petals
Explanation
This comic features a grandmother and grandchild walking through a wooded area. The grandmother says, "Every time I see these petals, it reminds me of Grampa." The child says, "Please stop." The grandmother continues with a suggestive remark -- "Long as his thigh" -- implying she is comparing the petals to an intimate physical attribute of her late husband.
The caption at the bottom reveals the real punchline: "Pro tip: never walk in Grammy's field of Orchis italica." Orchis italica is a real plant commonly known as the "naked man orchid" or "Italian orchid," because its flowers are shaped like tiny human figures, complete with what appear to be male genitalia. So the grandmother's reminiscence about her husband is triggered by flowers that literally look like naked men.
The humor comes from the uncomfortable position of the grandchild, who clearly does not want to hear their grandmother's sexual reminiscences, combined with the real-world botanical joke. SMBC frequently mines humor from the collision of wholesome family settings with inappropriately candid discussions of sexuality.