Rare Species Guide

  Synonyms

Helianthus rydbergii, Helianthus nuttallii var. rydbergii

  Basis for Listing

Helianthus nuttallii is an aggregate species that occurs throughout much of western North America. It consists of three subspecies but only one, ssp. rydbergii, occurs in Minnesota. This entity is a characteristic prairie plant of the northern Great Plains and appears to reach the eastern or perhaps the southeastern limit of its range in western Minnesota. There are ten documented occurrences in Minnesota, but only one has been seen since 1962. This might indicate a recent decline in populations, although there are other ways to interpret this history. The genus Helianthus, and this species in particular, has a reputation for being difficult to identify. Although there is some truth to this, the perception of taxonomic confusion is perhaps a greater hindrance than the reality. Even accomplished botanists often shy away from this group of plants. So, there is some reason to believe that the species still occurs in Minnesota but is being overlooked. Helianthus nuttallii ssp. rydbergii was listed as a special concern species in Minnesota in 1984.

  Description

Helianthus nuttallii ssp. rydbergii is a native perennial sunflower that grows to a height of 1-2 m (3.3-6.6 ft.). The leaves are all or mostly opposite. The petioles are 0.5-1.5 cm (0.25-0.59 in.) long, and the leaf blades are 4-20 cm (1.6-7.9 in.) long and 0.8-4 cm (0.3-1.6 in.) wide. The base of each leaf blade is wedge-shaped (cuneate), and the margins are entire or shallowly serrate. The shape of the blade is lanceolate to nearly ovate, the tips are acute to obtuse, and the lower surface has short, stiff hairs. There are 1-6 flowering heads, each on a peduncle 1-18 cm (0.4-7.1 in.) long. Each head is subtended by 30-38 bracts (phyllaries), which are lanceolate to lance-ovate in shape, 8-16 mm (0.31-0.63 in.) long, and 1.5-3 mm (0.06-0.12 in.) wide. The bract margins are ciliate and the tips are acute to acuminate. There are 10-21 ray flowers, each with an expanded yellow petal (laminae) 2-2.5 cm (0.75-0.98 in.) long. The disk portion of the head is 1.5-2 cm (0.59-0.75 in.) in diameter and yellow (Shilling 2006). Outwardly, this sunflower might not appear strikingly different than the other sunflower species seen on Minnesota prairies. It is perhaps most similar to H. grosseserratus (sawtooth sunflower), a common species, but H. nuttallii ssp. rydbergii has shorter petioles and smaller leaf blades, which do not exceed 4 cm (1.6 in.) in width. Also, the leaf margins of H. nuttallii ssp. rydbergii usually lack serrations, or have only shallow serrations (Heiser et al. 1969).

  Habitat

Based on herbarium labels from specimens that have been collected in Minnesota, it is clear that H. nuttallii ssp. rydbergii occurs in native prairies. It is difficult to be more specific, but the prairies where it has been collected seem to range from mesic to wet-mesic.

  Biology / Life History

Helianthus nuttallii ssp. rydbergii does produce rhizomes (Heiser et al. 1969), but the rhizomes are short and do not allow the plant to spread within its habitat. It seems likely that reproduction is accomplished only or primarily through seeds, which are spread by birds and possibly other animals. Under experimental conditions, fertile hybrids have been created between H. nuttallii ssp. rydbergii and two common associates: H. maximiliani (Maximilian's sunflower) and H. grosseserratus (Long 1966). It is not known if such hybrids exist in nature, although the breeding system of all the sunflowers is similar, so it is reasonable to expect some gene flow between sympatric species if the phenologies overlap. The conservation or management significance of any possible hybridization is not known.

The lack of population or habitat data in Minnesota may reflect the difficulty that field biologists have in identifying prairie sunflowers. More often than not specimens are unreliably identified or not identified at all. That is likely the case in Minnesota, where considerable attention has been given to the flora of remnant prairies, but only one specimen of H. nuttallii ssp. rydbergii has been vouchered since 1962.

The best time to search for H. nuttallii ssp. rydbergii is when it is in flower during the months of August and September.

  Conservation / Management

At this time, there is very little known about the population dynamics of H. nuttallii ssp. rydbergii in Minnesota. There is even little known about specific habitat and community affinities. Still, there are some general assumptions that can be made. It seems almost certain that this species occurs in fire-maintained habitats (prairies or prairie-like habitats within a prairie matrix) and is therefore probably fire-adapted. This is a very important consideration since land managers tend to use prescribed burns to manage prairies. Yet not all prairie species are equally benefitted, or benefitted at all, by fire. Even when fire is the appropriate prescription, the timing of the fire would be critical. It is also likely, based on what is known of similar species of Helianthus, that land use activities such as livestock grazing and hay harvesting would be detrimental to this species.

  Conservation Efforts in Minnesota

The only site where H. nuttallii ssp. rydbergii probably occurs in Minnesota is Foxhome Prairie in Wilkin County. It was found there in 1979 but there has been no attempt to relocate the population since that time. The site has been managed by The Nature Conservancy for the perpetuation of the prairie habitat so there is some reason to hope H. nuttallii ssp. rydbergii still survives there.

  References and Additional Information

Heiser, C. B., Jr., D. M. Smith, S. B. Clevenger, and W. C. Martin, Jr. 1969. The North American Sunflowers (Helianthus). Memoirs of the Torrey Botanical Club 22(3):1-218.

Long, R. W. 1966. Biosystematics of the Helianthus nuttallii complex (Compositae). Brittonia 18:64-79.

Schilling, E. E. 2006. Helianthus. Pages 141-169 in Flora of North America Editorial Committee, editors. Flora of North America north of Mexico. Volume 21. Oxford University Press, New York, New York.