Irregular Growth Trees are trees that grow in such a way that defies their natural growing habits.
Irregular Growth Trees, as stated above, are trees that grow in ways that defies the known growing habits of their type. Most irregular growths are found as trees that have been blocked off by a Rock/Cliff and result in a tree that has branches protruding in only one way, similar to Zombie Wood. Another way irregular's can be seen is in a state where all of the branches point in the same direction (this is most common in Cherry Trees.) but this is extremely rare. Irregulars are also seen when branches of the tree descend to odd proportions. Many irregulars are seen when a leaf is caught in the middle of the tree itself. This glitch is usually uncommon but sightings have increased dramatically since the Halloween Update when Spook Wood and Sinister Wood were introduced. Sometimes, branches or trunks on trees can disappear, making the tree look like it's floating. It is a problem with offsets.
Here is a gallery depicting several trees that are out of proportion: