The flowering seasons as they are used on this website are not the strict calendar seasons (ie, summer starting on June 21, etc).
A rather loose definition is used. Therefore you may find a species listed under both Spring and Summer if it blooms, say,
from late May through mid June.
On this website, Spring refers to species that bloom from mid-March through to about mid-June.
Summer covers the period from about early June through to the end of August.
Fall covers the period from sometime in late August through to the end of November.
And Winter is the period from December through March.