This World of Weeds article features Venice mallow (Hibiscus trionum). This attractive annual species is also called “flower-of-an-hour” because each flower remains open for just a few hours before being shed. Venice mallow is in the Malvaceae family, and is related to velvetleaf and common mallow, as well as cotton. The species name trionum refers to the normally three-lobed appearance of the leaves.
Ecology
Venice mallow is an introduced plant that has become naturalized in much of the United States. A native of southern Europe, the plant was introduced as an ornamental before escaping cultivation to become a weed. The species is most prevalent in the eastern part of Kansas, but can occur anywhere within the state. It can be found in gardens, cultivated fields, waste areas, pastures, and roadways.
Identification
Venice mallow is an annual species reproducing by seeds. It normally grows 0.5 to 2 feet tall, with its many branches often spreading out from the base prior to curving upward. The stems are covered with stiff, short hairs. Leaves are alternate and borne on stalks (petioles), with hairs on the lower surfaces but generally smooth on the top surfaces. The cotyledon (seed) leaves are heart-shaped, and the first true leaves are rounded with gently lobed edges (Figure 1). Subsequent true leaves are normally deeply divided into three lobes, but may have as many as seven lobes (Figure 2). The center lobe is longer than the side lobes, but all are gently lobed to toothed along the edges.
Figure 1. Venice mallow seedlings with cotyledons and first true leaves. Photo by Patrick Geier, K-State Research and Extension.
Figure 2. Venice mallow plant growing in a soybean field. Note the papery calyx near the center of the photo. Photo by Sarah Lancaster, K-State Research and Extension.
Flowers are up to 2.5 inches wide, are short-lived, and originate from petioles arising from the junction of the stem and leaf petioles. The flowers have five petals, are white to pale yellow, and have a reddish to dark purple center (Figure 3). Seeds are produced in a globe-shaped capsule surrounded by a papery husk called a calyx (Figure 2). At maturity, the calyx and capsule dry and rupture, exposing the small (0.1 inch) seeds, which are kidney-shaped, dark brown, and rough.
Because Venice mallow is an indeterminate plant, flowers and seeds are produced throughout the growing season until a killing frost in the fall. Like its close relative, velvetleaf, Venice mallow seeds have an extremely hard seed coat. Consequently, seeds may lie dormant for many years, even decades, before emerging.
Figure 3. Flower of Venice mallow with a reddish center and bright yellow anthers (pollen sacs). Photo by Patrick Geier, K-State Research and Extension.
Management
Historically, Venice mallow was most often problematic in soybean production areas in Kansas. In recent years, newer herbicides and herbicide-resistant soybean varieties have reduced its prevalence in the crop. Herbicides which effectively control Venice mallow in soybean include 2,4-D (Group 4), glyphosate (Group 9), and glufosinate (Group 10) in herbicide-resistant varieties, as well as Group 14 herbicides (sulfentrazone, saflufenacil, flumioxazin) and Group 15 herbicides (pyroxasulfone) in conventional soybean. Group 4 herbicides (2,4-D, dicamba, fluroxypyr) can be used if Venice mallow occurs in corn or sorghum.
Integrated weed management techniques should be the first line of defense when dealing with any weed situation, and this is particularly true in crops such as sunflowers, cotton, or dry beans. With fewer herbicides available to control Venice mallow and other weeds in these crops, prevention as well as mechanical and cultural control practices are even more important. With the longevity of Venice mallow seed viability, attention should be given to the prevention of seedset and movement of seed from infested fields to clean fields.
For more information, see the “2025 Chemical Weed Control for Field Crops, Pastures, and Noncropland” guide available online at https://bookstore.ksre.ksu.edu/pubs/SRP1190.pdf or check with your local K-State Research and Extension office for a paper copy.
The use of trade names is for clarity to readers and does not imply endorsement of a particular product, nor does exclusion imply non-approval. Always consult the herbicide label for the most current use requirements.
Patrick Geier, Weed Scientist, Garden City
pgeier@ksu.edu
Sarah Lancaster, Extension Weed Management Specialist
slancaster@ksu.edu