Take subway Line 2 from Nanjing East Station to People's Square and change to Line 1 to Shanghai South Railway Station. Find bus 973 to the garden finally.

Thanks Cindy. Do you have any idea where else to view flowers in early April

Here are some other places to see flowers in late March and early April:
Taoyuan Village in Huinan Town, Nanhui, Shanghai - peach blossom
Binjiang Firest Park - Azaleas
Anyway, Shanghai Botanical garden is a better choice. For the Expo, there will be a grand floer exhibition here. Suggest you take a taxi to the garden when you reach Shanghai South Railway Station.