You were told that the mystery plant is a tree, and you were offered these clues to its identity:
My cup runneth over.
Spilling mast quite liberally,
Is a trademark of this tree.
Its wood isn't soft, not at all,
And it can grow six stories tall.
Mushy ground is okay,
Some flooding too, but it must not stay.
The mystery plant is overcup oak (Quercus lyrata). Known in some locales as "swamp post oak," "swamp white oak," or "water white oak," overcup oak is a white oak native to the Atlantic and Gulf coastal plains from southeastern Virginia to the Florida panhandle and westward to east Texas, with extensions northward into Missouri, southern Illinois, and southern Indiana. There are scattered populations in some other places, including northern Alabama, central Tennessee, Delaware, and Maryland. Overcup oaks can be found in quite a few National Park System units, including Colonial National Historical Park in Virginia, Congaree National Park in South Carolina, and Big Thicket National Preserve in Texas.
Here is how to work through the clues.
My cup runneth over (Hebrew Bible, Psalms 23:5) is a very helpful phrase. Connected in this way, the terms "cup" and "over" might alone be enough ID for people who are familiar with this tree. Incidentally, the "overcup" part of the name derives from the fact that the acorns of this tree have a warty-looking gray cup that encloses about two-thirds of the small, nearly spherical nut.
When used in the context of food for animals or birds, mast refers to accumulations of acorns or other nuts on the ground, normally under or near the trees that produce them. Beginning at about age 15-25, a mature overcup oak commonly produces a big crop of acorns every three or four years. Thus, spilling mast quite liberally is a trademark of this tree. The acorns of this tree have spongy, buoyant shells and are readily dispersed by moving water. Since this species particularly likes the moist, intermittently inundated soils of flood plains (mushy ground is okay), acorn dispersion by floodwater is typical.
Some flooding [is okay], but it must not stay. The overcup oak grows best on better-drained soils, but can flourish in flood plains, depressions, and other wetlands that are inundated as much as 30-40 percent of the time. Unusually lengthy flooding episodes can be harmful or lethal, however, especially when they occur in late spring after the trees have leafed out. Untimely flooding also kills seedlings.
An overcup oak is a hardwood, not a softwood. (Its wood isn't soft, not at all.) Though the heavy, brownish-colored wood is hard, and can be marketed for lumber if in prime condition, it is prone to defects (fire injury, heartwood decay, insect damage, checking, etc.) that can make the wood undesirable even for such incidental applications as railroad ties and pallets. Because overcup oak is sometimes left uncut during logging operations, it can become a problem tree in managed forests.
In the course of a lifetime, which can span as much as 400 years, an overcup oak can attain a height of 60-65 feet. In other words, it can grow six stories tall. In mixed stands, few overcup oaks are able to reach such an impressive height. Being slow growing and shade intolerant, overcup oaks are prone to overtopping by faster growing trees. Though drought resistant, they are also easily damaged and stunted by fire.