Once the daylight hours lengthen, flowers develop on nutsedge.
Last week I spent Wednesday through Friday visiting the Conservation Innovation Grant plantings that Tara Baugher set up back in 2008. One weed problem I saw in a number of orchards and was told by others that seems to be a bigger problem this year was yellow nutsedge (Cyperus esculentus). Nutsedge is an erect grass looking perennial that is actually a member of the sedge family. There is also a purple nutsedge but that is not as prevalent in Pennsylvania. Yellow nutsedge grows on low ground and moist fields, along riverbanks, and in ditches and heavily irrigated crops. Undoubtedly, our wet growing season has favored nutsedge growth. It thrives in all soil types, including black peat soil, and it grows well in soils with a pH of 5.0 to 7.0.
Yellow nutsedge does not tolerate shade; consequently it was not a major problem in old standard orchards. Typically you will see nutsedge along the edges of the herbicide strip or just in from the grass drive rows. However, with new high density plantings which allow more sunlight into the lower portion of the canopy, nutsedge may move under the trees.
Yellow nutsedge leaves arise from a central triangular stem that grows in clumps and may extend to a height of one to three feet. The leaves are 1/8 to 1/2 inch wide with a yellow green color and a smooth and shiny or waxy upper surface. The distinctive leaves have a characteristic v-shape bending sharply at the leaf midvein. The underside of the leaf is lighter green and dull rather than shiny. Flowers develop from early to late summer once daylength reaches 12 to 14 hours.
At that point a triangular floral stem arises from the center of the basal clump and produces a yellow flower cluster as an umbel. Flowers may produce 1,500 seeds. Seed dispersal is one of the long distance mechanisms for dispersal of the plants. However, more commonly the rooting structures botanically called tubers but commonly referred to as "nutlets" are responsible for its spread in vegetable and certain agronomic crops. The tubers can 'hitchhike' on soil clods on tillage equipment as it moves from one field to another.
Control Measures in Early August
When days begin to shorten in early August (now) nutsedge plants initiate new rhizome growth downward resulting in the formation of the nutlets by early September. These nutlets are dormant during the winter and then sprout in early spring when conditions favor their growth usually from late May to mid-July. When the shoot reaches the soil surface it forms a basal bulb structure and within a few weeks a new plant grows from the bulb. The bulb also develops roots growing downward that will form new tubers/nutlets and the cycle repeats itself next year.
While there are herbicides that can be used to control or suppress nutsedge early in the spring, a key tactic of control new or established infestations is to prevent the formation of new tubers in August. The use of glyphosate to burn down the plants at this time of year will reduce tuber formation. The rate will depend on which glyphosate product you intend to use. Glyphosate labels have specific instructions for post-emergent treatment of nutsedge at this time of year. Application at this time will not control tubers that have not sprouted. Be sure to read the label of the glyphosate product you intend to use and include any surfactant such as a NIS and/or ammonium. The preharvest interval for most glyphosate products is 14 days for pome fruit and 17 days for stone fruits.
Control Measures Next Spring
Therefore there are a few selective herbicides that can be used to help combat nutsedge. In apples, Sandea 75%DF from Gowan with the active ingredient halosulfuron-methyl can be used as either a post-emergent or pre-emergent to control yellow nutsedge. Unfortunately, besides apples, it is only labeled on blueberries and is not labeled on other tree fruit crops. Rimsulfuron can also provide suppression of yellow nutsedge and is labeled for all tree fruit.
The best method to control nutsedge with Sandea is a post-emergent as a single spray at a minimum of 0.75 oz/A when nutsedge plants are in the 3-5 leaf stage early in the spring. If needed, a second treatment may be applied later in the season to any secondary nutsedge emergence. Post-emergent treatments will not control tubers that have not sprouted. Do not treat orchards where the trees are established less than one year. Sandea can also be applied as a pre-emergent treatment, applications to bare ground work best. If small weeds are present, include a broad spectrum post-emergent material as well. Include a non-ionic surfactant in the mixture. Applications at this time should be made before plants emerge and just prior to enough rainfall or irrigation to move the herbicide down into the root zone of the nutsedge. This is followed by a second application at the 2-4 leaf stage.
Sequential applications of glyphosate at lower rates than rates used in August can be applied when plants are in the 3 to 5 leaf stage (less than 6 inches tall). Repeat the applications as needed to control subsequent emerging plants or regrowth of existing plants.
Rimsulfuron products can also be used to provide suppression of yellow nutsedge. These products include Solida® from Cheminova, Matrix® VFN and Matrix SG from DuPont and Pruvin from Adama. There are two application timings. Preemergent applications should be made when enough rain is expected after the application to move rimsulfuron into the rooting zone of the nutsedge. This should be followed by a second application when emerging nutsedge is 2 to 4 inches tall. Application when nutsedge is 6 inches tall or taller will not suppress nutsedge. Note that if your soil organic matter is 6% or higher the pre-emergent application will not be effective.
Rimsulfuron can also be applied post-emergent with the first application being made when leaf blades are 2 to 4 inches tall. The second application should be made 14 days later. Certain grass and broadleaf weeds are also selectively controlled. Trees and vines must be at least 1 year old when treated. There is a 7 day PHI for pome fruit, and 14 day PHI for stone fruit, tree nuts, and grapes. The re-entry limit is 4 hours. Best results are obtained when the soil is moist at the time of application.