I think what Nikki Grimes means by “the oak tarries long in the depth of the seed” is that these oak tarries are within the seed and they are eager to grow. They want to grow into a beautiful oak tree and it could possibly be a metaphor for her wanting to grow. “But swift is the season of the nettle and weed” I think this means that the oak seed cannot grow because it isn’t the right time and there are all these ugly weeds and nettle growing so the oak doesn’t have room or the right environment to grow. This could also be a metaphor for Nikki Grimes and how he/she wants to grow into someone better, but there are all these things going on around her/him and it’s stopping her/him from growing.