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Weed Alert!

Glyceria declinata Brébiss.
(syn. Glyceria fluitans subsp. declinata (Brébiss.) O. Bolòs, Masalles, & Vigo,
G. fluitans var. declinata (Brébiss.) Ghisa,
G. notata subsp. declinata (Brébiss.) E.J. Weeda,
G. plicata subsp. declinata (Brébiss.) E.J. Weeda,
Glyceria plicata var. declinata (Brébiss.) Druce)
(Eurasian waxy mannagrass, small mannagrass, sweet grass)

Summary: Known invader sighted in new areas
Glyceria declinata has invaded deep vernal pools, swales, ditches, and stock ponds along the entire eastern side of California's Central Valley, from Shasta County south to Fresno County. It is also present in vernal pools in Mendocino County. It is adapted to long periods of inundation and typically grows from the Lolium multiflorum ring that lines the high water mark, to the bottom of the pools where Eleocharis, Eryngium, Psilocarphus, and Downingia are found.

(Click on thumbnail images for a closer view)

Floating leaves

Clump w/rare
Orcuttia viscida

Sparse infestation,
maturing plants

Dense infestion,
maturing plants

Prostrate plants

May 2006--was
2% cover in 2001

Fibrous roots,
copious seed

Feeding Canada

Note palea &
lemma tips

Lolium (left),
Glyceria (right)

As a native in
a Spanish playa

Distribution in
vernal pools
All above images by John Gerlach, except:
glyde11.jpg ("Note palea & lemma tips"/Fred Hrusa)
glyde09.jpg ("As a native in a Spanish playa"/Juan Gallego Fernández)

Perennial, culms 10-45 cm long. Leaf-sheaths tubular for much of their length; ligule an eciliate membrane 4-9 mm long, acute; leaf-blades flat, or conduplicate; 3-18 cm long; 1.5-8 mm wide. Leaf-blade surface smooth. Leaf-blade margins scabrous. Panicle open; linear, or lanceolate; equilateral, or nodding; 4-30 cm long. Spikelets ascending, or appressed; solitary. Glume apices obtuse. Lemma and palea apices dentate. (Summarized from [1].)

While G. declinata is described as a perennial species, all observations from the Central Valley of California indicate that it is either an annual genotype or growing as a facultative annual. In the Central Valley the spikelets mature from late April through May and shatter at maturity coating the ground below the plant with seed.

To the casual eye, G. declinata can easily be mistaken for the non-native Lolium multiflorum, as is illustrated in the image above that shows both in the same photograph.

Scientific and Common Names:
The genus name Glyceria is derived from the Greek name for sweet, for the taste of the grain which is relished by waterfowl and which has been harvested by humans for millennia.

Because the Jepson Manual [2] omits G. declinata and incorrectly describes the lemma tip of G. occidentalis as jagged or irregular, there has been some confusion regarding whether G. declinata from the Central Valley is in reality G. occidentalis. Glyceria declinata is correctly described in Munz [3]. Another complication with correctly identifying Glyceria species is that the morphological characteristics used to identify them are very similar and variable. As part of the development of the Poaceae section of The Flora of North America [4], Ian Wipple, Mary Barkworth, and Shaun Bushman developed chloroplast molecular markers to help distinguish species among herbarium collections of several Glyceria species [5]. They tested herbarium samples from Humboldt County, the Sierra Nevada foothills, and a degraded vernal pool in Sacramento County and all samples were determined to be G. declinata. Using their markers, Harald Meimberg and John Gerlach recently completed an analysis of six vernal pool populations from Sacramento County and Stanislaus County and all populations were determined to be G. declinata (unpublished). From these two studies and from observations of lemma characteristics of field samples from other counties, it appears that the only Glyceria species present in the Central Valley is G. declinata.

Native Range:
Europe: northern, central, southwestern, southeastern, and eastern. Africa: Macaronesia. Glyceria declinata is widespread in Europe and in areas with temperate climate it is considered a colonizer of mud flats and banks along slow moving rivers and streams and along the shores of lakes and ponds. It generally grows in the less inundated areas between the more inundation-tolerant G. fluitans and upland vegetation.

In the Mediterranean climate areas of Spain and Portugal G. declinata is often a dominant native species of vernal pools [6] and playas and a weed of rice fields in Spain and Portugal where it appears to behave as an annual species.

Range as an Invader:
In California, it is known to occur in Shasta, Tehama, Butte, Yuba, Placer, Sacramento, San Joaquin, Stanislaus, Fresno, Mendocino Sonoma, Alameda, and counties. In the USA it is also recorded from Louisiana and New York [7].

It also occurs in Australia and New Zealand.

Reproduction and Methods of Dispersal:
Glyceria declinata was first identified in California in 1953 by Beecher Crampton from a plant he collected from a vernal pool in Stanislaus County [8] and, according to vernal pool botanists [9], during the last twenty years it has rapidly spread through rice fields, vernal pools, cattle ponds, and roadside ditches.

In 2001 during a vernal pool vegetation classification project, a survey crew found it in a large vernal pool on the Lane Ranch in San Joaquin County but identified it as G. occidentalis [10]. In 2001, the cover of Glyceria in the pool was 2%. When John Gerlach surveyed the pool in May 2006 the plants had lemma morphology consistent with that of G. declinata and the cover was greater than 90% in spite of being recently grazed. The extensive and complete cover and the lack of flowering native species suggests that this is not just an annual fluctuation in population size.

Waterfowl are strongly attracted to maturing plants and strip the seed from the culms with their bills and they probably function as primary long distance seed dispersal vectors (pers. observation). Seed dispersal by waterfowl has probably increased dramatically as nonmigratory populations of Canada geese (Branta canadensis) have exploded in California during the last 20 years. Humans and wildlife disperse the seed over shorter distances as seed readily adheres to wet pants and boots.

The invaded vernal pools are some of the most important natural resources in California and contain both federal and state listed endangered plant and animal species. Dense G. declinata invasions appear to eliminate or significantly reduce populations of all native annual plant species, such as endangered Orcuttia viscida (Sacramento orcutt grass) from the vernal pools. It was rated a weed of "Moderate" impact by the California Invasive Plant Council [11].

Its population dynamics in vernal pools are erratic but it often establishes dense stands of large plants which both shade-out endemic species and eliminate the bare ground they require for germination and establishment. It produces an enormous amount of fine root mass on or just under the surface of the soil and a large leaf mass. These biomass changes probably effect nutrient cycling in the vernal pools and negatively impact vernal pool hydrology through increased transpiration.

While it has been collected from rice fields in Butte County for 20 years, it has recently become problematic enough to be considered a winter and spring weed and its dense populations make it more difficult to cultivate the fields for planting.

Control methods in vernal pools with sensitive status species will be a complex regulatory problem and field eradication efforts will require persistence. All methods should target existing seed and adult plants and reduce the spread of seed by animal and human vectors. Hand weeding prior to spikelet shattering is effective for small populations in high value vernal pools and must be continued annually until the seed bank is completely exhausted. Surrounding vernal pools and swales should also be weeded to prevent the rapid reintroduction of seed. Reducing seed spread can be accomplished by excluding humans and domestic animals from G. declinata populations from the beginning of spikelet shattering until all seed has been dropped and the vernal pools are completely dry. It is critical that all equipment used in both natural and created vernal pools is absolutely free of adhering seed or soil. Additionally, artificial ponds that attract waterfowl should be eliminated to reduce site desirability. Stock ponds may need to be replaced with other water sources that do not attract waterfowl or support G. declinata. Glyceria species are generally regarded as species with high soil and water fertility requirements so precautions should be taken to prevent the addition of nutrients in any form.

[1] Clayton, W.D., K.T. Harman, and H. Williamson (2006). World Grass Species: Descriptions, Identification, and Information Retrieval. http://www.kew.org/data/grasses-db.html. (accessed 20 August 2006).

[2] Hickman, J.C. (ed.) (1993) The Jepson Manual: Higher Plants of California. University of California Press, Berkeley.

[3] Munz, P.A. (and Keck, D.D.) (1968) A California Flora and Supplement. University of California Press, Berkeley.

[4] M.E. Barkworth (In preparation). Flora of North America.

[5] Whipple, I.G., M.E. Barkworth, B.S. Bushman (2006) Am. J. Bot. (submitted).

[6] J. A. Molina (1996). Sobre la vegetación del los humedales de la Peninsula Ibérica (1. Phragmiti-Magnocaricetea). Lazaroa 16:27-88.

[7] Plants Database, http://plants.usda.gov. Accessed August 2006.

[8] University of California Davis Herbarium specimen UCD36270.

[9] L. Aheart, J. Dittes, C. Witham, pers. communication.

[10] Barbour, M.G., A.I. Solomeshch, R.F. Holland, C.W. Witham, R.L. Macdonald, S.S. Cilliers, J.A. Molina, J.J. Buck, J.M. Hillman (2005).Vernal pool vegetation of California: communities of long-inundated deep habitats. Phytocoenologia 35: 177-200.

[11] http://portal.cal-ipc.org/files/PAFs/Glyceria%20declinata.pdf

--John Gerlach/Environmental Science Associates
--Edited: Barry Rice/Global Invasive Species Team; August 2006

Updated August 2006
©The Nature Conservancy, 2006