Solutions for Act I Scene 1 - Evergreen

Each specimen described is actually a letter of the English alphabet, with juvenile and adult variants being lower case and upper case respectively. That is:

Specimen 1: Strong similarities between juvenile and adult specimens - the main distinguishing feature for the fully grown specimen is its upper bough reaches the same height as its trunk.
This is the letter K. The lower case k looks very similar to the upper case K except the length of its 'arm'.

Specimen 2: Starkly different from specimen 1, this specimen metamorphoses completely between seedling and adult. It almost doubles in height, acquires a secondary root, and its hoop-shaped bough becomes more rigid and is lifted into its centre.
This is the letter A. The lower case a and upper case A share very little. The specimen text describes how one might grow - with the a's rounded bowl becoming the angular space above the A's crossbar.

Specimen 3: This interesting specimen raises the level of its canopy with respect to its trunk as it grows without significantly gaining height - the most mature of these have branches only at the very top of their trunk.
This is the letter T. The cross stroke of the lower case t could rise up its stem to become the upper case T.

Specimen 4: Apart from growing in size vertically and becoming less supple, this specimen sprouts a secondary trunk from its secondary root, which reaches the same height as the primary.
This is the letter N. The shoulder of the lower case n, when straightened out could become the left half of the capital. The secondary trunk describes the upper case N's right stem.

Specimen 5: The seedling of this specimen is most noted for the brilliant blossom that it delicately suspends above its stem. As it ages this blossom is lost, and it becomes a particularly plain, but rather tall, reed.
This is the letter I. The lower case i notably has a tittle, which is described in the puzzle text as a blossom, delicately suspended above the stem.

Specimen 6a/b: The recurve shape is characteristic in these vines, partly because it is shared between the juveniles and adults of the species. The fully grown specimen is significantly larger, perhaps twice the size of the juvenile.
These are the letters SS. The S's characteristic ess shape is captured somewhat in the adjective 'recurve', which has just enough vague meaning to be a good clue for the shape. It also doubles as a small hint for the answer, which (on combining the clued letters) is:

Katniss is a kind of pond weed (under genus sagittaria), which ties into the botany theme, but is probably most notable as the name of the protagonist of the Hunger Games series. The fact that the title of this puzzle sounds similar to the Katniss of literature's surname (Everdeen) is further confirmation of the answer.

PS the author wishes to convey his apologies to botanists and typographers alike.

The answer is: katniss