Mega Trees are a species of tree indigenous to Dimension 35-C. In the Pilot episode, Rick and Morty seek out these trees for their seeds, which Rick claims are highly important to his research. The trees reappear in the episode, "Tales From the Citadel", on Farmer Rick's farm.
Mega Trees are tall, straight trees with bulbous roots and tentacle-like fronds emerging from the tops of their trunks. The trunks are greenish at the base, and become reddish towards the top, featuring a glossy smooth surface with what appears to be veins underneath. Aside from their fronds, they have no branches. The trunk ends in several coiled strands of purple growth, with each frond ending in a Mega Fruit. The fruit collected in the pilot contained two seeds. The trees are clearly and intentionally phallic, with the fruits themselves resembling human male gonads, and the bulge at the base of the trunk resembling a scrotum.
It's currently unknown whether or not the fruits are edible (or at least safe to consume), but given the lack of apparent predators, as well as the lack of evidence that it's ever been used for consumption, it's safe to assume not.