Sorrow is my own yard (1)
Immediately, with the first line, our speaker has connected her emotional state to the natural world. Actually, this connection has already been noted in the title, when we're told that the lament is taking place in springtime. From this line on, we know any descriptions of nature to come will be just as much about our speaker's mind and her emotions as they are about what her yard actually looks like.
where the new grassflames as it has flamedoften before but notwith the cold fire (2-5)
Not only does the natural world provide us with an image of her sorrow, but her extreme sadness also totally transforms the natural world (at least as she perceives it). Because of her grief, she sees the grass/fire as burning cold. Hmm. That doesn't sound right.
The plum tree is white todaywith masses of flowers. (9-10)
When our speaker says "today," she subtly reinforces the sad fact that the present is different than the past. Duh, we already knew that, right? But in this case it's particularly worth emphasizing, because all the differences that our speaker experiences this spring seem to go back to the loss of her husband. This isn't about the mere passage of time. It's about the loss of a loved one. We get the feeling that when her husband was alive, she wouldn't have looked at the flowers and thought "masses of flowers." She probably would have described them in a way that seems prettier and lighter. The flowers, on the other hand, probably haven't changed much at all.