Not sure if hops are poisonous to all dogs, but there have been a handful of recorded cases where eating hops has caused the dogs temperature control system to go crazy and their body temp goes up to the point it kills them, IIRC. I spent a while looking in to it and all of the cases I found involved a dog eating the hop residue from a boiler or fermenter, not fresh flowers. But it was enough to put me off growing them anyway, and I dispose off hop residue in places not accessible to my dogs.
Subsequently added:
Heres the thing I found.
This is from a recorded case of hop poisoning
This is a warning that the ingestion of used hops from home brewing can be fatal to dogs. Some breeds have a toxic reaction that triggers the onset of Malignant Hyperthermia (MH), a life-threatening condition in which the body uncontrollably overheats. Do not mulch your used hops - seal them in a container and dispose of in the trash!
There have only been a handful of confirmed cases reported - around 9 dogs. Seven of the dogs were Greyhounds, one Labrador Retriever, an unknown breed, and most recently a Golden Retriever. 7 out of the 9 dogs died.
The first symptom to become obvious to an owner is heavy panting. Tachycardia (rapid heart rate) will also be present, up to 200 beats per minute. The temperature may rise as quickly as 2 degrees F every five minutes. Immediately take your dog to a emergency pet hospital.