The distinction between aquatic and terrestrial plants is often blurred because many terrestrial plants are able to tolerate periodic submersion and many aquatic species have both submersed and emersed forms. There are relatively few obligate submersed aquatic plants (species that cannot tolerate emersion for even relatively short periods), but some examples include members of Hydrocharitaceae and Cabombaceae, Ceratophyllum, and Aldrovanda, and most macroalgae (e.g. Chara and Nitella). Most aquatic plants can, or prefer to, grow in the emersed form, and most only flower in that form. Many terrestrial plants can tolerate extended periods of inundation, and this is often part of the natural habitat of the plant where flooding is common. These plants (termed helophytes) tolerate extended periods of waterlogging around the roots and even complete submersion under flood waters. Growth rates of helophytes decrease significantly during these periods of complete submersion and if water levels do not recede the plant will ultimately decline and perish.
There are many invasive plant species. Some of them are Air Potato, Autumn Olive, Beach vitex, Brazilian peppertree, British yellowhead and Canada thistle. The scientific name for air potato is Dioscorea bulbifera L which was first discovered in United States during the 1770s where it was first cultivated as a food crop but then it was noticed that forms dense vines. Autumn olive (Elaeagnus umbellata Thunb.) is also referred to as Japanese silverberry. This plant species has been shown to displace native species when it first originally came from Asia to the U.S. for the purposes of erosion control and ornamental uses. Beach vitex (Vitex rotundifolia L. f.) which is also known as chasteberry, roundleaf chastetree and Monk's pepper, is an invasive plant that was introduced around the year 1990 to the U.S. from Asia for erosion control. This plant species tear down the nesting habitat of sea turtle and displaces native species. Brazilian peppertree (Schinus terebinthifolius) is another invasive species that is commonly known as Christmas berry, Florida holly and broad leaf pepper tree. It was introduced to the U.S. in the 1840s from South America for ornamental uses. However, it began to form dense monospecific stands that inhibit native species from growing, possibly doing this by producing chemicals. In reaction to the damage that Brazilian peppertree caused, several local and federal agencies like Florida Water Management District (SFWMD), the U.S. Department of Agriculture (USDA) agreed to produce a specific species of insect in order to reduce the occurrence of this invasive plant species in certain areas.