It is a biennial crop, but grow as an annual crop because it produce sprouts in the first year, but only flowers and seedpods in second year. So you have to sow seeds every year to get new sprouts.
Also, it is better to sow Brussels sprouts in summer to harvest in fall since the sprouts mature in hot weather or dry condition will develop a bitter taste.
http://www.gardening.cornell.edu/home...
https://extension.illinois.edu/veggie...

For more information please see our Brussels sprouts entry
https://www.plantvillage.org/en/topic...