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Weed Science

Tropical Signalagrass 1

Tropical signalgrass (Urochloa subquadripara) is a blanket-like grass which has become a big problem in sod, golf courses, and lawns.

Although it is killed by frost, for much of southern Florida it continues to spread vegetatively in successive years.  On the golf course, tropical signalgrass stands out in the fairways because the patches are taller and often darker than 419 bermudagrass.  In the home lawn, tropical signalgrass blends in well enough that it can spread undetected for years, but up close it is ugly, and if it is killed by frost it leaves behind an unsightly brown mess.  Sod farmers have struggled with tropical signalgrass since the late 1980s.  It is essentially resistant to atrazine and asulox.

Tropical signalgrass has gotten away with its act partly because people have not known what it is.  It is called smallflowered Alexandergrass in Weeds of Southern Turfgrasses.  Most golf course superintendents call it bullgrass, which could be confused with another weed more commonly called bullgrass, or bull paspalum, Paspalum setaceum.  Some golf course superintendents call it crabgrass.  Sod farmers often call it signalgrass.

Inflorescences (seedheads) of tropical signalgrass (left) and crabgrass (center).  Both are branched, but the racemes of tropical signalgrass emerge from nearly right angles, and are distantly spaced.  In contrast, the racemes of crabgrass fan out from nearly a point.  The inset at the far right shows that spikelets of tropical signalgrass (on the left) are more rounded, and widest near the middle, while the crabgrass spikelets at the far right are pointed and widest below the middle.

To be more specific, tropical signalgrass roots from the nodes (unlike true bullgrass) and has more papery leaves compared with the soft leaves of crabgrasses. The spikelets are relatively long, 3.5 to 4 mm, compared with our crabgrasses, or Digitaria spp., whose spikelets are narrower and not more than 3 mm long. The inflorescence or seedhead consists of modified racemes that emerge at right angles to the culm, and they are spaced much like the signal flags used on ships. In contrast, the racemes of the crabgrass infloresence are "digitate," emerging from the same point of attachment, like the figures on a hand. The leaf of tropical signalgrass, Urochloa subquadripara, is generally glossy and pointed.

The environmental range of tropical signalgrass is very broad. It can compete in dense lawns of St. Augustinegrass, as well as closely mown fairway bermudagrass. Tropical signalgrass, once established, persists in the driest blocks of sod, and it can also thrive in wet areas.

For getting rid of it, there is presently nothing available for St. Augustinegrass home lawns.

It is relatively easy to control tropical signalgrass on the golf course, but some of the products used on it are ineffective. Illoxan is ineffective and does not even discolor tropical signalgrass in research plots. Drive (quinclorac) is labeled for control of signalgrass, but in my research has very little effect, discoloring the tropical signalgrass very minimally. In contrast, tank mixtures of MSMA + Sencor (metribuzin) have been quite effective. Anecdotal reports by golf superintendents have suggested that particular adjuvants have helped, but I find that MSO (methylated seed oil) is sufficient.

In research plots that I established at Oak Tree Country Club, with the cooperation of Bill McKee, 13 days after the first herbicide application there was no significant effect from either Drive or Illoxan. MSMA treatments (including treatments with Sencor) showed an average 65% injury to signalgrass. It is interesting that tropical signalgrass has become a major problem during the last decade in which Illoxan has grown to be the chemical of choice for goosegrass control on golf course fairways, and even greens.