All 89 bee plants
Horse chestnut pollen
Aesculus hippocastanum
Brick to rusty red, in the red range.
FloweringMay to June
PollenGood
NectarYes
SeasonSpring, Summer
WhereGarden, Woodland
The classic red pollen seen on bees in May. The flower even changes colour once its pollen is spent.

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.