The meaning of Piercy’s poem is simple. It uses metaphor to show how the female gender role can confine women and stop them from reaching their full potential.
“The bonsai tree / in the attractive pot / could have grown eighty feet tall” A bonsai tree can grow to be quite large. But when it’s in the pot it can only grow so big, especially when it’s branches and leaves are being trimmed away at.
This is also true for many women throughout history who were regulated to domestic life. They were kept in a nice, pretty house but it kept them from growing. “Growing” mainly means it kept them from expanding their knowledge and succeeding in the outside world.
The women may have been well-taken care of, but much like the bonsai tree, they couldn’t grow beyond what the “Gardener” allowed.
Again, the bonsai tree gets to live in a pretty pot and the woman gets a pretty house. Because of this many would tell them they are lucky as seen with this line: “how lucky, little tree, / to have a pot to grow in.” But this isn’t true because they aren’t able to choose their own path.
I think the poem is saying that there should be a matter of choice. But instead, women were trained from the very beginning to be confined the home. In order for them to expand their minds and intellect they need the opportunity to succeed outside of that.
This is why there is a mention of the “Crippled brain,” by limiting the options the brain can only know domestic skills. The poem is showing that women have the potential to go beyond that, and they should have the right to do so.