The asparagus plant is dioecious meaning that it produces both male and female flowers on separate plants. Each flower has two sets of sexual organs with one set aborting as the flower matures. This leaves either all male or all female flowers on a plant. Asparagus relies on insects for pollination.
If you are going to save asparagus seeds you will need two miles separating different varieties as they easily cross pollinate. Day caging is a method for growing outbreeding varieties that normally cross pollinate when grown close together. This works for different varieties grown in the same garden or on the same allotment site. When the asparagus starts flowering place a cage over each variety. In the mornings remove one cage to allow insect pollination, replacing the cage at night. Repeat this process in rotation between all the cages and the varieties until the flowering ends.
When growing asparagus for seeds choose the best looking female plants with at least one male plant close by. The male flowers are bell shaped and greenish with the female flowers much smaller. The female flowers produce reddish berries which contain six seeds. You will need to protect the berries from the birds who find them tasty.
The ripe berries need to be collected from the plants before they fall to the ground. The berries then need to be rubbed over a screen to release the six seeds, which then need to be washed in several changes of water. Leave the seeds to dry completely for several days and store the dry seeds in a marked envelope for up to five years.