She hasn't told her garden yet

I haven't told my garden yet --
Lest that should conquer me.
I haven't quite the strength now
To break it to the Bee --

I will not name it in the street
For shops would stare at me --
That one so shy -- so ignorant
Should have the face to die.

The hillsides must not know it --
Where I have rambled so --
Nor tell the loving forests
The day that I shall go --

Nor lisp it at the table --
Nor heedless by the way
Hint that within the Riddle
One will walk today --

When Emily writes here she is hinting toward her upcoming death,

 one of her favorite topics. It would only be natural for her to inform her garden,

 seeing as she was said to be a "gardener first and a poet second." 

She apparently spent a lot of time either working in or enjoying her garden,

 and amongst her fascination over death, she does not want her garden

 to know what will one day happen to her, as it has happened to those around her.

 Here she underlines both her fascination over death and its mystery 

and her love for all things nature, which would have brought welcome variety to her otherwise simple life.