Like tall nettles, thistles, takes ideas about life from nature.
The defiant character of the plant is emphasized in the very first word ("Against") of this poem. They not only resist all animal ("rubber tongues of cows") and human ("hoeing hands of men") attempts to destroy them but even attack the natural world, indicated by the words "spike the summer air". Further, they make it difficult for themselves to reproduce because their seed-pods open only under 'pressure'.
Every plant, without exception, is similarly defiant, according to the second stanza. Moreover, it is determined to revenge itself upon whatever destroyed the parent plant. The violence of its determination is expressed in the term "resurrection". This paradox, that it brings forth life in death, gives it a mythic quality and makes it seem an otherworld species. Its unremitting and unfeeling hostility is conveyed by the words "grasped fistful of splintered weapons", that refer to its many sharp thorns, which can inflict painful wounds.
The poet further emphasizes the fierceness of the thistle by linking it to those legendary warriors, the Vikings, who came from the cold Scandinavian countries and raided the coasts of northern and western Europe from the eighth to the tenth centuries. He does this explicitly in the words "Icelandic" and "Vikings" but also indirectly through reference to the origins of the word "thistle" (which, with the word "Viking", derives from Old Norse or Icelandic, the language of Norway and its colonies down to the 14th century). He even suggests thistles are a means of revenge used by the Vikings against those who took their lives so many centuries ago. The long-dead bodies of Vikings have resisted assimilation and complete disintegration in foreign soul and they still exist as a "stain", or have merely "decayed". The thistles...