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| Poa annua |
Perhaps you
have seen this invasive “winter grass” invade your lawn and flower beds at home
in past years. It is a clumpy, spring-green tuft of grass that first appears in
October, continues to grow denser and put out an abundance of seed mid to late
winter, and finally disappears as the weather warms up in May and June. Annual bluegrass, Poa annua, is native to Europe, but now found world-wide. It can grow 6-8 inches tall and can tolerate
mowing heights below 1/8 inch as seen on golf course putting greens. It has been estimated that Poa can produce as
many as 14,000 to 63,000 viable seeds per square foot in a year on a golf
course green! Seeds can remain dormant
for a few years, but with regular irrigation, most do not show much of a
dormancy period.
So, why is
Poa a problem, it is green after all. The biggest problem with Poa annua in the Central Valley is that it is a true annual
that will abandon you by early summer in full sun areas because it cannot
tolerate the heat. Germination will
start as early as August when soil temperatures start to drop below 70
degrees. Germination will continue
throughout the winter and by December (or before) you will start to see the
appearance of the open-panicle seedheads popping up. By February and March you will see the
yellow-green patches with seedheads across the lawn areas and throughout the
flower beds. Two to three months later
most of the plants will only be brown tufts scattered about. Just think about the poor golf course
superintendent that has to manage greens that are mowed daily at extremely
short heights, excessive summer heat, and golfers trampling over the turf
throughout the day. His job is to keep
alive what Poa he has and try to minimize its further spread.
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| Poa annua flowerhead |
So what can
be done? If possible, one should try to
limit the spread of Poa by not moving the seeds with mowers from bad turf to
good turf. Mowing when the grass is dry can
help somewhat, but certainly washing off the mower between lawns is obvious
during periods of seeding. Secondly, a
pre emergent herbicide should be applied mid to late August to reduce the
likelihood of seed germination. One must
also consider if the lawn is going to be overseeded or spot-seeded late
September or early October. The pre
emergent will possibly cause a poor or weak stand of new seedlings as the
herbicide will reduce root development.
There are a few options for selectively controlling Poa annua in existing lawns, but they are not available to
homeowners and generally they are specific for either warm or cool season
grasses.
Controlling Poa annua in lawns can be tricky,
but it’s a no-brainer to apply a pre emergent now in flower beds to interrupt
the life cycle this winter and save you a lot of work pulling weeds this
spring.
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