Primulas growing in your garden may be thousands of kilometres away from their natural habitat. Though some pollination must occur by wind much pollination occurs by insects. The insects which are able to pollinate the species you are growing may be absent from your garden. In order to ensure a steady supply of seed, I recommend that you hand-pollinate your Primulas. Growing primulas from seed helps maintain the genetic variability and vigour of the species and may lead to exciting forms to select and propagate vegetatively.
 

Best seed set is the result of crossing pin and thrum flowers. Some plants are self-sterile, so always cross genetically different plants. Plants which were created through division are clones and are genetically identical. See John Richards book Primula for an excellent detailed explanation of Primula heterostyly (pin and thrum). If you need to familiarize yourself with the parts of a flower, see the Primula Features page.

When hand-pollinating, the goal is to get pollen from a flower of one plant to the stigma of a flower on another plant. The easiest method, described below, is to use a thrum flower as a source of pollen to pollinate a pin flower. Pollen is ripe when it is a strong yellow color and distinct grains are visible. The stigma is receptive when it looks sticky and moist. Usually a warm, dry day provides the best conditions for pollination. Repeat pollination with pollen from a second plant will ensure viable pollination.

If the stigma (pin) does not protrude beyond the face of the flower as in the example, gently split apart the pin flower to reveal the stigma.  If the stigma (pin) extends beyond the face of the flower and your pollen bearing (thrum) flower is close enough, try bending the stem of one plant so that the flowers touch and the stigma extends into the anthers of the other flower to get coated in pollen. If you wish to pollinate a thrum flower, you must pull apart the flower to reveal the stigma. Note that a flower will still produce seed even though the petals are completely removed.