Here some nice gardens around Los Angeles area.

Robert Irwin’s Central Garden
1200 Getty Center Dr

Descanso Gardens
The great garden with oak forests and lots of native plants is great place to relax and learn about LA, after exploring you can stop by in their cafe for refreshing drink or small food.
1418 Descanso Dr, La Cañada Flintridge, CA 91011

The Japanese Garden
The Japanese Garden 6.5 acres (26,000 m²) is great weekend gateway, this place was carefully design to increase enjoyment. Garden has great gift store as well with jewelry from Japan & the Asia-Pacific region.
6100 Woodley Ave

The Huntington Library, Art Collections, and Botanical Gardens
Expansive complex housing art galleries with famous works, rare-book collections & lush gardens.
1151 Oxford Rd

UCR Botanic Gardens
University’s garden offers 40 leafy acres of 3,500+ plant species & 200+ bird types. Open daily.
900 University Ave

Rancho Santa Ana Botanic Garden
Non-profit property boasts diverse native gardens & plants, regular classes, a nursery & gift shop.
1500 N College Ave

Sherman Library & Gardens
Serene botanical garden with patios, conservatories & walkways, plus a research library & cafe.
2647 East Coast Hwy