Glyceria declinata
(small sweet grass)

G. declinata is a perennial grass associated with shallow water. It is native throughout Europe and in Morocco. It has been introduced to, and is invasive in, North America, Australia and New Zealand.

In California, G. declinata has invaded deep vernal pools, swales, ditches and stock ponds, and is reported to be rapidly spreading in California's Central Valley (Cal-IPC, 2012). The invasion has led to a degradation of vernal pool ecosystems, the habitat of many federal and state protected endangered and threatened species (Gerlach et al., 2009), and  has also become a problem in rice plantations in Louisiana.

In Australia, Jessop et al. (2006) described the species as occurring along creeks and in seasonally flooded areas. However, G. declinata is not considered a serious threat there and is acceptable as an impurity in seed imported into Australia subject to certain conditions (AQIS, 2012). In New Zealand, G. declinata is found in wetlands, bogs, freshwater margins, lakes and streams, but is only considered problematic in Northland where its total control is required in designated community control areas, with the long term goal of preventing deterioration of areas with significant ecological and economic values.

Most Glyceria spp. grow in generally wet areas in many parts of the world. Species are found in Europe, Africa, temperate and tropical Asia, Australasia, the Pacific, North and South America (Clayton et al., 2012). G. declinata is native throughout Europe and in Morocco, and has been introduced to North Ameria, Australia and New Zealand.

Gerlach (2006) states that in areas with a temperate climate, G. declinata is considered a coloniser of mud flats, 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 flood-tolerant G. fluitans and upland vegetation.

Hubbard (1984) described the habitat of G. declinata in Britain as ‘on muddy or dried-up margins and in the shallow water of ponds, ditches and streams’. Gerlach (2006) reported that in areas of Spain and Portugal with a Mediterranean climate G. declinata is often a dominant native species of vernal pools, and that it occurs as a weed of rice fields in Spain and Portugal, where it appears to behave as an annual species. In California, the species is reported as invasive in deep vernal pools, swales, ditches and stock ponds (Cal-IPC, 2012). In Australia, too, the species occurs in wet and seasonally inundated places (Simon and Alfonso, 2012). In New Zealand, Edgar and Connor (2010) describe its habitat as ‘in damp ground in swamps, on stream margins, along drains and in damp pasture’.

Genetics

G. declinata is a diploid with a chromosome count of 2n=20 (Borrill, 1957b). G. declinata is highly self-fertile and the viability of the seed produced compared favourably with that of the open-pollinated seed from the original parents. Borrill (1957b) concluded that the species can be regarded as ‘largely inbreeding’. When he tried crossing (in both directions) the diploid G. declinata with the tetraploids G. fluitans or G. plicata, these were completely unsuccessful as no signs of seed development were seen. However, Hubbard (1984) notes the presence of a very rare hybrid between G.declinata and G. fluitans in Hertfordshire, England, UK.

Reproductive Biology

Borrill (1957a; 1957b) studied the breeding systems and fertility relationships of G. declinata and two other British species of Glyceria. He found that all five of the populations of G. declinata studied flowered in the first season (flowers emerged between 20 July and 1 August), presumably after planting the previous autumn. 

In the Central Valley of California the spikelets mature from late April through May and shatter at maturity, coating the ground below with seed (Gerlach, 2006).

Borrill (1957b), in three British species of Glyceria, observed that anthesis occurs in the morning, the terminal spikelets being the first to mature followed by the next spikelet down, and so on. Within each spikelet the florets mature strictly from base to apex. In G. declinata the first sign of anthesis is the slight separation of palea and lemma on a basal floret, followed by the opening of up to six florets in the same spikelet. The lemma and palea diverge widely as the filaments elongate and the short feathery stigmas protrude slightly. Dehiscence of the small anthers is by two longitudinal slits; after shedding pollen the anthers curl up and become shorter. The stigmas appear to be fully receptive when the pollen grains are shed. Pollinated or not, the florets remain open for up to 10 minutes.

Physiology and Phenology

Borrill (1957a) studied the variations in morphology of five populations of G. declinata sourced from different locations but grown from seed under uniform conditions. Plant bulk (dry weight, number of culms and shoots) formed the main feature of the differences between populations; floral differences were less pronounced. By contrast, different races of G. fluitans were more variable, and differed from each other in spikelet, leaf and culm length.

Gerlach (2006) reported that ‘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.’

Distinguishing between G. declinata and other species morphologically is not always easy. Hubbard (1984) described G. declinata as 'distinguished from other British species of Glyceria by the 3-toothed or 3-5-lobed tips of the lemmas and by the sharply 2-toothed tips of the paleas'. In the USA, in distinguishing G. declinata from the similar native species G. occidentalis, Whipple et al. (2007) described G. declinata as ‘usually shorter and more decumbent than G. occidentalis, its panicle branches tend to be shorter, straighter, and have fewer spikelets than those of G. occidentalis, and its lemmas have two or more equal lobes on either side of the tip rather than inconspicuous, unequal lobes.’

Due to the variable regulations around (de)registration of pesticides, your national list of registered pesticides or relevant authority should be consulted to determine which products are legally allowed for use in your country when considering chemical control. Pesticides should always be used in a lawful manner, consistent with the product's label.

Cultural Control and Sanitary Measures

Excluding humans and domestic animals from populations of G. declinata ‘from the beginning of spikelet shattering until all seed has been dropped’ may help prevent the spread of the species (Gerlach, 2006). For vernal pools in California, the same author recommends exclusion of humans and domestic animals until the pools are completely dry, and that ‘all equipment used in both natural and created vernal pools is absolutely free of adhering seed or soil’ (Gerlach, 2006).

To prevent the spread of G. declinata by waterfowl, Gerlach (2006) suggests that artificial ponds which attract waterfowl should be destroyed or otherwise made unattractive to waterfowl, for example by changing water sources. Additionally, ‘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’ (Gerlach, 2006). 

Physical/Mechanical Control

In vernal pools in California, Gerlach (2006) recommends hand weeding prior to spikelet shattering as effective for small populations ‘in high value vernal pools’, although it must be carried out annually ‘until the seed bank is completely exhausted’. 'Surrounding vernal pools and swales should also be weeded to prevent the rapid reintroduction of seed' (Gerlach, 2006).

Chemical Control

Braverman (1996) tested herbicides for the control of G. declinata in rice in Louisiana and found that glyphosate, clethodim, sethoxydim or sulfosate all gave over 90% control. He also found that a range of post-emergent treatments increased rice yields substantially.

Hinds-Cook et al. (2007) tested herbicides for the control of Glyceria spp. (species not specified) in grass seed (Italian ryegrass, Lolium multiflorum, seedling perennial ryegrass, Lolium perenne, or seedling tall fescue, Schedenorus arundinacea) production fields in Oregon, where Glyceria spp. have developed resistance to ethofumesate. They found that the HPPD enzyme-inhibitors mesotrione, pyrasulfotole-bromoxynil and topramezone all gave good control of Glyceria spp.

Intergrated Pest Management

Gerlach (2006), in considering control in California’s vernal pools, warned that control methods will require persistence and may be a ‘complex regulatory problem’ when pools contain vulnerable or sensitive species.

25/01/2013: Original text by:

Ian Popay, consultant, New Zealand, with the support of Landcare Research.