Disclaimer: Even though I have my fair share of experience, I am not a tax professional by any stretch of the imagination.
To a degree, gardening expenses have nothing to do with the square footage of space that is exclusive to the renter.
The percentage of your cost basis that is depreciated is based on the square footage of floor space that is "exclusive to the renter". Therefore, common areas shared with the owner are not included. For example, if the house has only one kitchen and it is used by the tenants and the owner, then the floor space of that kitchen is not exclusive to the renter and is therefore not included in the percentage of floor space that "is" exclusive to the renter.
Generally, if 10% of your floor space is exclusive to the renter, then you would include 10% of the property value in your total cost basis also. However, since land is not a depreciable asset, the land value really has no effect on the amount of depreciation that is allowed. The land value comes into play when you sell the property,since an equal percentage of the land would be treated as business property.
So, lets say you pay a third party to cut your grass weekly and you pay them $50 per cut. If 10% of your floor space is exclusive to the renter, then you may be able to claim 10% of your lawn cutting fee as a rental expense; which would come out to $5 per cut with my numbers. But I still question that.
Now let's play devil's advocate, as from an IRS perspective there is one thing I would question with this. Is 10% of your land exclusive to the renter? Can you prove it? Probably not. Based on that from an IRS agent perspective, they would probably disallow the deduction. At least, that's what I would expect.
There are three "golden rules" to keep in mind when dealing with an IRS audit.
1) You are guilty until proven innocent.
2) The burden of proof is on the accused (that would be you!) and not the accuser.
3) If it's not in writing, then it did not occur.
