the art of plant sex 

The presence of self-incompatibility is seen in sunflowers which is why they rely heavily on pollen movement between plants by insects and bee colonies.  These organisms benefit the plant because they carry pollen from plant to plant, allowing cross-fertilization to take place.

The bloom period is during the summer where the seed abundance is high. 

Flowers contain both male and female producing parts.  In order for reproduction to take place, the pollen (which will produce the sperm) has to come into contact with the stigma of the pistil (which contains the ovule with the egg).  Pollen is pushed out the top of the style as it grow, it is then ready to be picked up by any insects that happen to visit the flower in search of pollen or nectar.  Once the style has pushed the pollen out by its piston-like action, it splits open, exposing the stigmatic surface, which thus far has remained virgin but is now ready to receive pollen. Bees are most common visitors. 

The pollen then grows down the style, and in a short time sperms are discharged, one of which will fuse with the egg and thereby initiate the development of a new seed.  However if by chance the flower does not receive any pollen from an outside source, the stigma curves around and eventually contacts its own pollen so that self-pollination may then take place, thus ensuring seed set.  From here the seeds can be harvested and then planted, beginning the growth process once again.

Extensive crossing and hybridization has lead to an increase in the range of flower colors including shades of yellow, brown and white.