the idea fully
A similar issue was presented to me the other day when, whilst trying to deal with a crawling zombie (one that I'd removed a locomotive limb from, not Steve), my melee weapon kept hitting grass, leaving said undead blighter unscathed apart from aforementioned amputation. So something like: weapons bypass grass (and do no damage to dirt?), only certain tools (spade/pickaxe) overflow damage from grass to dirt, grass is only harvestable by fists - something like that.