Purple nutsedge, (Cyperus rotondus) is a troublesome weed in Arizona at elevations of 3000 feet or less. It is often found in irrigated crops and irrigated turf.

Part of the problem with purple nutsedge as a weed is that "what you don't see can hurt you!" This is due to the fact that nutsedge produces an abundance of underground storage organs (nutlets) which are botanically described as tubers. Just like a potato, the tubers are storage organs which have 4-6 "eyes". Each "eye" allows shoots to resprout and reach the soil surface.

The first flush of purple nutsedge first appears just about the same time that the bermudagrass greens up in the spring. This occurs because 1 or 2 of the eyes from nutlets sprout and elongate towards the soil surface. Once they reach they reach the soil surface, a basil bulbil is formed, just like an onion bulb (actually compressed leaves). The bulbil produces the triangular ranked foliage that is typical of nutsedge. Quickly, the bulbils send out short white underground runners which reach the soil surface and form more bulbils, and more nutsedge plants.

So in the early spring, the purple nutsedge is maximizing the amount of competitive cover it can develop, and at the same time, minimize the amount of nutlets that sprout. By the last half of summer, the nutsedge starts to form more underground tubers. This perpetuates the longevity and dedication of this beasty weed. By the beginning of fall, the existing foliage begins to fade and decline, only to reappear next year.

WEED CONTROL OR SUPPRESSION OF PURPLE NUTSEDGE
There are three things to consider in weed control/suppression of purple nutsedge:

1. Dedication

2. Observation

3. Repeat applications

Although some herbicides may be translocated too or be picked up by the nutlets, the overall population of nutlets in an established turf simply means that there will be a lot of growing points potentially reaching the surface. Therefore, the regrowth (observation) which follows after a treatment warrants additional treatments (repeat applications), once you see a new flush of growth (observation) observing the turf.

The trick becomes simply to keep the foliage of the weed to a minimum which thereby (1) minimizes food production and new nutlet formation and (2) forces existing nutlets to sprout. Repetition of removing the foliage (dedication) creates an unfavorable condition for the nutsedge.

The herbicides realistically remove the foliage chemically. I have done experiments that show some inhibition or delay of nutlet sprouting, but they do eventually sprout and reach the surface.

The chemicals that are most used for purple nutsedge suppression include MSMA and IMAGE (imazaquin). MSMA is of course, a contact herbicide which burns back the foliage. Regrowth occurs either from the basal bulb or from emerging growing points from the nutlets. Inclusion of a surfactant increases the rate and degree of damage to the nutsedge leaves, but it also damages the bermuda somewhat as well. The label rate of MSMA for purple nutsedge is 2.0 lbs. AI/A.

IMAGE (imazaquin) has some foliar, but mostly soil uptake. My experience has been to apply IMAGE in the early morning, let it dry on the leaves for 3-4 hours, and then immediately water it in. The IMAGE slowly kills the leaves in about 10-14 days at the 0.5 lb. AI/A label rate. In an additional 14-21 days, a repeat application is often necessary. IMAGE will cause bermudagrass to sometimes turn apple green for 3-4 days after application, an it will reduce clipping production for up to 14 days afterwards. IMAGE is not labelled for use on cool season grasses. It will knock out Kentucky bluegrass, ryegrass, fescues and bentgrasses completely at the label rates. IMAGE is produced by AMERICAN CYANAMID COMPANY.

An experimental chemical from MONSANTO is currently in use via an experimental use permit (EUP) in Arizona. Unlike the IMAGE herbicide, the MONSANTO product is taken up almost exclusively through the foliage. Therefore, it should not be irrigated after application (as the same with MSMA). The MONSANTO product (proposed name is MANAGE) can be used on both bermudagrass and other cool season grasses. MANAGE is reported to have good activity on both purple and yellow nutsedge. Yellow nutsedge is Cyperus esculentum, which is more predominate at higher elevations -- where cool season turfs predominate. There is however, some yellow nutsedge in Phoenix.

Next months issue includes control of yellow nutsedge and tips on how to I.D. yellow and purple nutsedge from each other.