These markers are made of wood, and although they have been varnished they will eventually decay if left in the ground.
You might consider pulling the markers from the soil for the winter and cleaning them off. Then before using them again in the spring apply a coat of oil or varnish before you insert them into the ground. They should last many years like that.
You can recoat them as needed with any standard varnish. Or you can apply an oil, like General Salad Bowl Finish or Watco Butcher Block Oil, available at Rocklers, Woodcraft, Home Depot, Lowes or True Value. Any decent hardware store will carry similar finishes. Just ask for products that are considered safe for food contact by the FDA.
