I have one of their leaflets for this year, which highlights each month & the plants/flowers you could expect to see.
For April it says : Skunk cabbage, foxglove tree and magnolias. - for May it says: Tulips, handkerchief tree, wisteria, giant alliums and lilacs.
I would think a lot depends on the weather - as things may bloom earlier than expected if we are lucky enough to get any sunshine!
I would also suggest you make sure it's open when you plan to visit, as this year it was closed on Thurs and Fridays up until 30th April, - from then on it was open every day. (10am-6pm)
Edited: 2:46 pm, September 25, 2012