the start date?
the end date?
The Hitachi Seaside Park website has a page with this information. The season is relatively short, from mid-April to early-May.
I Googled again and found https://jw-webmagazine.com/dive-into-the-ocean-of-4-5-million-blue-flowers-e7de15969e77/ -
The best time to see Nemophila Harmony is usually late April to early May.
Hitachinaka City began planting flowers like the Baby Blue Eyes in Hitachi Seaside Park in 2002. In the short years since the event has become one of Ibaraki’s most well-known sights. The Baby Blue Eyes are sown every year in November, covered during the winter under a frost-free sheet, and carefully watered and weeded.
They usually bloom from about mid-April to early May, but sometimes weather conditions affect the season. In 2018, for instance, the peak of the bloom came ten days earlier than usual. Generally, the best time to visit the Hitachi Seaside Park is in April when the daffodils and tulips line up with the Baby Blue Eyes flowers.
During the 2 or 3-week peak of the Baby Blue Eyes flowers, the city provides free shuttle buses from Ajigaura Station. A temporary additional city bus runs from Katsuta station to support the number of visitors. It’s fairly easy to access Hitachi Seaside Park in Ibaraki Prefecture from Tokyo, but it does take some time out of your day. A one-way trip by public transportation easily takes more than two hours if you go from Tokyo.
Opening hours of Hitachi Seaside Park during the Baby Blue Eyes bloom are from 9:30 am to 5:00 pm. Be mindful that the park is usually closed on Mondays, but opening days change during peak seasons. Please check their website carefully when planning your trip. Admission to Hitachi Seaside Park is 450 JPY for adults, 210 JPY for seniors, and free for children under 12 years old.