Polygala cruciata L.
Polygala cruciata var. aquilonia
Basis for Listing
Polygala cruciata (cross-leaved milkwort) is a small inconspicuous plant that occurs primarily in the Atlantic and Gulf coastal plains but has scattered and infrequent occurrences inland. There is a discrete secondary range in the Great Lakes region (including Minnesota) that is disjunct from the main range (Peattie 1922). This isolated range is likely a relict of a larger continuous range that existed shortly after deglaciation. Populations in the Great Lakes region have always been small and scattered but have recently suffered significant declines resulting from habitat loss.
Polygala cruciata is a distinctive annual with purplish or pink flowers in a dense spike-like raceme. The most prominent parts of each flower are not the petals but actually a pair of deltoid petaliferous sepals or wings. The bracts that subtend each flower persist as subulate hooks after the fruit has fallen. Only P. sanguinea (blood milkwort) bears any resemblance to this species, but it has alternate leaves, whereas P. cruciata has whorled leaves. Although they may occur in whorls of 3, the linear leaves of P. cruciata are more commonly 4-whorled, hence the species epithet that means cross-shaped (Gleason and Cronquist 1991).
The Minnesota populations have occurred primarily on wet sandy shores of shallow lakes in the Anoka Sand Plain, and in sandy or peaty meadows or swales. These habitats may be in low depressions or at the margins of emergent wetlands. Habitats are typically open and sunny with acidic soils and fluctuating water tables. Other rare species associated with these habitats include Rotala ramosior (toothcup), Scleria triglomerata (tall nutrush), and Xyris torta (twisted yellow-eyed grass).
Biology / Life History
Polygala cruciata is an annual that flowers in July and August and possibly into September. Flowers are insect-pollinated, and seeds are gravity-dispersed. One published study conducted on a wet pine savanna in Mississippi suggested that P. cruciata was fire dependent and declined during an extended absence of fire (Hinman and Brewer 2007). It is unknown if the results are relevant to Minnesota, but the known habitats in Minnesota are also fire-dependent.
Conservation / Management
Polygala cruciata occurs in shallow groundwater-maintained wetlands and formerly the sandy shores of small groundwater-fed lakes and ponds. These habitats are very fragile and susceptible to any activity, such as ditching, tiling, and storm water runoff from urban developments and agricultural fields that alters groundwater or surface water flow patterns. The habitats are also easily damaged by the invasion of non-native species such as Phalaris arundinacea (reed canary grass), Lythrum salicaria (purple loosestrife) and Typha angustifolia (narrow-leaved cattail) These habitats typically exist as isolated remnants less than 20 ha (50 acres) in size, which makes them even more vulnerable (Kost 2000).
Best Time to Search
The best time to search for Polygala cruciata is when in flower during July and August.
Conservation Efforts in Minnesota
One extant population of this species occurs in Cedar Creek Ecosystem Science Reserve, which is managed for research purposes by the University of Minnesota. Another site near the city of Blaine is a state Scientific and Natural Area.
Lynden Gerdes, 2008 (MNDNR) and Welby R. Smith, 2021 (MNDNR)
(Note: all content ©MNDNR)
References and Additional Information
Gleason, H. A., and A. Cronquist. 1991. Manual of vascular plants of northeastern United States and adjacent Canada. Second Edition. New York Botanical Garden, Bronx, New York. 910 pp.
Hinman, S. E., and J. S. Brewer. 2007. Responses of two frequently-burned wet pine savannas to an extended period without fire. Journal of the Torrey Botanical Society 134(4): 512-526.
Kost, M. A., and M. R. Penskar. 2000. Natural community abstract for coastal plain marsh. Michigan Natural Features Inventory, Lansing. 6 pp.
Peattie, D. C. 1922. The Atlantic Coastal Plain element in the flora of the Great Lakes. Rhodora 24(280):57-70.