Don't ask for seasonal flower colors to complement the skirting.Once you start these tasks you will increase the pride of ownership among your tenants and will see that they will begin to invest in the appearance of their homes through painting, removal of foil from windows, etc.  

It may be wise to increase curb appeal, trimmed lawn, simple nice flowers, clean coat of pain, paved driveway/sidewalk, etc as well as a nice interior.

If a tenant asks about planting flowers, I tell them they are responsible for weeding.

I did a refinance with them lasts year and it went smooth as silk.  

You need to look at rental rates in your area, and if you have to weasel clause in your lease like the tenants must mow the lawn, water the plants/flowers, trim the plants, that could scare away tenants.

Under those 4 windows you need some beautiful flowers right across under every window purple, red or something that stops just below the windows, that alone is your cheapest curb appeal. 

Sealing limestone from the inside can create more issues down the road because the limestone leaks naturally and water will become trapped between the limestone and whatever you put on it and the sealant will start to fall off or leak.It's better to tackle the water issue from the outside: Sealing the walls from the outside (lots of digging), making sure water drains away from the house when it rains, proper gutter systems that lead the rain water away from the flower beds.Sometimes you also have to install a sump system.

Someone buried an old washing machine drum in the flower bed in front of the apartment -- redneck planter perhaps?  

Tell her or him to stop trying to be a unique flower & manage that individual's expectations.

I am not making an excuse for them but being cheerful and happy everyday in a job were you do nothing but screen people's problems and excuses isn't going to be all flowers, rainbows and ponies, if you know what I mean.