It is easy to miss maple flowers, especially on large trees. From a distance, the branches appear to blush and that is all. Look closely and you’ll see a profusion of anthers leaping out of short, red petals.
I visited the marsh area of the Arboretum to see what was in flower. I saw some Siberian squill (Scilla siberica) underneath the redwoods, nodding their blue flower-heads.

Closer to the wet ground were Japanese butterbur (Petasites japonicus) which looked like little cabbages carrying bouquets of star-shaped, white flowers. The plants are small now but the leaves will eventually grow to be rhubarb-size.

I found a European relative of the Japanese butterbur, common butterbur (Petasites hybridus), growing in the swamp proper. The flower-heads look like they belong on a coral reef.

And, finally, some bright-looking flowers that reflected the strong afternoon sun: lesser celandine (Ranunculus ficaria). This is a plant that embraces spring with all of its heart-shaped leaves and nine-petaled flowers.

Interesting to note that none of the marsh plants above are native to New England.
Thanks to Sheila for the ID help.