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*Preemergence Annual Bluegrass Control in Turfgrass*

Dr. Patrick McCullough, University of Georgia Extension Weed Specialist



Annual bluegrass (*Poa annua*) is a major problematic weed in turfgrass that
begins to germinate this time of year.  Compared to most turfgrasses, annual
bluegrass has a lighter green color, coarser leaf texture, and produces
unsightly seed heads.



Contrary to its name, both annual (live for one season) and perennial (live
for many seasons) biotypes of annual bluegrass may be found in turf.
 Perennial biotypes will be more prevalent:



·        On highly maintained lawns that receive frequent irrigation and
high nitrogen fertilization.

·        In shady or highly trafficked lawn areas with compacted soil.



While the two biotypes may not be easily distinguished from each other,
annual types are more upright in growth and produce greater seed than
lower-growing perennial types.



Annual bluegrass seed germinates in late summer/early fall once soil
temperatures fall below 70° F.  Seedlings mature in fall, overwinter in a
vegetative state, and produce seed in late spring and early summer.  Annual
bluegrass is a prolific seed producer as individual plants may produce over
360 viable seeds even when closely mowed.  Seed may lie dormant in soil for
many years before germinating.  Annual bluegrass flowers and produces viable
seed in spring and at virtually any mowing height.  Annual bluegrass grows
well under short day lengths and cool conditions, and may out-compete other
turf species during late fall and early spring.  Annual bluegrass often dies
from summer stresses but may survive if irrigated and pests are adequately
controlled, especially perennial biotypes.



*Cultural Control*

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Several cultural practices improve annual bluegrass control.



   - Deep and infrequent irrigation encourages turfgrass root development
   which improves the ability of desired grasses to compete with annual
   bluegrass.
   - Withhold water until desirable turfgrass species exhibit initial
   drought stress symptoms.  Overwatering, especially in shady areas, will
   predispose turfgrass to annual bluegrass invasion.
   - Avoid practices that promote soil compaction.  Relieve compaction with
   regular aerifications in spring and fall.
   - Voids left in turf with exposed soil following aerifications may permit
   annual bluegrass invasion during periods of peak germination.  Time
   aerfications in early fall to allow turf to recover before annual bluegrass
   germinates.
   - Reduce nitrogen fertilization during peak annual bluegrass germination
   and during periods of vigorous growth (cool weather).  High nitrogen at
   these times encourages annual bluegrass spread and survival in to winter and
   spring.  Fertilizing dormant turfgrasses when annual bluegrass is actively
   growing will exacerbate infestations.**
   - Lower mowing heights encourage annual bluegrass invasion.  Height of
   cut for lawns should be no less than 2 inches.  **
   - Mow lawns at least once per week during periods of vigorous growth to
   prevent scalping.  Scalping thins out turf enabling weeds such as annual
   bluegrass to establish. While returning clippings is recommended to recycle
   nutrients to the soil, removal of clippings may be useful when annual
   bluegrass is present and producing seed heads. Removing clippings at this
   time will reduce the spread of viable seed through the lawn.**



*Chemical Control*

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Preemergence herbicides may prevent annual bluegrass infestation via seed
and limit current infestations from further spreading.  However,
preemergence herbicides will not eradicate established plants and will not
effectively control perennial biotypes of annual bluegrass from spreading
vegetatively.  Application timing of preemergence herbicides for annual
bluegrass control is very important, and thus herbicides must be applied in
late summer/early fall before annual bluegrass germination.  A second
application can be applied in spring to control germinating plants.  Fall
applied preemergence herbicides cannot be used if reseeding or resodding is
needed to repair areas of damaged turf within several months after herbicide
applications.



Several preemergence herbicides effectively control annual bluegrass in fall
and winter which are similar to products used for summer annual weed
control.  These herbicides include dithiopyr (Dimension), oxadiazon
(Ronstar, Starfighter), pendimethalin (Pendulum, others), and prodiamine
(Barricade, others).



Combination herbicide products are also available which may improve efficacy
of applications.  These products include oxadiazon plus bensulide
(Anderson’s Crab and Goose) and benefin plus oryzalin (Team 2G or Team
Pro).  Many preemergence herbicides are available under a wide variety of
trade names and formulations, and thus, turf mangers should carefully read
and follow label instructions before applying products.**

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Most preemergence herbicides will provide similar initial efficacy if
applied before annual bluegrass germination and sufficient rain or
irrigation is received.  Preemergence herbicides require incorporation from
irrigation or rainfall so that weeds may absorb the applied material.  In
order to effectively control annual bluegrass, preemergence herbicides must
be concentrated in the upper 1/3 inch of the soil profile.  Herbicide
retention on leaf tissue can be avoided by irrigating turf immediately after
applications for effective soil incorporation.**

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Preemergence herbicide applications on non-irrigated sites have less
potential for successful residual control compared to irrigated turf.  Turf
managers should return clippings on non-irrigated sites to help reduce
herbicide retention on leaf tissue and incorporate herbicide concentration
into the soil.  If clippings are collected as part of routine maintenance,
turf managers should consider returning clippings until at least half to one
inch of rainfall is received in order to move the herbicide off leaf tissue.



Turf managers applying liquid formulations of preemergence herbicides to
non-irrigated sites should use high water volumes (>100 gallons per acre) to
reduce foliar contact and increase soil water concentration with the
herbicide.  Applying dry granular products on non-irrigated sites may reduce
contact with turfgrass leaf tissue for more effective soil incorporation.
Granular products may be easier to handle and apply with less equipment
necessary than sprayable formulations.  Granular herbicides should be
applied when morning dew is no longer present to avoid interference from
turfgrass leaf tissue.

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-- 
Dr. Teri Hamlin
North Region Agriculture Education
Georgia Department of Education
204C Four Towers University of Georgia
Athens, Ga 30602
706-542-3679 / 706-540-0032
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