This is most often carried out by bees but sometimes by other insects, and small animals, or even wind or water.
Planting many flowering plants near vegetable crops is important to encourage bees and insect pollinators.
This also has the effect of bringing beneficial insects to the garden as an aid to pest control. Plants with small flowers are best for this like daisies, yarrows, and members of the Apiaceae family with their umbel (umbrella like) flowers.
Flowers also make the vegetable garden look colourful too.Many plants are self pollinating and others being pollinated by wind or movement will be easily pollinated in your garden but sometime others need a little help.
Members of the cucurbitaceae family often miss out on natural pollination. We can give them some help by hand pollinating their flowers.
First you need to recognise the female and male flowers of these plants.
Female pumpkin and squash (and most cucurbits) have baby pumpkins (or whatever) beneath them the males don't.
Seen here in the picture the male flowers on the left side, female on right - pumpkin top squash below. Sorry the pumpkin flower is past flowering.
If this pumpkin was not fertilized (pollinated) by bees then the pumpkin will go yellow, wither and fall off. This may happen because of weather factors but usually it is due to a lack of pollinators.
They may need to be hand pollinated - pollen (yellow powder) from the male flower is placed - either with soft brush or by removing the male flower from the stem, then the petals and introducing the pollen to the central female flower parts (the pistils) with the male stamen.
The photos below will illustrate this better than words:
Find a male flower
Remove the petals
Find a female flower (on a vine of the same kind of pumpkin)
Look inside the flower
Introduce the pollen bearing male flower into the female flower.
This procedure is best carried out early in the morning and before the flowers are fully opened and should result in setting of baby pumpkins.
Occasionally you will still get failed fruit set and this can be caused by irregular watering or adverse weather conditions beyond your control.
This method can be used for other cucurbits cucumbers, melons, zucchini, squashes and gourds. Some other plants can also be hand pollinated when insects seem to be missing them.
This procedure is also important to follow if you are planning on saving seeds from your crops and you are growing two or more members of the same family.
In this case you must find the two flowers and pollinate them just before they open to avoid them being pollinated by insects with out your knowledge. After hand pollination in this case you must cover the female flower with a fine mesh bag to keep out insects.