CPC National Collection Plant Profile

Howellia aquatilis

Ed Guerrant

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CPC National Collection Plant Profile

Howellia aquatilis

Common Names: 
howellia, water howellia
Growth Habit: 
CPC Number: 


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Howellia aquatilisenlarge
Photographer: Ed Guerrant

Howellia aquatilis is Fully Sponsored
Primary custodian for this plant in the CPC National Collection of Endangered Plants is: 
Edward Guerrant, Ph.D. contributed to this Plant Profile.

Howellia aquatilis

How can any plant found in California, Washington, Idaho and Montana, especially one with over 150 known occurrences, possibly qualify as Threatened under the Endangered Species Act? Howellia aquatilis is such a plant, and it richly deserves protection. Over two thirds of the known sites are restricted to just one of only six clusters of populations, and the entire species occupies a total area of less than 200 acres of a very particular, ecologically fine-tuned, and easily disrupted habitat. It was discovered in 1897 by brothers Thomas and Joseph Howell on Sauvie Island, which is near Portland, Oregon, a state in which it has since become extinct. This aquatic annual species is distinctive enough that it is the only member of its genus.

In its short life, plants produce two kinds of flowers. Early flowers remain underwater and never open. Even so, they consistently produce seeds, which are necessarily the result of self-pollination. Flowers produced later do emerge above the surface of the water and open into what look like tiny white lobelias. The above-water flowers, which are also primarily self-pollinating, do have the potential to accept pollen from other plants. Due to the predominance of inbreeding, Howellia has an extremely uniform genetic makeup throughout its entire range.

In addition to limited genetic diversity, the species' very particular environmental requirements may also limits it long term future prospects. Although this plant is aquatic, its seeds do not germinate under water. Since seeds germinate in the fall and overwinter as seedlings (another curious property), Howellia requires a dry autumn followed by a wet spring in order to establish for the year. In addition to seasonally fluctuating ponds, Howellia requires fertile, highly organic soils, which are generally maintained by deciduous trees surrounding the ponds. Therefore, disturbances to the surrounding forest community also impact this threatened species. Research indicates that Howellia does not form a persistent seed bank, making this annual especially dependant on year to year reproductive success in order to persist.

Distribution & Occurrence

State Range
State Range of  Howellia aquatilis
  • Restricted to small pothole ponds or orphaned river oxbows that are generally less than 3 ft (1 m) deep, but occasionally up to 6 ft (2 m) deep. Ponds are typically in a matrix of dense forest vegetation, and are nearly always surrounded by broadleaf deciduous trees. Bottoms of wetlands contain clay and organic sediments. Habitats are filled by snowmelt run-off and spring rains, and then dry out to varying degrees by the end of the growing season (Shelly 1992).
• Found in elevations between 3m (10 ft) in WA to 1350 m (4420 ft) in MT (Shelly 1992).
• Almost always bordered with one of the following broadleaf trees: Popullus trichcarpa, P. tremuloides, Fraxinus latifolia. Most wetlands have a well-developed shrub component composed of plants such as Cornus stolonifera, and Spirea douglasii (USFWS 1996)

  CA, ID, OR (extirpated), MT, WA.

CA: Mendocino County
ID: Latah Co.
OR: formerly Willamette Valley
WA: Puget trough, Columbia Basin
MT: Swan River Drainage

Number Left
  • Most sites containing H. aquatilis are < 1 acre (FWS web page)
• As of 2000 (Rush and Gamon 2000, USFWS 1996, CDFG 2001):

The main geographical areas where H. aquatilis is found include:
1-- ID (Latah Co.) 1 occurrence
3 --WA (Spokane, Clark and Pierce Co) 45 occurrences in Spokane Co., 5 occurrences in Pierce Co. and One (1) in Clark County.
1 - MT (Lake and Missoula Co.) A total of 101 occurrences in Lake and Missoula Counties.
1 - CA (Medocino Co, rediscovered in 1996). 6 occurrences.


Global Rank:  
Guide to Global Ranks
Federal Status:  
Guide to Federal Status
Recovery Plan:  

State/Area Protection
  State/Area Rank Status Date  
  California S1.2 7/1/2001  
  Idaho S1 GP2 12/21/2001  
  Montana S2 T 5/29/1991  
  Oregon SH EXT. 2/1/2001  
  Washington S2 T 10/1/2001  

Conservation, Ecology & Research

Ecological Relationships
  Howellia aquatilis produces two types of flowers: submerged cleistogamous flowers and emergent chasmogamous flowers. Cleistogamous flowers are characteristically self-pollinating; chasmogamous flowers are predominantly self-pollinating, but can be cross-pollinated. (Lesica et al. 1988) At least in Montana, submerged flowers appear in early May, shortly after the dormant seedlings have begun to grow, and emergent flowers bloom when stems reach the surface, from late June until August. Seed dispersal from underwater fruits begins in early June and extends into late summer as emergent fruits ripen. The two different flowers extend seed production over most of growing season (Lesica 1990 in Shelly 1992), but it appears as though cleistogamous flowers produce the majority of seeds (Lesica 1988).

This species only grows in zones within wetlands that are seasonally inundated yet dry out in late summer or early fall. This annual inundation may help keep competing vegetation from becoming too well established (WNHP, 1999). Seed germination occurs in the fall in dried, aerobic portions of the pond and plants then over-winter as dormant seedlings (Lesica 1990 in Shelly 1992). Since seeds do not germinate without exposure to the high oxygen levels, the population levels in a particular year are directly influenced by the extent to which the pond dries out at the end of the previous growing season (Shelly 1992).

Seeds produced by submergent flowers likely sink immediately. While seeds from emergent flowers may float a short distance, they do not float for long and there is likely not long distance dispersal within ponds. Broken stems with fruits have been observed floating in water, providing some longer distance dispersal within the same wetland, but the species is mostly restricted to quiet water (Shelly and Moseley 1988 in Shelly 1992). It is possible that waterfowl and mammal (deer, bear, moose) ingestion distributes seeds between ponds (Shelly 1992), but this phenomenon has not been investigated.

Fire may have historically influenced H. aquatilis distribution. Intense fire late in the growing season has the potential to extirpate a population. With this fire regime, clusters of small populations would be would be less likely to be eliminated than one large population. Fire early in the growing season may offset organic mat build-up in ponds, benefiting the population. The primary effect of fire would be to remove trees, influencing the drying regime of the ponds (Shelly 1992).

In Montana, the restriction of H. aquatilis clusters of populations in closely adjacent ponds suggests metapopulation dynamics. Metapopulation dynamics may be critical for the survival of species that inhabit a shifting mosaic of habitats. It may also help the population persist in the midst of environmental stochastity, since multiple populations can serve as a source of colonists (USFWS 1996).

In Montana, the pothole ponds inhabited appear to be at an early succession stage. Aquatic grasses, sedges, pondweeds, and burreeds characterize these ponds. With increasing sedimentation and accumulation of organic matter, and the lowering of the water table, these habitats eventually develop into sedge meadows, and H. aquatilis is not present. In ponds that are more successionally advanced, and remain wet for the majority of the growing season, Typha latifolia is frequent. H. aquatilis occurs with T. latifolia, but these ponds support less vigorous populations of H. aquatilis (Shelly 1988).

  • Drainage of aquatic habitat for urban and agricultural development (Meinke 1982)
• Invasion of noxious weeds: e.g. reed canary grass (Phalaris arundinacea), and purple loosestrife (Lythrum salicaria. (WNHP 1999, USFWS 1994)).
• Invasion of reed canary grass, Phalaris arundinacea (Lesica 1997)
• Timber harvest activities on adjacent uplands (WNHP 1999). Timber harvest may increase run-off by lowering winter snow sublimation and evaporation rates and decreasing vegetative interception of overland flow (Lesica 1992). Alternatively, removal of trees may increase evaporation rates but decrease water loss due to transpiration. Timber harvest and road building may also increase mineral soil input (Lesica 1992).
• Cattle disturb shorelines and associated vegetation, and trampling of bottom sediment adversely affects the seed bank (USFWS 1994).
• Removal of native vegetation surrounding ponds:
- increases annual speed of pond drying
- increases annual water depth, decreases drawdown due to increased run-off and decreased evapotranspiration (Shelly 1992).
- Increases density of associated vegetation in pond, resulting in increased competition for light and other resources, plus the lowering of the water table. Most ponds around which logging has occurred have more dense vegetation. (Shelly 1992).
• Short and long term climate change (Shelly 1992). Long term shifts in average climate conditions such as many consecutive wet or dry years may move away from optimal conditions (adequate wetland recharge that follow summer and early fall drying the previous year) (USFWS 1996)
• Aquatic vegetation succession (Shelly 1992)
• The low genetic diversity severely restricts the ability of H. aquatilis to adapt to changing conditions (Shelly 1992).

Current Research Summary
  • Germination trials preformed on MT populations by Lesica (1992) found that seeds had significantly lower germination rates after 8 months of dried storage. Additionally, plants grown from older seeds had reduced vigor (Lesica 1992). However, different monitoring studies indicated that seeds retained germinability for at least 2 years (Schassberger and Shelly 1991, Shelly 1992). These conflicting germination results may reflect variability in duration of seed viability, perhaps correlated with the extent of wetland drying in a year (Shelly 1992 based on work done by Lesica 1991, USFWS 1996).
• Germination trials and observations. Howellia aquatilis has relatively large seeds, and is capable of germinating in both the dark and at low temperatures, which are characteristic of plants that generally do not form persistent seed banks (Lesica 1990). H. aquatilis germination was 2-3 times greater under an above-freezing, fluctuating temperature regime, possibly the consequence of a mechanism that prevents germination of seeds that are buried too deep (Lesica 1990). While H. aquatilis requires submersion to grow and reproduce, it cannot germinate under anaerobic conditions (Lesica 1990).
• The seed bank peaks in September, immediately after seed dispersal. By mid-October, approximately 25 percent of the seed bank has germinated, and the seed bank is reduced to 40% of the September peak. Disease and predation reduce the seeds back to 10 percent of the September peak by mid-May (Lesica 1990).
• Optimal development for H. aquatilis occurs in ponds that dry down in wet years but do not dry up too early in dry year (Lesica 1990, 1992).
• Relatively shallow organic and mineral sediments with high levels of nutrient availability provide optimal H. aquatilis habitat. Disturbances that cause a decrease in pond fertility will have strongly adverse effects on H. aquatilis populations (Lesica 1990). Germination trials in a variety of soils indicate that H. aquatilis has difficulty completing its lifecycle in non-organic substrates (Lesica 1992).
• Water pH and water conductivity are often highly related, and are probably important in determining the distribution of H. aquatilis, but not the abundance in an area (Lesica 1990).
• Howellia aquatilis is negatively associated with dissolved solids, indicating that this species is probably restricted to fresh water (Lesica 1992).
• Abundant graminoid cover may exclude H. aquatilis from some ponds (Lesica 1992).
• Individuals/population can vary dramatically annually. Monitoring showed that in 1985 and 1987, 10,000 plants established, but in 1986 there were fewer than 100 plants (Lesica et al. 1988).
• Transplant experiments were preformed to determine the viability of transplanting seedlings into ponds that still contain water (H. aquatilis will not germinate in anaerobic environments). Two years after the plantings, plants were still present at the two transplantation sites. However, one of the sites experienced an 85% decrease in population side between the first and second year of monitoring. Additionally, while the sites contained reproductive plants after the first year, no new plants were apparent after the second year. The possibility of further decline, followed by local extinction, is high for both ponds. If the populations persist, these results would suggest that limited dispersal events keep H. aquatilis from establishing in suitable ponds. If they do not persist, it is possible that transplanted ponds were unsuitable for H. aquatilis over the longer term (Roe and Shelly 1991).
• Protein electrophoresis was used to examine the genetic structure of 4 populations. 8 enzymes on 18 loci were encoded. All the loci were monomorphic for the same allele, both within and between populations. H. aquatilis appears to have only one homozygous genotype throughout its entire range It is likely that these populations have not had enough time since separation to diverge genetically. It is difficult to determine if these plants are rare because they are genetically impoverished or vice versa (Lesica et al. 1988).

Current Management Summary
  • Listed as Threatened under the Endangered Species Act. Critical habitat was not designated, because the Fish and Wildlife Service was concerned about the publication of site-specific maps of critical habitat (USFWS 1994).

Research Management Needs
  • Continue inventory studies (WNHP 1999)
• Continue population and transect monitoring (Shelly 1992).
• Research seed bank dynamics further. Seed bank production is likely to be higher in years when the pond retains more water, but subsequent effects of high water level on seed bank persistence is unknown (Shelly 1992). Research the longevity of seed viability, especially in regards to wetness/dryness of the environment (USFWS 1996, Rush and Gamon 2000).
• Detailed study of population dynamics in relation to pond drying and other climatic influences (Shelly 1992, Rush and Gamon 2000). Also, population dynamics in relation to snowpack depth, annual precipitation and temperature patterns (USFWS 1996).
• Further study the effect of predation and disease on seed bank and population (Shelly 1992)
• Study the successional pathways and rates of emergent freshwater ponds. Determine if artificial habitat maintenance can be used to maintain populations (USFWS 1996).
• Study the degree of threat posed by reed canary grass (Phalaris arundinacea) (Rush and Gamon 2000).
• Investigate the relationship between nutrient availability and the abundance of Howellia (Shelly 1992).
• Maintain a forested buffer with a minimum width of 300ft around aquatic habitats. Logging and vegetation disturbing activities in this buffer should be minimized, but activities may be necessary to maintain the buffer (management for mountain pine beetle kill, prescribed burning) (Shelly 1992). The potential effects of prescribed burning should be examined prior to burning areas with H. aquatilis. Early season burns likely do not have an adverse effect, because the ponds still contain water (USFWS 1996).
• Create and maintain corridors between ponds for animal movement to facilitate seed dispersal and maintain metapopulation dynamics (Shelly 1992).
• Determine the mechanisms by which seeds are dispersed between ponds (Rush and Gamon 2000)
• Check suitable but currently unoccupied ponds to determine if species is colonizing new sites (USFWS 1996).
• Preservation of sites that have a broad scope of ponds at different stages of succession, providing suitable colonization habitats as ponds that currently contain H. aquatilis become inappropriate (Lesica 1988).
• Restore populations in areas where the plant was know to occur historically (Lesica 1988).
• Model rates of colonization and extirpation (Rush and Gamon 2000).

Ex Situ Needs
  • Seeds cannot be saved using conventional means (i.e.. Drying and freezing.) Determine successful means of maintaining viable seed or plant material.
• Collect and store seeds or plant material.


Books (Single Authors)

Abrams, L.; Ferris, R.S. 1944. Illustrated Flora of the Pacific States: Washington, Oregon, and California. Stanford, CA: Stanford University Press.

Eastman, D.C. 1990. Rare and Endangered Plants of Oregon. Beautiful America Publishing Company. 194p.

Meinke, R.J. 1982. Threatened and Endangered Vascular Plants of Oregon: An Illustrated Guide. Portland, Oregon: U.S. Fish & Wildlife Service, Region 1. 326p.

Nakamura, Gary; Kierstead Nelson, J. 2001. Illustrated Field Guide to Selected Rare Plants of Northern California. University of California, Agriculture and Natural Resources. Publication 3395. 370p.

ONHP. 2001. Rare, Threatened and Endangered Plants and Animals of Oregon.

Books (Sections)

Pavlovic, N.B. 1994. Disturbance-dependent persistence of rare plants: anthropogenic impacts and restoration implications. In: Bowles, M.L.; Whelan, C., editors. Recovery and Restoration of Endangered Species. Cambridge University Press. Cambridge. p 159-193.

Rush, T.E.; Gamon, J. 2000. Habitat characteristics and associated difficulties with inventory and monitoring an annual aquatic plant, Howellia aquatilis Gray. In: Reichard, S.H.; Dunwiddie, P.W.; Gamon, J.; Kruckeberg, A.R.; Salstrom, D.L., editors. Conservation of Washington's Rare Plants and Ecosystems. Washington Native Plant Society. Seattle. p 224.

Conference Proceedings

Isely, D. Water howellia - Known ecology of the California disjuncts. North Coast Chapter of the California Native Plant Society. Ecology and Management of Rare Plants of Northwestern California--Rare Plant Symposium Presentations; February 6-8, 2002; North Coast Inn, Arcata, CA. 2002.

Electronic Sources

(2002). Rare Plants in Washington, and Research. The Rare Plant Care and Conservation Program at the University of Washington's Center for Urban Horticulture. http://depts.washington.edu/rarecare/index.htm. Accessed: 2002.

CDFG. (2001). Special Vascular Plants, Bryophytes, and Lichens List. Biannual Publication, Mimeo. 141 pp. California Department of Fish and Game, Natural Diversity Database. Accessed: 2001.

ICC. (1995). Idaho EO database. Idaho Conervation Center. Idaho Fish and Game Department.

ICDC. (2001). Online Blue Book. Idaho Conservation Data Center. http://www2.state.id.us/fishgame/info/cdc/cdc.htm. Accessed: 2002.

ONHDB. (2000). Oregon Natural Heritage Program Database. Portland, Oregon.

TNC. (2002). Where We Work: Swan River Oxbow Preserve - Swan Valley, Montana. The Nature Conservancy - Montana. http://nature.org/wherewework/northamerica/states/montana/preserves/art346.html. Accessed: 2002.

WNHP. (2000). Washington Natural Heritage Program Database. Olympia, Washington.

Journal Articles

1879. (Original Publication). Proceedings of the Am. Acad. of Arts and Sci. 15: 43.

Isely, D. 1997. Rediscovery of Water Howellia for California. Fremontia. 25: 29-32.

Lesica, P. 1992. Autecology of the endangered plant Howellia aquatilis: implications for management and reserve design. Ecological Applications. 2, 4: 411-421.

Lesica, P. 1997. Spread of Phalaris arundinacea adversely impacts the endangered plant Howellia aquatilis. Great Basin Naturalist. 57, 4: 366-368.

Lesica, P.; Leary, R.F.; Allendorf, F.W.; Bilderback, D.E. 1988. Lack of genetic diversity within and among populations of an endangered plant, Howellia aquatilis. Conservation Biology. 2: 275-282.

McCune, B. 1982. Noteworthy collections - Montana: Howellia aquatilis. Madroρo. 29: 123-124.

McMahan, L.R. 2000. The Endangered Plants of Portland & Surrounding Areas. The Berry Botanic Garden Newsletter. 13, 1: 4-5.

Ripley, J.D.; Leslie, M. 1996. Defense Department's Biodiversity Initiative. Endangered Species Technical Bulletin. XXI, 5

Schultheis, L.M.. 2001. Systematics of Downingia (Campanulaceae) based on molecular sequence data: Implications for floral and chromosome evolution. Systematic Botany. 26, 3: 603-621.

Shelly, J.S. 1988. Distribution and status of Howellia aquatilis A. Gray (Campanulaceae) in Lake and Missoula Counties, Montana. Proceedings of the Montana Academy of Science. 48: 12.

Shelly, S. 1995. Eccentric aquatic (conservation of endangered Howellia aquatilis). American Horticulturist. 74: 11.

Shelly, S. 1995. Howellia aquatilis: Montana's First Federally Listed Plant Species. Kelseya, Newsletter of the Montana Native Plant Society. 8, 2: 1, 6.

Sjogren, P.; Wyoni, P.I. 1994. Conservation genetics and detection of rare alleles in finite populations. 8, 1: 267-270.

USFWS. 1994. The plant, water howellia (Howellia aquatilis), determined to be a Threatened species. Federal Register. 59, 134: 35860-35864.


Bursik, R.J. 1995. Update: Report on the conservation status of Howellia aquatilis in Idaho. Boise, Idaho: Conservation Data Center, Idaho Department of Fish and Game, Natural Resource Policy Bureau. through Section 6 funding from U.S. Fish and Wildl.

Gamon, J. 1992. Report on the status of Howellia aquatilis Gray. Olympia, WA: Washington Natural Heritage Program. Department of Natural Resources. p.46.

Lesica, P. 1990. Habitat requirements, germination behavior and seed bank dynamics of Howellia aquatilis in the Swan Valley, Montana. Helena, MT: Flathead National Forest. Conservation Biology Research. p.44 + appendix. Unpublished report.

Lesica, P. 1994. Monitoring Howellia aquatilis at Swan River Oxbow Preserve: 1993 progress report. Helena: Montana Nature Conservancy. p.5. Unpublished report.

Lichthardt, J.; Moseley, R.K. 2000. Ecological assessment of Howellia aquatilis habitat at the Harvard-Palouse River flood plain site. Boise, ID: Prepared for the Idaho Department of Parks and Recreation by the Idaho Department of Fish and Game, Idaho Conservation Data Center. p.14 + appendices. Unpublished report.

MTNHP. 2002. Montana Rare Plant Field Guide. Montana Natural Heritage Program.

Roe, L.S.; Shelly, J.S. 1992. Update to the statues review of Howellia aquatilis: field surveys, monitoring studies, and transplant experiments - 1991. Unpublished report to Flathead National Forest, Kalisell, Montana. Montana Natural Heritage Program, Helena. p.57.

Schassberger, L.A.; Shelly, J.S. 1991. Update to the status review of Howellia aquatilis, field surveys, monitoring studies, and transplant experiments. Unpublished report to Flathead National Fores, Kalisell, Montanta. Montana Natural Heritage Program, Helena. p.57. Unpublished report.

Shelly, J.S.; Moseley, R.K. 1988. Report on the conservation status of Howellia aquatilis, a candidate threatened species. Regions 1 an 6, U.S. Fish and Wildlife Service. p.43. Unpublished report.

Shelly, J.S.; Schassberger, L.A. 1990. Update to the statues review of Howellia aquatilis: field surveys, monitoring studies, and transplant experiments - 1989. Unpublished report to Flathead National Fores, Kalisell, Montanta. Montana Natural Heritage Program, Helena. p.50.

USFWS. 1996. Water howellia (Howellia aquatilis) recovery plan: public and agency review draft. Helena, Montana: U.S. Fish & Wildlife Service.

WNHP. 1999. Field Guide to Selected Rare Vascular Plants of Washington. Produced as part of a cooperative project between the Washington Department of Natural Resources, Washington Natural Heritage Program, and the U.S.D.I. Bureau of Land Management, Spokane District.


Reeves, Matt. 2001. Hydrologic Controls On The Survival Of Water Howellia (Howellia aquatilis) And Implications Of Land Management, Swan Valley, Montana. [M.S.]: The University of Montana. Missoula, Montana.

Rice, D. J. 1990. An application of restoration ecology to the management of an endangered plant, Howellia aquatilis. [M.S.]: Washington State University. Pullman, WA. 85p.

  This profile was updated on 7/8/2010
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