Things planted in the ground or otherwise permanently attached to the real property are usually properties of the landlord, even if planted by the tenant.
RCW 9A.56.020 (Theft — Definition, defense) provides:
(1) "Theft" means:
(a) To wrongfully obtain or exert unauthorized control over the property or services of another or the value thereof, with intent to deprive him or her of such property or services;
So, theft likely is not a probable charge against a landlord who removes plants in the ground from the landlord's land.
Whether there are violations of law or contract between landlord and tenant depend on the specific facts.
Generally, only the prosecutor has the power to decide whether to prosecute criminal cases.
You should review your specific facts with your attorney to find out your legal options.