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OP, as soon as something is attached to the property, it becomes the landlord's property and it stays.
If you are a great gardener, draw up plans and submit them to the and lord and ask him to give you an allowance to buy plants. The landlord pays, you take care, and the plants stay, but you don't feel bad about it.
If you buy something with your own money and want to keep it, plant it in a nice looking planter. If it is in a planter, you can take it with you when you go. You can really dress a place up with a few handsome planters full of bright flowers.
If you landscape nicely and then dig up a lot of mature landscaping when you leave a couple of years later, I wouldn't be too surprised if you get sued for the damage. It's not like no one is going to notice that it has been done.