What does "inking their cress" mean in a winter twilight?

The correct answer should be And since to look at things in bloom Fifty springs are little room, About the woodlands I will go To see the cherry hung with snow.

He says that 50 years are a short time period even though it might seem like a long time because one day you're a kid and the next time you look back it's already been 50 years past. That's why this means that life is short and should be enjoyed.