Genèse 1:12

And the earth brought forth, etc.: Even though "according to its species" is not stated concerning the grasses when they were commanded, [nonetheless] they heard that the trees were commanded thus; and they brought up an a fortiori argument (kal vechomer) regarding themselves, as is explained in the aggadah (homiletical teachings) in Chapter Shechitat Chullin (Chullin 60a).

And the earth produced grasses (and) herbage whose seed seedeth, and the tree making fruit after its kind. And the Lord saw that it was good.

AND G-D SAW THAT IT WAS GOOD. This affirms the existence of the various kinds forever.
There was no special day assigned for this command for vegetation alone, since it is not a unique work. The earth, whether it brings forth anything or is salt land, is one.

ותוצא הארץ, had the Torah written [in verbatim response to G’d’s directive]ותדשא הארץ , I would not have known that it was only one, as we explained on verse 11. The Torah wanted to make sure that we did not get the impression that the earth suddenly proliferated with a carpet of herbs. The remarkable thing was that although the earth produced only a single herb, ויהי כן, it endured, i.e. survived until it had many others added after the sun had become strong on the fourth day.
Alternatively, the meaning of this formulation is that from this moment on the earth always produced all these kinds of plants in their unadulterated form, each true to its species. There never was an occasion when nature suddenly produced a tree combining the properties of two different species. This only happened when man deliberately introduced an additional species through grafting it onto a tree. This is the reason why G’d commanded man not to crossbreed plants or animals. Both Adam and Noach were commanded this as our sages (Sanhedrin 60) state in conjunction with the verseאת חקותי תשמרו בהמתך לא ים, שדך לא תזרע כלאים, “do not crossbreed animals, nor seed your field with a mixture of more than one seed (trees).” (Leviticus 19,19). This referred to statutes G’d had already promulgated prior to the revelation at Mount Sinai. Concerning the mixing of seeds of herbs, no such prohibition exists for gentiles. Concerning the uprooting of such hybrids gentiles are not commanded to do this, and even Israelites are only obliged to do this in the Land of Israel. The entire subject revolves around deliberately upsetting the way in which G’d created His universe.[in the Sefer Hachinuch the applicability of part of this legislation to gentiles is described as מפי הקבלה, “based on tradition.” Ed.]
This is also why a scholar questioned what the halachah is according to Rabbi Chanina bar Pappa who had quoted the herbs arriving at the conclusion that they must maintain the purity of their species although the Torah had not written anything about למינו in connection with the herbs. The scholar wanted to know what the situation would be if someone had deliberately crossbred two species of herbs. According to the opinion of Ravina one could not hold such a person culpable for such an innovation. Alternatively, the question it that seeing that the herbs had voluntarily taken it upon themselves to maintain the same standards as did the trees and plants which carried their seed in them, would anyone violating that standard be held responsible for violating a law of nature which G’d had approved even though it was instigated by nature itself? The question was left open.

ותוצא הארץ דשא למינהו, “the earth brought forth a variety of herbs and grass, each according to their respective features;” according to the Talmud, tractate Chulin, folio 60, these herbs reasoned as follows: “if it had been the will of the Creator for all the plants to grow forth indiscriminately mixed with one another, why did He include the word: למינו, “in accordance with its individual features although trees due to their size are automatically set apart from one another, how much more must He be concerned that different types of herbs not be mixed up with one another. Upon hearing what the herbs had said, Mattatron, G–d’s general manager in supervising the smooth running of the universe, coined the phrase: יהי כבוד ה' לעולם, ישמח ה' במעשיו, “may the glory of the Lord endure forever, May He derive pleasure from His creatures.” (Psalms 104,31) This interpretation is somewhat difficult to reconcile with the tradition that Chanoch, (a descendant of Sheth, mentioned in Genesis 5,19, for the first time was identical with Mattaron. We must therefore assume that though the body of Chanoch had not yet existed on the third day of creation when the herbs were produced, his soul was already “waiting in the wings” to be assigned a suitable body to inhabit. It was therefore this soul that coined the phrase quoted. This is not as strange as it may sound, as we encounter something similar concerning the ספר תולדות האדם (Genesis 5,1) “this is the book containing a record of human history, etc.;” a line which was interpreted by our sages in Seder olam chapter 30, that the Almighty showed the first human being, Adam, (before his sin) all the human generations and the outstanding individuals of each generation. (Talmud, tractate Avdah Zarah, folio 5) Clearly, this is not to be understood at face value, seeing that Adam at that time was the only human being whose soul had already been assigned a body to inhabit.