I hope you get this resolved to your satisfaction and that this helps clarify some of the issues whether you retain counsel or try to handle it yourself.
Were your plants within the utility's right of way? I suppose they weren't if they were on/inside your fence. If they weren't, the co-op should be liable for your damages. Before hiring counsel, I think I'd write them a demand letter telling them what you want and include your story and pictures. One problem with taking a situation like this to court is how to value the destroyed plants and your expectancy for organic certification (which really would have to be more than a speculation). The threat of litigation or bad press might entice them to be more receptive to your demand, though. Unless you could show that they sprayed your area with the intent to harm you or show a violation of a statute that authorizes punitive damages, punitive damages would be hard to prove/recover (although an inflamed jury could put the co-op into a settling mood). So, you may be restricted to compensatory damages. I wonder if the wind was > 5 mph when they sprayed. Confronting them now could alter their behavior near your property in the future, so that alone might make it worth doing. I don't suppose your property is insured under a policy that would cover such damage.
Some random thoughts. (1) If the co-op owns the right of way and your plants were not in the right of way (even if they were, that probably wouldn't address the fact that the land on which you would be trying to get certification is not in the right of way), then the co-op would seem to owe you the same or a similar duty that any adjoining landowner would owe to you. As such, the co-op "should" have some liability for negligent spraying. I doubt if utilities are immune from that duty, but maybe someone on the forum knows for sure. That would be the first question that should arise if you consult counsel. (2) If the co-op merely holds an easement, is that easement across your property (did you or your predecessors in interest grant the easement)? If it is, then there might be a breach of easement element to the situation. Those power transmission utility easements usually give the easement holder the right to clear vegetation on the easement, but you'd have to look at the exact easement language to see how much leeway they have. The right to clear vegetation shouldn't give them the right to damage your non-easement property. (3) If it does hold an easement and the easement isn't across your property, it seems the analysis in (1) still would apply. (4) If the co-op hired a contractor to do the spraying, the co-op still bears whatever responsibility there is, and the contractor might be liable as well. (5) If the spraying destroyed your ability to seek organic certification for all or part of your land, then that would seem to be in the nature of a taking/condemnation of that property right for which the co-op should have to pay. Obviously, proving the expectancy of certification and reducing it to a dollar value would be key issues, but they might be less difficult in a settlement context than they would be in court. (6) One problem might be convincing the ignorant co-op management that their actions damaged you and that you're serious about compensation. Maybe they're sensitive and informed and the problem was the negligence of the guy running the sprayer. If the co-op is insured so that you end up dealing with the insurer, one might expect a second layer of ignorance and insensitivity. A lawyer might be the antidote. (7) If you reach a settlement with the co-op, it should include language governing how they will behave in the future with respect to your property. Without speculating too wildly about that issue, it might involve giving you effective advance warning of an intent to spray, not spraying the segment adjacent to your property at all and using mechanical maintenance instead, allowing you to maintain the right of way so that they do nothing with it, etc.
Ideally, one would analyze your dead plants to ascertain what was sprayed on them, but maybe it would be more practical to collect specimens of your plants and the plants in the right of way so they could be tested later if necessary to show that the same product killed both. (Or you could try asking the co-op what was used.) Maybe you could seal them well enough in plastic to stomach putting them in your refrigerator or freezer. Whether that would preserve the agent analysis, is another question, but the less appealing option is to have them analyzed now. One thing I wonder about is whether they used one of the very potent and persistent SU herbicides and, if so, what that residue would do to that area over the long term. As for the certification application, I imagine those with less scruples might neglect to disclose the incident (which I am NOT recommending), although the State ought to figure that land close to a utility right of way probably gets/will be sprayed. Good luck.
In theory, theory and practice are the same; in practice, they aren't -- lament of the synthetic lifestyle.