Every year is a little bit different when it comes time to figure out when to harvest, but in general mid-August through September is the window to harvest. Depending on moisture and temperature through spring and summer, hops will mature at different times year over year and each variety will mature at a different rate.
Our spring this year was very cold and wet so our harvest is a few weeks further into August than it was in 2018. We can visually inspect the hops as they ripen, some cones are larger than others by variety and some are smaller. When you break open the cones, you should be able to see the lupulin inside as small yellow balls between the leaves, and it smells great!
Once the hops look and smell ready, we send in a sample to Alliance Analytical Labs to analyze the acid levels and confirm they are within targeted range. Some years you can hit the range well and others it is a bit of a guessing game because one acid might be close to the range and another is right in the middle. Based on the data you can guess when an optimal time to harvest is within a few days or a week. If you have multiple varieties you can line up an order of harvest based on data or if they are all in range you get in gear and harvest them all as soon as possible.
Another key factor to determine harvest time is % dry matter, or the amount of moisture in your hops. We usually shoot for 21-24% dry matter which is another data point received from a lab or you can calculate your own with a scale and a microwave.
If you want to bring out your inner biologist and grab that microscope, the lupulin changes shape as the hop matures. The more spherical the lupulin is under a microscope, the closer it is to harvest time.