Tenant wants a garden

[Matt Faix]

We have a tenant that moved into a duplex about a month ago and asked if she could use a small portion of the yard to make a garden. She moved out of a house that she and her ex-husband owned, so she is accustomed to having a house and maintaining it. To-date she is a great tenant, and has done a terrific job with her portion of lawncare/shrub maintenance. I'm fairly confident that she would maintain a small garden. 

A couple questions on the topic though:

1. The property is a duplex and currently all outside space is common area, and shared maintenance. Does the garden then become exclusively hers to enjoy and maintain?
2. What happens when the tenant leaves? Would you require that they remove the garden and plant grass back where it was? (I can hear the laughter now, not a chance this would actually happen), or rather charge them a removal fee from the security deposit.  
3. Do you offer some alternative? - ie planters on a porch that can be easily removed.

Has anyone ever had this come up? If so, I'm curious to hear your thoughts. Thanks!