sluggish dazed spring approaches (line 14-15)
This whole "approaching" thing is really important. The idea of spring is introduced here, but the actual arrival of spring doesn’t seem to be happening in real time. The speaker sees spring in the distance, but, as for when exactly it will be here, well, that part gets left aside. Imagine how different it would be if the line read: "sluggish dazed spring is here!" Ouch. That’s probably why he writes the poems, and we just write about them, but you see the difference, right?
Now the grass, tomorrow the stiff curl of wildcarrot leaf (lines 20-21)
Here’s another moment where time gets tricky. If the grass is here now, maybe spring is happening already. But, it also remains in the future, in the tomorrow of the speaker’s imagination.
It quickens: clarity, outline of leaf (line 23)
Again, quickens can mean "comes to life," but we can also feel how things are speeding up. The pace of the poem quickens; winter, when things are frozen or slow moving, gives way to spring, when things move and grow quickly.