September is hop harvest season. Fresh hops are picked off the bine and dried. The drying makes the perishable wet hop much more stable and useable all year-round. These dried hops are called dried whole hops, and this is the traditional way of handling hops, you just put them whole into the brew kettle. Recently more and more hops are being processed into pellets which makes them even more stable, in fact they can keep their quality properties for a much longer time. At the same time they also take up less space as they are more space efficient, meaning that it takes less to achieve the hopping qualities you need.
Recent years have introduced a new phenomenon: wet hopping (sometimes also referred to as fresh hopping or green hopping). Freshly-harvested wet hops are used directly in the brewing process skipping the drying process. This imparts much stronger, and different, flavour and aroma qualities to the beer than what dried hops would do.
Because wet hops is a perishable product it must be used very shortly after it is picked off the bine. This typically means that the brewers that do make wet hopped beers get shipments overnight from hop harvesters so that they can make their beer the day after. Because of this wet hopped beers is a seasonal product. In many ways this is similar to the beaujolais nouveau only this time for beer.
As far as I can tell this trend started in the hop-growing regions of California and the Pacific Northwest. There are even wet hop festivals! It is indeed a very nice way to celebrate the hop harvest.
I tried to get hold of some hop rhizomes this spring, but soon found that I had started searching for them too late, so I have to wait until next year to make my own wet hopped beer. But if you happen to have a hop plant, consider making a wet hopped beer. Now is the right time, unless you want to wait another year.