Three years it shall be forbidden to you. Many commentaries say, with regard to the essence of this mitzvah, that it is based on an Amoraite law to create tricks/shortcuts in order to hasten the ripening of the fruit of the tree before its time. Maimonides, in Guide for the Perplexed, wrote -- and we'll see a more rational explanation here -- that this mitzvah is in order to remember the Creation of the World, since everything (total) was created on Day 1, and everything (individual categories) emerged, each on its own day. Plants of the ground were hidden until Day 3; and even though they bloomed on Day 3, they aren't even seen until Day 4 when the lights of the sky caused the fruits to ripen -- because all ripening is dependent on sun and moon, as we learn [Deuteronomy 33:14] "With the bounteous yield of the sun, And the bounteous crop of the moons." So we find that they were hidden and impossible to eat until Day 4. Based on this, we are instructed to keep the mitzvah of orlah in memory of the creation of the world. Our plants, too, should be hidden and sealed for three years, since the essence of orlah is being hidden and sealed.