Horticulture (plants, flowers, trees, and so on) imagery pops up all over the play, so it's worth thinking about. In the king's opening speech, he vividly describes how civil warfare threatens to crush the "tender florets" that spring from English soil, planting the English countryside instead with dead bodies (1.1.1). Yikes! Later, Hotspur describes how the Percy family helped Henry depose King Richard II when they "put down Richard, that sweet lovely rose, / An plant[ed] this thorn, this canker, Bolingbroke" (1.3.7). By suggesting King Henry (formerly called Henry Bolingbroke) is a "thorn" and then a "canker" (an icky plant disease), Hotspur imagines England as a kind of garden that's on the cusp of ruin and decay.
Interestingly enough, this idea originated in the play Richard II, when Henry's father, John of Gaunt, referred to the corrupt King Richard II as a lousy gardener who failed to properly tend to England (R2, 2.1.) In Richard II, the realm was portrayed as a kind of fallen Eden that had been ruined by Richard's sleazy policies. OK, fine. So what? Well, we notice that the horticulture imagery in Henry IV Part 1 gestures, simultaneously, at England's vulnerability to corruption and decay and also at its capacity for regeneration and growth. So, while the play is consumed with the destruction caused by civil warfare, it never forgets that England will ultimately survive and thrive, like a garden.
At other times, horticultural references refer more specifically to family lineage and loyalty. When King Henry says his son, Prince Hal, is disloyal to his "blood" family and behaves as though he is "grafted" to his loser companions (3.2.1), he alludes to the seeming unnaturalness of Hal's relationship with commoners and criminals. ("Grafting" is a horticultural practice where tree tissues are fused with another tree.) Later, King Henry uses similar language when he praises Hotspur for his bravery and honor, suggesting the young man is "[a]mongst a grove, the very straightest plant" (1.1.4). Henry, of course, puns on Hotspur's familial roots in the royal House of Plantagenet. He also implies that Hotspur's a distinguished young man who stands out as the "straightest," among the rest of the conniving Percy family.
King Henry's not the only one who likes his plant metaphors. When Hotspur learns that his father won't be joining the rebel forces at the battle of Shrewsbury, he says Northumberland's absence is like a "perilous gash, a very limb lopp'd off." Here, Hotspur figures his father as one "limb" or branch on the proverbial family tree. His absence, in other words, is damaging to the rebels' cause, like a wound. We later learn that Northumberland's absence will literally be a "perilous gash" – the rebels are defeated at Shrewsbury and Hotspur is fatally wounded by Hal. There's a lot to say but the point we want to make here is this: the play's many allusions to "family trees" remind us that family relationships are very much at the heart of Henry IV Part 1. Check out our discussion of "Family" if you want to know more.