# *Poa annua*

**Annual bluegrass**

# Poaceae

*Poa annua* is an annual or biennial grass species that is abundant
worldwide. It is found at low elevations in California, especially near
the coast, and will flower continuously throughout the year when
moisture permits. It is thought to be native to Europe. *Poa annua* is
tetraploid; while it is widespread in all temperate regions throughout
the world, its putative diploid ancestors are restricted to parts of
Europe (Darmency and Gasquez 1997). *Poa annua* exhibits wide
morphological variability including annual erect and perennial prostrate
ecotypes (Darmency et al., 1992). Sterile triploid plants have been
reported in areas of Europe where populations of *P. annua* and *P.
supina* co-occur (Duckert-Henriod and Favarger 1987).

*Poa annua* is a common turfgrass species in golf turf; as such, it is
currently being bred to produce more seeds per panicle and have greater
pest resistance (Johnson and White, 1997a). The species is usually
considered to be annual because of characteristics including continuous
flowering throughout a growing season. However, a range of growth and
flowering habits actually exits, including true annuals, winter annuals,
biennials, and perennials. True annual genotypes appear to be rare
(based on Johnson and White's (1997a) survey of *P. annua* genotypes).
Vernalization may be the primary floral induction mechanism in this
species. Ten to twelve weeks of cold temperatures (4°C) was found by
Johnson and White (1997a) to induce a range of *P. annua* genotypes to
flower; those plants that received less than ten weeks of cold treatment
formed fewer, less fertile, and more deformed flowers.

The annual form of *P. annua* exhibits a winter annual life cycle, dying
out during warm and dry seasons. However, perennial populations are also
common. Many of these perennial types flower only in the spring
(May-June), then persist and are vegetative the remainder of the year
(Johnson and White, 1997b). The species is predominantly
self-pollinated. It is a grazing-tolerant grass; after the removal of
herbivores, cover decreases. In fact, large herbivores (e.g., reindeer)
have been implicated in the replacement of species on the sub-Antarctic
island of South Georgia by *P. annua* (Chown and Block 1997).

Because annual bluegrass invades golf courses and is considered an
undesirable turfgrass, there are a number of control methods that have
been tested in greenhouse and field experiments which are briefly
reviewed below. It is important to keep in mind, however, that results
from these intensively managed environments (i.e., regularly mowed and
fertilized) cannot be directly extrapolated to natural systems.

*Colletotrichum* spp. cause crown rotting anthracnose on *P. annua*
(Horvath and Vargas, 1997). Knott (1996) reported that a combination of
the herbicides monolinuron and pendimethalin (at a rate of 560 + 660 g
a.i. ha-1, respectively) were nearly 100% effective in controlling *P.
annua*. In a study of the efficacy of household compost, Ligneau and
Watt (1995) found that a 3-cm layer of household compost completely
suppressed emergence of *P. annua*. Leachate from the compost suppressed
emergence by 95%.

*Xanthomonas campestris* pv. *poannua* is a facultative parasite causing
bacterial wilt of annual bluegrass. However, its efficacy in controlling
*P. annua* may be limited, with infection rates as low as 11% in field
studies (Zhou and Neal 1995). Johnson and Murphy (1996) found that plant
growth regulators (paclobutrazol and flurprimidol) only mildly
suppressed the perennial subspecies of *P. annua*. However, a root- and
crown-infecting fungus, *Magnaporthe poae*, is one of the most
destructive pathogens of annual bluegrass, causing "summer patch"
(Melvin et al., 1993). Additionally, Crutchfield and Potter (1995)
provided evidence that root feeding larvae of the Japanese beetle
(*Popilia japonica* Newman) can significantly reduce root biomass of
annual bluegrass.

**Literature Cited:**

Chown, S. L. and W. Block. 1997. Comparative nutritional ecology of
grass-feeding in a sub-antarctic beetle: the impact of introduced
species on *Hydromedion sparsutum* from South Georgia. Oecologia
111:216-224.

Crutchfield, B. A. and D. A. Potter. 1995. Feeding by Japanese beetle
and southern masked chafer grubs on lawn weeds. Crop Science
35:1681-1684.

Darmency, H. and J. Gasquez. 1997. Spontaneous hyubridization of the
putative ancestors of the allotetraploid *Poa annua*. New Phytologist
136:497-501.

Horvath, B. J, and J. M. Vargas. 1997. Isozume variation of
Colletotrichum spp. causing crown rotting anthracnose of amenity
turfgrasses. (Annual Meeting of the American Phytopathological society,
rochester, New York, USA, August 9-13, 1997.) Phytopathology 87:S44.

Johnson, B. J. and T. R. Murphy. 1996. Suppression of a perennial
subspecies of annual bluegrass (*Poa annual* spp. *reptans*) in a
creeping bentgrass (*Agrostis* *stolonifera*) green with plant growth
regulators. Weed Technology 10:705-709.

Johnson and White. 1997a. Flowering responses of selcted annual
bluegrass genotypes under different photoperiod and cold treatments.
Crop Science 37:1543-1547.

Johnson and White. 1997b. Vernalization requirements among selected
genotypes of annual bluegrass (Poa annua L.). Crop Science 37:1538-1542.

Knott, C. M. 1996. Evaluation of herbicides for weed control in runner
beans (Phaseolus coccineus). Tests of Agrochemicals and Cultivars
17:38-39.

Ligneau, L. A. M. and T. A. Watt. 1995. The effects of domestic compost
upon the germination and emergence of barley and six arable weeds.
Annals of Applied Biology 126:153-162.

Melvin, B. P., M. G. Nair, J. M. Vargas, and A. R. Detweiler. 1993.
Controlling annual bluegrass (Poa annua L.) summer patch disease with
Faeriefungin. Hortscience, 28(3):195-196.

Zhou, T. and J. C. Neal. 1995. Annual bluegrass (*Poa annua*) control
with *Xanthomonas* *campestris* pv. *poannua* in New York State. Weed
Technology 9:173-177.