You may dispose of a lulav in any way that is not degrading. So, you may drop it into a field or put it on a lawn--unless animals might eat it or step on it or if it will be subject to poor treatment before it decays.  Don't dispose of a lulav or etrog directly into the garbage. Burn, bury, or wrap them in a bag or one layer of plastic and you may throw it into normal garbage.