The speaker is watching the very start of spring, and the poem turns on a clever bit of wordplay. The first plants to appear are all "rockets": dame's-rocket is one by name, and the others are lumped in with it as early greens shooting up. The title, "Waiting to launch," extends the image. These plants are like rockets on a pad, ready to go, while the bigger growth, the wild onions at the wood's edge, hasn't arrived yet.
Then the poem shifts from plants to the speaker. Standing in the field, they notice a dark spot and seem to hope it might turn into a bear. It never does. That small letdown mirrors the season: something is about to happen, but not quite. The mood is patient and quietly hopeful, with a little humor in the speaker's willingness to stand around wishing for a bear. It captures the restless in-between feeling of early spring, when you're alert to every sign of change.