Answer: thorns
When Aurora falls into her 100-year slumber (or coma) after pricking her finger on a spindle, the fairies likewise make the entire kingdom sleep. Thorns and briars grow around the kingdom to protect it from outsiders.
"Sleeping Beauty" (1959) was the Disney Studio's last feature cartoon to be inked and painted by hand, the subsequent films using Xerography, the photocopying technology of the Xerox Corporation. During the scene in which the briar forest erects itself, however, the animators did employ Xerography to save time and money. After "Sleeping Beauty", there were massive layoffs in the ink and paint department, which had been mostly occupied by women, who had generally been forbidden, both formally and informally, from being actual animators and sketchers, the more desirable and to Walt Disney's mind important job. (Kay Nielsen was one exception to this unwritten rule, which came from Walt himself.) Connoisseurs of animation have noted a striking visual contrast between the hand-painted and outlined animation cels of the movies through 1959 and the ones in "101 Dalmations" (1961), which mechanically outline the actual sketches and irrevocably alters the classic Disney animated look and feel.
It should be noted that the animators used photocopying for the growth of the thorns in "Sleeping Beauty" without telling Walt Disney himself, but he later stated that he had noticed that shortcuts were taken. Unhappy with the changes taking place, he grew increasingly alienated from the animation studio and most of his energy was focused on television and on the new Disneyland theme park in Anaheim, California.