Understanding the New Year of the trees through insights on the creation of the earth, the Garden of Eden, and the curse of thorns and thistles.
We read in Tractate Rosh Hashana Chapter 1:1:
There are four New Year Days:
The first of Nissan is New Year for (the ascension of) Kings and for (the regular rotation of) festivals;
The first of Elul is New Year for the Cattle-tithe, but according to R. Eliezer and Simeon, it is on the first of Tishrei.
The first of Tishri is New Year’s Day, for ordinary years, and for sabbatical years and jubilees; and also, for the planting of trees and for herbs.
On the first day of Shevat is the New Year for trees, according to the School of Shammai; but the School of Hillel says it is on the fifteenth of the same month.
In the Mishna, we are introduced here to four cycles of time:
The first – of man and locations, expressed through the highest of men (the king) and the highest of locations (the Temple). 
The second – of animals, its culmination the tithing of the cattle – bringing it into kedusha, the realm of holiness.
The third – the cycle of time itself – years, sabbatical years and jubilees, which is also, the cycle of the Earth – the planting and the herbs.
The fourth, with which we are concerned – the cycle of trees.
The question we should automatically ask ourselves is why the trees are given their own category – what is it about them that deserves its own new-year? Why aren’t they written together with the planting and the herbs, as could have been expected, in the cycle of the earth?
In order to properly understand the relations between the trees and the earth – we shall look back at creation, Genesis 2:5:.
“Now no tree of the field was yet on the earth, neither did any herb of the field yet grow”.
As the Sages have pointed out, the verse is phrased in a way that takes us to an earlier time, to an event that will happen eventually – ‘was yet on the earth’. The trees of the field (bushes) and herbs of the field (grass) had not yet grown, and this is given double reasoning in the continuation of the verse: “Because the Lord Godhad not brought rain upon the earth, and there was no man to work the soil”. Notice we are told of what has yet to happen. What do we need this description for? Why do we need to know that there aren’t any bushes or grass yet?
The next verse (Genesis 2:6) goes on to describe what there already was:
“And a mist ascended from the earth and watered the entire surface of the ground.”
And so, although the rain has yet to come, there is nonetheless a mist that rises from the earth and this mist waters the ground. It would seem, in light of the above – that for some reason this mist is not enough to water the bushes nd grass.
Creation goes on and God creates man, plants the Garden in Eden, and places man inside it:
“And the Lord God formed man of dust from the ground, and He breathed into his nostrils the soul of life, and man became a living soul. And the Lord God planted a garden in Eden from the east, and He placed there the man whom He had formed.” (Genesis 2:7)
Man is not just placed in the garden, he is also held responsible for it as it says in the next verse (Genesis 2:8):
“Now the Lord God took the man, and He placed him in the Garden of Eden to work it and to guard it”.
It would seem now that man has been placed in the garden, that we would see a realization of the expectations described in the beginning of this description, that the bushes and grass start to grow. However, in this garden too, we are told only that the trees grow:
“And the Lord God caused to sprout from the ground every tree pleasant to see and good to eat, and the Tree of Life in the midst of the garden, and the Tree of Knowledge of good and evil”. (Genesis 2:9)
Furthermore, a closer look will show that verse five describes trees and herbs of the field, but here God planted in Eden a garden.
So when did the bushes and grass grow?
In Chapter Three after the description of Adams sin:
And to man He said, “Because you listened to your wife, and you ate from the tree from which I commanded you saying, ‘You shall not eat of it’, cursed be the ground for your sake; with toil shall you eat of it all the days of your life. And it will cause thorns and thistles to grow for you, and you shall eat the herbs of the field.
With the sweat of your face you shall eat bread, until you return to the ground, for you were taken therefrom, for dust you are, and to dust you will retur”… And the Lord God sent him out of the Garden of Eden, to till the soil, whence he had been taken.
It would seem that only after the sin does the herb of the field grow! Only from the time that man is expelled from Eden and is sentenced to work the earth -as part of his curse. He was also sentenced to stop eating the fruit of the trees and start eating from the herbs of the field. This curse is based on man being taken from Eden and returning to the ground, the soil ‘whence he had been taken’. It is as if God has severed his connections to man, ‘fired’ him from his position as gardener, and now man is no more than a worker of the soil from which he came, disconnected from the spirit of life that was breathed into him.
From the time man is expelled from Eden as part of the curse of man and Earth, the ground starts to function improperly, growing thorns and thistles, and with them, the herbs of the field. Likewise man is demoted from his high status of eater of the sweet fruit of the trees, to the low level of a beast -eater of the herbs of the field.
In summary: just as man was expelled from the Garden of Eden to the ground from which he was taken, so too is he expelled from the fruits of that garden: the fruits of the trees to the ‘fruits’ of the ground the herbs of the field, which become the base of man’s nutrition. 
So back to the question we started with – why do the trees have their own New Year? It seems we can now give a clear answer.
The grass and bushes are plants of the ground, the natural ‘blanket’ of the earth, and naturally share the same time cycle with the Earth, the time cycle of sabbatical years and jubilees: the sabbatical of the Earth. The trees, on the other hand, are not part of the earth; they were planted by God. This idea is expressed also in the shape of trees that expand as they grow further from the ground. When man eats from the sweet fruit of trees, he actually tastes the taste of Eden, even while still living in a world after sin. The time cycle for trees is the time of Eden, paradise, the time of redemption.
 Of course, the festivals can also be viewed as a cycle in time, but the foundation of this cycle, in my opinion, is location.
 If we go back to verse 2:3, “Now no tree of the field was yet on the earth, neither did any herb of the field yet grow…” and so on, we can now see that it is actually a foreshadowing of Adam’s eventual sin!