Well I have to go with two of my favourite subjects, spring sickness and stratification.
Stratification, damp chilling of seeds. Two pics below showing what it does. The first is a batch of daylily seeds refrigerated for six weeks in a dry condition, so not stratified, then planted at room temp. Pic taken after four weeks at room temp:
Now for the properly stratified seeds, from the same batch as above but instead refrigerated in damp vermiculite for six weeks before planting. Same number of seeds as above and pictured also four weeks after planting at room temp - only one had not germinated by then (I think it was missing the embryo so a no-go to start with):
These particular seeds typically have roughly 25% non-dormant seeds and 75% dormant ones and therefore if not stratified before planting germination will be erratic over several weeks or even months.
