North Central And Northeast

Trees: March to June
Grasses: May to August
Ragweed: August to October (except northern tips of Maine and Michigan)

MidAtlantic

Trees: March to June
Grasses: May to August
Ragweed: August to October


Southeast

Trees: January to May
Grasses: April to October
Ragweed: July to December (except Florida)


Southwest

Trees: February to May and September to January
Grasses: April to November
Ragweed: August to December (except the West)


North and Central

Trees: March to June
Grasses: May to October
Ragweed: August to October


Midwest

Trees: March to July
Grasses: May to November
Ragweed: July to November (except high mountains)


Pacific Mountain Region

Trees: April to June
Grasses: May to November
Ragweed: July to October


Southern California
Trees: February to June
Grasses: May to November
Ragweed: June to November


Pacific Northwest

Trees: March to May
Grasses: May to November
Ragweed: July to October