Issue No. 13, Article 7/June 22, 2007
Morningglories: Gardener's Friend, Farmer's Adversary
Morningglory species, with their large, brightly colored flowers, are favorites of many gardening enthusiasts. These plants grow well in a variety of soil types, produce abundant foliage during the hot and sunny days of summer, and aggressively cover trellises, poles, fences, and the like. While morningglories are a favored ornamental for many homeowners and gardeners, some species can be troublesome weeds in Illinois corn and soybean fields.
Several species of annual morningglory occur in Illinois agronomic cropping systems, including tall (Ipomoea purpurea), ivyleaf (I. hederacea), and pitted (I. lacunosa) morningglory. I. pandurata, a perennial morningglory species that is widespread across Illinois, goes by several common names, including bigroot morningglory and wild sweet potato. Two other perennial species--field bindweed (Convolvulus arvensis) and hedge bindweed (Calystegia sepium)--also are members of the plant taxonomic family of the morningglories (Convolvulaceae) but do not belong to the genus Ipomoea. The family name Convolvulus is derived from the Latin verb meaning "to entwine," while the genus Ipomoea comes from the Greek ips ("a worm") and homoios ("resembling"), which refers to the wormlike twining of the plants as they grow around stationary objects.
The distribution of the three annual morningglory species varies somewhat across Illinois. Tall and ivyleaf morningglory are perhaps the most widely distributed, while pitted morningglory is most commonly found across the southern half of the state. The three perennial species occur across most areas of Illinois. Both annual and perennial morningglory species occur in cultivated fields and nondisturbed sites.
Another vining plant that may be mistaken for a morningglory species is wild buckwheat (Polygonum convolvulus). The leaves of wild buckwheat are heart-shaped, similar in appearance to those of tall and pitted morningglory. One easy way to distinguish wild buckwheat from the annual morningglories is the presence of an ochrea at the junction of the leaf petiole and stem on wild buckwheat, a common characteristic of the Polygonaceae (smartweed) family.
Identification of the annual morningglory species can be accomplished as early as cotyledon-stage plants. Tall and ivyleaf morningglory seedling plants have butterfly-shaped cotyledons with rounded lobes, while the butterfly-shaped cotyledons of pitted morningglory are slender and more deeply notched, with pointed lobes. The true leaves of tall morningglory are heart-shaped and covered with hairs that lie flat on the surface. True leaves of pitted morningglory are also heart-shaped but are generally smaller than the leaves of tall morningglory, with few to no hairs. The leaf margins are often tinged with purple and taper to a more pronounced pointed tip. Ivyleaf morningglory true leaves are very hairy and deeply 3-lobed.
Postemergence control of annual morningglory species in soybean can be challenging. These weed species can increase in size very quickly with adequate soil moisture and warm air temperatures, often exceeding labeled sizes in a short time. Emergence of annual morningglories occurs over a relatively long period compared with many other summer annual weed species, and emergence is often enhanced following a precipitation event. So achieving acceptable morningglory control in soybean with a single postemergence herbicide application can be difficult.
Postemergence herbicide options for morningglory control in soybean include both contact and translocated herbicides. Contact herbicide options include products containing the active ingredients fomesafen (Flexstar), lactofen (Cobra), and acifluorfen (Ultra Blazer). These products require thorough spray coverage of the target foliage to achieve optimal control, and they work best when morningglory plants have no more than 4 true leaves. Translocated herbicide options for control or suppression of morningglory include glyphosate, cloransulam (FirstRate), imazamox (Raptor), and chlorimuron (Classic).
Symptoms of herbicide injury on morningglory following application of a translocated herbicide consist of an initial stunting and yellowing of the leaves. Injury symptoms are often slower to develop with translocated herbicides than with contact herbicides. 2,4-DB, at 1 to 2 fluid ounces per acre, is sometimes tank-mixed with either contact or translocated herbicides to improve morningglory control, but it is rarely applied alone postemergence. Table 2 provides information from product labels about product application rates and morningglory sizes. Many of these products are labeled for tank-mix applications with other postemergence soybean herbicides, so application rates and morningglory sizes may vary from what is presented here. Be sure to consult the product label for any recommended spray additives, as recommendations can vary according to current growing conditions, tank-mix partner, weed spectrum and size, and other factors.
Soybean weed control practitioners are often frustrated when attempting to control morningglory postemergence exclusively with glyphosate. Glyphosate, at 0.75 to 0.77 lb ae per acre, is much more effective when morningglories are small (about 1 to 3 inches) than when applications are delayed until plants exceed 8 to 12 inches. If larger morningglories are present and the initial plan was to apply glyphosate at 0.75 to 0.77 lb ae, you may want to consider some alternatives that might improve overall morningglory control.
Three potential options for improved morningglory control include increasing the glyphosate application rate from 0.75 to 1.12 lb ae per acre; making sequential applications of glyphosate, spaced approximately 10 to 14 days apart; and adding a tank-mix partner to glyphosate. Field research conducted at the University of Illinois (as well as field research from several other universities) has demonstrated improved morningglory control from each of these options compared with a single application of 0.75 lb ae glyphosate. In some trials sequential glyphosate applications improved morningglory control more often than tank-mixes, whereas in other trials tank-mixes were equal to or better than sequential glyphosate applications. Overall, sequential applications or herbicide tank-mixes are about equal with respect to the number of instances in which one tactic has improved control relative to the other.
Source: Defelice, M.S. 2001. Tall morningglory, Ipomoea purpurea (L.) Roth--Flower or foe? Weed Technology 15:601-606.