Umi no Nakamichi Kaihin Koen is a flower park that boasts seasonal flowers in bloom as well as beach attractions. Unfortunately, comos were not quite in full bloom when I visited, but kochia bushes were showing several beautiful bright colors of the season. Kochi, otherwise known as broom weed, is abundant in a particular part of the Ibaragi Prefecture and famous for its round and bushy appearance. I also recommend the Flower Museum and the Animal Forest along with many other exhibits.
You can see the details here (Only in Japanese, though).
Here is the park guide in English.
