i've been there in early may, and that was too early. i think mid-may would be best.
I have been to Hitachi Seaside Park in early May (probably was somewhere between 3-10) and was just the right time. I guess April 11th is too early. You can probably enjoy tulip and canola flowers.
I'm not sure about Ashikaga Flower Park. They say on their website that they have different kinds of wisteria flowers, some of which bloom in mid April. There may be chances to see the beautiful flowers there.