The author could be alluding to Shakespeare's Macbeth (Act IV, Scene 1, with the witches) :
A shew of eight Kings, and Banquo last, with a glasse in his hand.
Macb. Thou art too like the Spirit of Banquo: Down:
Thy Crowne do's seare mine Eye-bals. And thy haire
Thou other Gold-bound-brow, is like the first:
A third, is like the former. Filthy Hagges,
Why do you shew me this? - A fourth? Start eyes!
What will the Line stretch out to'th' cracke of Doome?
Macbeth (Act IV, Scene 1, emphasis mine)
Here, Shakespeare is using the verb stretch in a very similar manner, having a character inquire of the witches whether something (in this case, a family dynasty of kings, each succeeding his father, which have appeared in an apparition) will last until doomsday.
The "Filthy Hagges" (i.e. the witches) could be being referenced or alluded to by Frost's gloomy imagery that could be an apt metaphor to describe a stereotypical witch's lair.
Earlier in this scene in Macbeth, we see some tree and forest related imagery, which could map to Frost's use of arboreal language:
Thunder 3 Apparation, a Childe Crowned, with a Tree in his hand.
What is this, that rises like the issue of a King,
And weares vpon his Baby-brow, the round
And top of Soueraignty?
All. Listen, but speake not too't
3 Appar. Be Lyon metled, proud, and take no care:
Who chafes, who frets, or where Conspirers are:
Macbeth shall neuer vanquish'd be, vntill
Great Byrnam Wood, to high Dunsmane Hill
Shall come against him.
Macbeth (Act IV, Scene 1, emphasis mine)