First, you don't have a lien on your property; you are encroaching on your neighbor's property. However, you may be able to continue doing so.
Your BEST solution is to sit down and talk with the neighbors and work something out. It's only 15 sq. ft. Maybe you could pay them $100 for permission to use that patch as long as you own the house. You could also work out a yearly rent. Whatever you work out, record the agreement under your parcel number with the county.
If you can't come to an agreement, consider adverse possession or suing the seller for failing to disclose the fact that his property line was NOT where it seemed and he had a permissive eeasement he never recorded.
Adverse Possession
If the previous owner's use of the area was non-permissive, and went on for the statutory number of years, the land now belongs to you. You sue the neighbors to quiet title to the area. Call your seller and double check that he got permission from the neighbors to plant the area. If he agrees that he did, you are SOL. BUT, this may put him on the hook for failing to disclose. Get his answer in writing either way.
Demand that the neighbors pay for a survey to locate the proeprty line exactly. You may find that they can't claim as much of the land as they thought (or they may be able to claim more). Surveys cost about $600, so the neighbors may back off.
If the seller says he didn't get permission, then the adverse possession time requirement started running when he planted the area. It has continued running through today. Check with a local attorney to see how long the requirement is.
If you sue to quiet title, you will have to pay for the survey.
Failure to Disclose
You may have an action for failure to disclose a known permissive easement benefitting the land. Call your buyer's agent and ask what the state laws require as far as disclosure of easements benefitting or burdening land. Then hire an attorney.
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No matter who owns the land, the nieghbors can't force YOU to remove the bushes. They have to remove them themselves. They get to keep the bushes. If they tear out any bushes on your land, they may be liable for large damages.
In Washington, destroying landscaping on someone else's land makes you liable for triple damages + attorney fees + costs. Mature landscaping is worth A LOT of money, too. Point this out to them and put them on notice that you dispute the property line location. They have to buy a survey before they dig up one plant or risk big damages for "wanton" trespass and destruction of landscaping.
Good luck.
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This is not legal advice and you are not my client. Double check everything with your own attorney and your state's laws.
