Manomin Ecology: Environmental Factors That Impact Manomin Growth

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Research focused on crop restoration, cultural revitalization, and treaty living. Researchers who believe in collaboration and knowledge sharing. Learn more about the Manomin Research Project here.

By Samantha Mehltretter with Niisaachewan Anishinaabe Nation

Manomin has long been central to Anishinaabe diets (Vennum 1988, 1). The complex carbohydrate complements the lean protein in their diet from fish, and historically helped families make it through harsh winters (Vennum 1988, 42). While more widely known in English as wild rice, manomin is not classified as a rice but rather a cereal. In fact, it is the only cereal native to North America (Dore 1969, 6; Vennum 1988, 12). The annual aquatic grass, genus Zizania, grows in shallow lakes and rivers from what is now known as Northern Manitoba and Saskatchewan to southern states like Florida and Texas (Vennum 1988, 32-33). Multiple species of Zizania span this large area (Dore 1969, 16-22). Zizania palustris, the species of interest in our study, is found in the north-midwestern United States (Wisconsin and Minnesota), southern Manitoba and northwestern Ontario, with some stands in southern Ontario. Not all species of manomin have the same habitat requirements, but all are sensitive to changes in their environment (Lee and Stewart 1981, 2140).

Manomin is an annual emergent macrophyte, which means it is an aquatic plant (macrophyte) that grows mostly above the water’s surface (emergent) and starts from a new seed each year (annual). While manomin’s energy initially comes from its seed, once leaves develop in submerged leaf stage [1], photosynthesis is used to help the plant grow [2]. This lifecycle makes manomin very sensitive to water depth. If the water is too deep in manomin’s early life stages, its submerged leaves may not get enough sunlight to mature. Manomin is also sensitive to changes in depth. A sudden rise in water level, especially during floating leaf stage [3], can result in the plant uprooting because the leaves are buoyant (float well), and the roots are not firmly secured in the sediment (Raster and Hill 2017, 270). Further, once aerial shoots have grown well above the water’s surface, a water level drop can leave the stalk unsupported and cause it to collapse (Vennum 1988, 21). Several researchers have investigated what depths are most suitable for manomin productivity, although there is considerable variation in the results reported. An early publication from Moyle (1944, 177) suggests manomin grows best in 30 – 90 cm of water. Pillsbury & McGuire (2009, 730) found the densest populations of manomin occurred in just over 70 cm +/- 25.5 cm, and Tucker et al. (2011, 117) stated water depths should be less than 50 cm. Pip & Stepaniuk (1988, 283) mention manomin can grow in water depths as great as 150 m, but that it grows best in 20 – 100 cm of water. Generally, it appears manomin prefers water levels less than 1 m, with shallower waters improving stand density so long as the sediment is saturated (Raster and Hill 2017, 270; Tucker et al. 2011, 117).

Water quality can also impact manomin growth. Similar to when manomin is in deep water, turbid water (murky water you cannot see through) can limit the sunlight available for photosynthesis while manomin is in its submerged stage (Myrbo et al. 2017, 2747). Beyond turbidity, early papers focused on the impact of water quality on manomin. Specific ranges or limits on water chemistry to support manomin growth were suggested (Moyle 1944, 178). For example, alkalinity should be less than 80 mg/L and preferably less than 40 mg/L [4], and sulphate concentrations should be less than 40 mg/L and preferably less than 5-10 mg/L. More recently, however, researchers suggest that the water chemistry is only as important as its impact on sediment chemistry (Aiken et al. 1988, 41). This is largely because emergent macrophytes, like manomin, draw nutrients from the sediment, which is in contrast to most submerged aquatic plants that get their nutrients from the water (Day and Lee 1989, 1381). While manomin is in its submerged stage, however, water quality, specifically the amount of oxygen and carbon dioxide dissolved in water, is a key factor in manomin’s maturation (Aiken et al. 1988, 42).

Manomin can grow in a variety of soils (Dore 1969, 50; Aiken et al. 1988, 42), but prefers “soft-textured sediments as silts, muds, and oozes” (Dore 1969, 50). Manomin needs a substrate (sediment bottom) that is not too firm and that the roots can penetrate. At the same time, the substrate cannot be too “watery” or the plant may be easily uprooted by waves or changes in water level (Day and Lee 1989, 1384). The sediment must also have sufficient organic material (“ooze”) and available nutrients like phosphorous, nitrogen, calcium, magnesium and potassium (Day and Lee 1989, 1386). Knowledge Keepers at Niisaachewan Anishinaabe Nation have referred to this as “loon bottom,” associating bird waste with fertilizer. If there is too much organic matter, however, the thick accumulations can reduce the amount of oxygen in the sediment, which reduces manomin productivity. Having enough oxygen in manomin sediments is important because without it, some bacteria can convert chemical compounds like sulphate [5] into compounds like sulfide, which are toxic to manomin (Myrbo et al. 2017, 2745). One story, recounted in Dore (1969, 51), shares how a bush pilot saw a moose running through shallow waters with aquatic plants, and noticed a trail of churned sediment and muddy waters from the disturbance. The following season, the pilot noticed that directly along the moose’s tracks grew healthy manomin stands. The moose stirred up the sediment and uprooted the aquatic plants, leaving more aerated (oxygenated) and open sediment for manomin to grow in (Dore 1969, 51).

Similar to sediment oxygen levels, the sediment pH (how acidic it is) impacts the types of compounds found in the sediment. For example, phosphorous is an important nutrient for manomin growth (Lee and Stewart 1984, 1613; Lee 1986, 2044; 1987, 1437; Day and Lee 1989, 1386). It is, however, only available to manomin in certain forms.  When manomin soils are very acidic (low pH), phosphorous will combine with other elements (like aluminum or iron), which makes the phosphorous unavailable to plants (Mitsch and Gosselink 2000, 184). This means that even if there is a lot of phosphorous in the sediment, if the environment is too acidic manomin cannot use the phosphorous to help it grow. Conversely, if the soils have a high pH (too basic), then phosphorous will combine with calcium and magnesium (Mitsch and Gosselink 2000, 186), and is also not available to manomin. When sediment is neutral (close to a pH of 7) or only slightly acidic phosphorous changes into a form that manomin can use (Mitsch and Gosselink 2000, 186).

Manomin’s growth is also influenced by biota (living things) in its environment. Manomin must compete with perennials (plants that do not die each year) for space. Remember, manomin has to start from seed each year, and needs ample sunlight to grow. Once perennials are established it is hard for manomin seedlings to access enough resources (e.g. sunlight, nutrients, space) to survive. It would be like trying to re-build your home each year in a community where the prime real estate is taken by people who have large established homes that they have lived in for years. This situation is referred to as a closed ecosystem, because the biodiversity is established and not likely to change without a disturbance (Dore 1969, 53). Annuals like manomin need sufficient and suitable space and resources to regrow each year. An invasive species of cattail (a hybrid of Typha latifolia an Typha angustifolia) is an example of an emergent perennial macrophyte that has taken over manomin habitat (Dysievick, Lee, and Kabatay 2016, 5). Knowledge Keepers at Niisaachewan Anishinaabe Nation (NAN) refer to this species as “bog.” Cattail is more tolerant of deep water and fluctuating water levels, and so manomin does not compete well against this invasive perennial (Dysievick, Lee, and Kabatay 2016, 9). Cattails not only outcompete manomin for the best “real estate” (shallow waters), but it also alters the sediment by depleting it of nitrogen, and increasing litter (dead plant material) and decomposing biomass, which reduces the amount of oxygen in the sediment (Dysievick, Lee, and Kabatay 2016, 28). In contrast, a species of pondweed (Potamogeton robbinsii) actually helps make nutrients available to manomin when it decomposes, thus enhancing manomin yields (Lee 1987, 1436). Similarly, water lilies can help restrict the growth of submerged plants in manomin stands. As manomin starts to reach the water’s surface in June, it will have grown above the lilies before the lily pads have fully matured in July. Aquatic vegetation that would otherwise grow amongst the manomin stands cannot grow beneath the expansive water lily leaves that restrict sunlight (Dore 1969, 52). This helps manomin prosper as fewer aquatic plants means less competition for resources.

Plants are not the only biota influencing manomin growth. Pests, including insects, fungus and animals may also impact manomin (Dore 1969, 58-65). The larvae of moths, as an example, eat manomin seed before it is fully ripe, and once mature, the moths lay their eggs in the pistillate of the manomin plant (where the seed eventually forms) (Dore 1969, 58). Both activities prevent manomin from fully maturing and yielding rice for harvest. Muskrats will eat manomin shoots before the plant has a chance to mature, and many waterfowl enjoy eating the rice as well (Dore 1969, 64).

Other factors that affect manomin growth include weather, land use and boating. While manomin’s aquatic habitat helps dampen large fluctuations in temperature (Dore 1969, 54), once manomin is 2 m or more above the water’s surface it becomes sensitive to windstorms, hail, frost and heavy rains (Vennum 1988, 26). If these events break the stalks or strip unripened grains, fewer seeds are available for harvesting or germination in subsequent year(s). Changes in land use have also contributed to manomin decline, largely in areas with increased residential development (Pillsbury and McGuire 2009, 732). Further, motorboat activity amongst manomin stands may destroy stalks, create wakes that uproot the plant, or increase turbidity restricting sunlight during the early life stages of manomin (Pillsbury and McGuire 2009, 732).

Although manomin can grow in a multitude of environments, it is also sensitive to changes in those conditions. Physical, chemical and biological factors play a role in whether manomin stands will prosper or grow at all, and there is often variety from year to year (Moyle 1944, 181). A considerable amount of research has been done on the different factors affecting manomin growth, especially with relation to water levels, nutrient availability and pollutants. Manomin, however, is also a plant that adapts, and some stands are influenced by these factors differently. As such, our proposed research aims to use the expansive academic knowledge on manomin and apply it to the historic sites near Niisaachewan Anishinaabe Nation (NAN, in what is now northwestern Ontario) to identify the most important factors affecting these stands. Then, we can co-develop river sharing and crop management strategies with NAN to restore this valuable crop’s growth on the Upper Winnipeg River.


[1] In submerged leaf stage, the manomin seed has used its stored energy to grow into a small seedling with leaves. The seedling and its leaves, however, are still completely underwater. The plant needs sunlight to penetrate through the water column and reach its submerged leaves so it can produce energy through photosynthesis to grow.

[2] Photosynthesis is the process by which plants use sunlight, carbon dioxide and water to produce energy. Refer to our earlier blog post for the manomin life cycle and other introductory information: Lehman, M. with Niisaachewan Anishinaabe Nation (2019, November 1). An Introduction to Manomin. NiCHE.

[3] In floating leaf stage, the seedling reaches the surface and its leaves grow like long ribbons that lie along the water’s surface.

[4] This is in mg/L as CaCO3.

[5] Early literature discussed that manomin does not grow in high sulphate waters (Moyle 1944). More recently, however, it has been determined that sulphate isn’t toxic to manomin, but rather its reduced form sulfide is (see Mybro, et al. 2017 for more detail).


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Dore, William G. 1969. Wild-Rice. Ottawa: The Queen’s Printer for Canada.

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———. 1987. “Ecological Relationships of Wild Rice, Zizania Aquatica . 5. Enhancement of Wild Rice Production by Potamogeton Robbinsii.” Canadian Journal of Botany 65 (7): 1433–38.

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Image provided by the Manomin Research Project.

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