I think it's like greenhouse. But there are always grass and only grass spawning in farm. I'm afraid grass will destroy crops so I have to place something where I don't plant.
I know it's very late game but if and when you get that ugly Golden Clock and place it on your Farm, it will also prevent the Weeds from spawning and spreading on your Ginger farm, too.
Other options before then are to put Pathing down where you won't be growing Crops and checking on your Ginger Farm every 2-3 days.