We visited Ashikaga Flower Park in winter, but the illumination was incredible. Top 3 Illumination in Japan apparently.
Another famous fuji
The top attraction at this sprawling 94,000 m2 garden is the wisteria, or fuji in Japanese. They bloom in late April to early May, right around the national Golden Week holidays, during which the park is absolutely packed. Try to go on a non-holiday or arrive very early to beat most of the crowds.
The parks has several varieties of wisteria, including white, pink and yellow varieties and a species of purple wisteria with extra petals called yae-fuji. Each blooms at a slightly different time, so you'll have a chance to see at least one type in full bloom whenever you visit during the season. There are long tunnels of wisteria to enjoy and one massive, elderly tree has grown to cover hundreds of trellises. All throughout the park, you can enjoy the delicate scent of wisteria, as well as wisteria ice cream and soba noodles! Make sure you visit the grassy field in the back corner of the park for a sunny snooze in the lounge chairs provided.
The park is open year round and also does a rather nice winter illumination, so consider it as a stop if you are in the area any time of year.
The Flower Park is home to Japan's largest collection of wisteria (fuji) trees. Their collection includes a 100+ year old wisteria that extends and incredible 1000 square meter canopy across multiple trellises. Although the admission is pricey during the wisteria blooming seas (late April to early May), the smell alone from the millions of flowers blooming is well worth it.