There is a hunger bar for each player, and there is another spot in the inventory for the item that is in your hand.

You till the ground for planting by pressing" r" when playing as the purple alien.

You can turn it back to grass by pressing" f".

You can plant seeds by pressing "r" when you have seeds in your hand and are playing as the yellow alien.

Except for the  Needle Moss that you get from the dead trees with moss growing on them,  you can grow that by making a trellis, then placing it by pressing the drop button, then pressing" r" when you have the moss in your hand.

You harvest the crops by pressing" r" when they are done.

You get seeds from bushes.