Of the nine arrowhead species that occur in New York, only spongy arrowhead is an annual (Haynes and Hellquist 2000, Weldy and Werier 2005). The name spongiosa means spongy (Fernald 1970) and is clearly in reference to the very spongy leaves.
There are about 20 known extant populations, 2 populations which have not been seen in over 70 years, and 1 population which is believed extirpated. Of the about 20 extant populations, 7 of them have 100 or fewer individuals. Overall, the habitat for this species is very limited in New York and some of it has been altered or destroyed.
One population was extirpated in the past 20 years when a channel was deepened and the associated mud flats were destroyed. A couple of populations may have experienced a small decline in the number of individuals present over the past 20 years but the data is not conclusive. Short term trends appear to indicate at least a small decline although a lack of data makes this assessment inconclusive.
At least two populations have not been seen for over 70 years. Surveys need to be conducted before these populations can be determined extirpated. One population has been extirpated in the past 20 years. It is unclear if the about 20 known extant populations have changed in the past 100 or so years but at least 7 of these populations are relatively small with 100 or under individuals. This may indicate they are declining. Overall, long term trends are not clear but point to at least a small decline.
Potential threats include habitat conversion or destruction and disturbances by boaters including large wakes. Potential threats also include development adjacent to a population which could result in the habitat changing.
Deepening of channels and disturbances that can change the habitat where spongy arrowhead occurs should be avoided.
Surveys should be conducted to sites where populations have not been seen in recent years to determine if these populations have become extirpated. There are a few specimens which come from sites outside of the area where spongy arrowhead occurs. The identity of these specimens should be checked. All known extant populations should be monitored on a regular basis.
In New York, this species occurs in freshwater to brackish intertidal mud flats, often with Sagittaria subulata. It is most abundant on these open mud flats but occasionally occurs in lower abundance in the taller vegetation associated with the adjacent and upslope brackish or fresh tidal marshes (New York Natural Heritage Program 2007). Brackish to fresh-water tidal mud flats and salt marshes (Haynes and Hellquist 2000). Brackish to nearly fresh tidal waters and salt marshes (Gleason and Cronquist 1991). Shallow water of the Hudson River (Haynes 1979). On tidal mud of brackish estuaries (Fernald 1970).
This species is restricted to estuaries along the Hudson River from south of Albany to a little north of the Tappan Zee Bridge (New York Natural Heritage Program 2007).
This species occurs from New Brunswick south near the coast to Delaware and eastern North Carolina (Haynes and Hellquist 2000, Weakley 2007).
Spongy arrowhead is an annual herbaceous plant. It grows in tidally flooded areas. During low tide the entire plant is often above water. At high tide the plants become mostly or entirely submerged. The leaves arise from the base of the plants in dense whorls and are spongy, very narrow, and strap-like. The tips of these leaves sometimes become somewhat expanded. The flowering stalks are shorter than the leaves and have a few whorls of long-stalked flowers towards their tips. The flowers are small and not showy (Fernald 1970, Haynes and Hellquist 2000).
It is easiest to identify this species when it is in flower or fruit. If the population is large enough entire plants with roots should be gathered for voucher specimens.
Sagittaria subulata grows in the same habitat as S. montevidensis ssp. spongiosa and is perhaps superficially similar. S. subulata can be distinguished by: its non-spongy, narrower (0.1-0.4 cm wide) leaves; pedicels narrower; fruiting heads smaller (0.6-0.8 cm in diameter); and many plants strictly vegetative vs. spongy, wider (to 2.0 cm wide) leaves; pedicels wider; fruiting heads larger (1.2-2.1 cm in diameter); and most plants reproductive for S. montevidensis ssp. spongiosa.
Sagittaria graminea has non-spongy leaves, sepals of carpellate flowers reflexed, scapes with 1-12 whorls of flowers, and the lower flowers pistillate and without sterile stamens vs. spongy leaves, sepals of carpellate flowers erect and appressed, scapes with 1-4 whorls of flowers, and the lower flowers pistillate and with sterile stamens for S. montevidensis ssp. spongiosa.
Sagittaria rigida has non-spongy leaves, sepals of carpellate flowers reflexed, scapes with 2-8 whorls of flowers, and carpellate flowers sessile vs. spongy leaves, sepals of carpellate flowers erect and appressed, scapes with 1-4 whorls of flowers, and carpellate flowers on pedicels for S. montevidensis ssp. spongiosa.
Limosella australis is quite different, being a member of the Scrophulariaceae family. It can be easily distinguished by its narrower, filiform leaves that are 1-2 mm wide vs wider spongy flattened leaves that are up to 2.0 cm wide for S. montevidensis ssp. spongiosa.
Echinodorus tenellus is a closely related species that grows in different habitats. It can be distinguished by its non-septate roots, non-spongy leaves, flowers in 1(-2) whorls with 3-6 pedicels per whorl, and achenes that are ridged and essentially beakless vs.septate roots, spongy leaves, flowers in 1-4 whorls with 1-3 pedicels per whorl, and achenes that are not-ridged and have a small beak for Sagittaria montevidensis ssp. spongiosa (Fernald 1970, Gleason and Cronquist 1991, Crow and Hellquist 2000, Haynes and Hellquist 2000).
The plants are annuals and are difficult to find early in the growing season. They are easiest to spot and identify when they are in flower or fruit which is from mid to late August through September. Therefore, the best time to survey for this species is during this time period.
The time of year you would expect to find Spongy-leaved Arrowhead vegetative, flowering, and fruiting in New York.
Sagittaria montevidensis ssp. spongiosa (Engelm.) Bogin
Sagittaria montevidensis ssp. spongiosa is sometimes placed in the genus Lophotocarpus (Smith 1900, Fernald 1970). Recent evidence (Kaul 1967, Argue 1974) indicates that Lophotocarpus is best placed in the genus Sagittaria. This is followed by most recent authors (Voss 1972, Gleason and Cronquist 1991, Haines 1998, Haynes and Hellquist 2000, Weakley 2007). Sagittaria montevidensis is represented by three taxa (Haynes and Hellquist 2000). Some authors (Haynes and Hellquist 2000) treat these taxa as subspecies: S. m. ssp. montevidensis (native to South America and introduced in the southeastern United States); S. m. ssp. spongiosa (native to northeastern United States and adjacent Canada); and S. m. ssp. calycina (native to central and eastern United States and Mexico). Others (Gleason and Cronquist 1991, Weakley 2007) treat these taxa at the species level: S. spatulata (=S. montevidensis ssp. spongiosa); S. calycina (=S. montevidensis ssp. calycina); and S. montevidensis (=S. montevidensis ssp. montevidensis). Some authors (Kartesz 1994) place S. m. ssp. calycina and S. m. ssp. spongiosa as two taxa under the species S. calycina (S. calycina var. calycina and S. calycina var. spongiosa) with S. m. ssp. montevidensis as a distinct species with no infraspecific taxa. Smith (1900) recognized two taxa (Lophotocarpus spongiosus and L. spatulata) that are currently lumped under S. m. ssp. montevidensis. When recognized in the genus Sagittaria, at the species level, the name S. spatulata has priority since it was the first name transferred to that genus. Only S. montevidensis ssp. spongiosa occurs in New York (Haynes and Hellquist 2000). Sagittaria montevidensis ssp.
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Information for this guide was last updated on: December 29, 2008
Please cite this page as:
New York Natural Heritage Program. 2023. Online Conservation Guide for Sagittaria montevidensis ssp. spongiosa. Available from: https://guides.nynhp.org/spongy-arrowhead/. Accessed March 23, 2023.