There have been some plants that are mentioned and have appeared over the course of the movies and TV series.
Ground Prickly - a plant that grows on the ground. It is show briefly as a vine that is along the ground with many round parts growing out of them with spines. It is on one of these that Littlefoot steps on in the TV episode The Meadow of Jumping Waters. In human terms it is called a burr.
Ground Stars - These appeared in The Hermit of Black Rock inside a cave where Guido and Petrie were trapped at the time. It is seen within a cave with an opening higher up so some sunlight can shine on it. It looked like a type of bush with leaves that resemble treestars.
These were some flowers that the gang, along with Ali, went to the Land of Mists to get to bring back so Littlefoot's Grandfather could eat them and be cured in the 4th LBT movie Journey Through the Mists. They bloom only at night. They greatly resemble modern members of the flower genus Datura.
Red Treestars - These appear in "The Meadow of Jumping Waters". They are mentioned by Ruby as being rare and very sweet. Petrie believes that eating tickly red treestars brings bad luck since he heard a story told to him by his uncle Pterano, though Spike finds them as tasty as any other plant. Red treestars are the dinosaur word for maple leaves.
These flowers are found growing on the island where the gang encounter Chomper for the second time. Their smell hides the smell of Chomper's friends so his parents are unable to smell them. Spike eats some with no visible ill effect, except for apparent bad breath. Petrie finds he likes the taste of them on some berries. They appear in the 5th LBT movie the Mysterious Island. They are probably foul smelling roses without the thorns or ancestors of the rose.
This is Ruby's favorite food. When frozen in winter they are called Hard Water Sweets. In the English Language they are called grapes.
Treestars - This is one of the favorite foods of the inhabitants of the Great Valley, the other being tree sweets. They are so called since they are shaped somewhat like stars. They appear in The Land Before Time movies, and the TV series. In human terms, they are leaves from a sycamore tree.
This refers to a certain type of fruit, and a pink flower that grows on trees. In The Land Before Time XI: Invasion of the Tinysauruses this refers to a specific flower on a specific tree. In the TV series this refers to any sort of fruit that grows on a tree that is edible.
There are also yellow-colored fruits that have appeared in a few episodes of The Land Before Time TV series. These fruits that grow on trees resemble the modern day papaya with its shape, yellow exterior and red interior. The odor of the tree sweets is fragrant to herbivorous dinosaurs but foul to carnivorous creatures. Thus, tree sweets act as a sort of repellent against predator attacks.
They were mentioned in "The Bright Circle Celebration". It's possible that they are just regular tree roots.