That's the question I've been Googling lately. Having never grown hops before, I'm a bit nervous about harvesting at the right time. Fortunately, there's a lot of information on the internet, and some of it appears to be helpful. If nothing else, here are some shots of our Cascade hops this week.
The aroma of the lupulin was very strong when I held the cone close to my nose. Each member of my family described the smell differently, describing it as pine, lemon, box elder bug, mint and generic green plant. Internet sources call for a spicy, citrus and pine smell, so I think we're pretty close right now.
This image was taken with the camera shooting into the eyepiece of our stereo microscope set at 30x magnification. The lupulin is definitely there, and appears even more crystalline at this magnification. Most of the cones were not as ripe as the cone pictured here, but harvest appears to be just around the corner.