All 89 bee plants
Poplar pollen
Populus spp.
Dull yellow brown, in the brown range.
FloweringMarch to April
PollenFair
NectarNone
SeasonSpring
WhereWaterside, Woodland
Bees also gather sticky poplar bud resin to make propolis.

Easy to mistake for these
Closest pollen colours in our set, measured the same way the pollen identifier scores a match. Colour alone will not settle it, so check the flowering months and what is actually growing near your hives.
Out at about the same time
Saw a load like this going in?
Sample the colour against all 89 plants, then keep a forage record per hive.