The “Milk weed” pod, also known as the Swan plant is shown when David places it on Elizabeth Shaw’s grave, and may have multiple connotations. As well as it being an offering, it has a symbolic gesture as to what may have become of her.
The pods hold many seeds and are egg shaped, we know David used Shaw’s eggs to create the Xenomorph. But the relevance doesn’t stop there, some variations of the genus require the pollen of other plants to reproduce. As you know Shaw couldn’t have children, at least not human ones. It took David’s meddling to make use of her eggs.
The relative of the Green Lantern is the Red Lantern, in Japanese culture this pod is used during the summer festival. Believed to help ferry the soul to the spirit world.
The garden David created for Elizabeth is from Arnold Böcklin’s Isle of the Dead, is missing a part of the painting, the ferryman, is symbolised by the flower and David. The ferryman is the one who brings the souls of the dead to the spirit world.
This plant also contains a milk like sap, which contains cardiac glycosides. Poisonous to cattle and humans, ingested in small amounts may cause vomiting and ingested in large amounts causes death. Could this be the method David used to kill her?
The Milk weed is also food to the caterpillars of the monarch butterflies, which lay their eggs on the plant. Unharmed by the poisonous sap, the caterpillars that chrysalises and turn into butterflies use the cardiac glycosides as a defense mechanism. Becoming poisonous themselves. The Xenomorph embryo deposited by the facehuggers uses the human body as a source of nutrition/food as it eats itself out of the hose once the gestation period ends.
Some other genus of Lantern are used in medicine as sedatives, which you could say the facehuggers have similar properties when attaching to the host, sedating them but keeping them alive.
The symbolism of rebirth shown by the butterfly, as mentioned in Alien: Resurrection by Dr Gedimen. The Newborn he described as a beautiful butterfly, emerging from the cloned Xenomorph Queen’s womb. Monarch is also another word for Queen.
*special thanks to Fassymioamor for providing the picture of Shaw’s grave, you can follow their blog on tumblr for great Alien: Covenant gifs.