Spring is one of the ideal times to hydroseed as it has a full season to grow.
Summer hydroseed requires more frequent watering during hot weather.
Fall is another ideal time to hydroseed. Results in little initial growth depend on weather but will fill in and thicken up in the Spring.
Winter (dormant seeding) is acceptable until freezing temperatures. An enhanced erosion control mulch is used to protect the seed until Spring and hold things in place for a longer period until vegetation is established.