Gardening tools, of course. You can never have too many.
Gardening book?
Stepping stones.
Garden statues, gnomes, snails etc.
A lunch date w/you.
A sign that says "Lordy, Lordy, Her-name-here, is turing 40!"
Pedicure gift certif
Bottle of wine
Silver charm (4 leaf clover for luck)
Magazine subscription
Potted flowers she can re-plant
A framed picture of the two of you
A book you know she would love
Take her to the movies, lunch, pedi, shopping
Take her wine tasting
Comfy slippers, cozy bathrobe
Dinner certif for $40
Your time! :)
Edit: 40 reasons why you love her