Spent flowers are ones that have opened into full bloom, after a few days, / week, (depending on the type of plant ) after the BLOOM has started to wilt, turn brown or in fact the petals fall off, you will find the little hard seed pod.
You either keep the plant tidy by removing the blooms when they are showing signs of looking tired etc.
Dont be confused with blooms that are wilting from lack of water, make sure you are removing flowers that have bloomed and done their job.
it's always good to keep the flowering plants tidy and remove the spent flowers, by doing this, you are tricking the plant into making more flower heads, IF you allow the plant to make seeds, it will stop flowering as that is it's job done, Flower, make seeds, scatter seeds and the whole process starts again the following year.
All annuals especially the ones we use for summer bedding plants need constant dead-heading or after the first flush of flowers, the plants want to form seed-heads and that's your hard work done because by allowing the plants to form seeds, they are done, they wont make many more flowering buds as they have used all their energy into making seeds instead of more flowers.
Hope all this makes sense to you, by looking at the individual flower-heads you will soon see the difference between a spent flower, And one already forming seeds.
Even IF you want to gather seeds for germinating the nezt year, I would only let maybe one plant make seeds, what could you possibly do with hundreds of seeds when you only need maybe 50 seeds.
Hope this helps you out.
Kindest Regards.
WeeNel.
