under TX law what is a home owner's right when the neighbor sprays herbicide killing my plants

A property owner may not use their property in such a manner that it effects a neighboring property owner's use of his property. What you describe constitutes a "nuisance." Trespassing is not just physically walking on property it can include sending noxious odors, smoke, chemicals, etc. Have you told your neighbor about the problem and ginve hi man opportunity to cease and desist? If he refuses you can sue him in court for the value of the damage to the property and for an injunction preventing him from continuing to do so.