5 weed species deserve a closer look

Yellow woodsorrel (Oxalis stricta) and hairy bittercress (Cardamine hirsuta) have challenged both ornamental plant producers and grounds managers for decades. Growers are probably already familiar with them.

Three rogue weeds are relatively new to ornamental production and exemplify the definition of a problem competitive weed: mulberry weed (Fatuoa villosa), chamberbitter or leafflower (Phyllanthus urinaria) and long-stalked phyllanthus (P. tenellus). They have a rapid growth rate, extensive seed production potential and produce large, densely fibrous root systems. These key characteristics are associated with competitive pressure in ornamental plant production systems.

Once mature, seeds of all five of these weed species are forcefully ejected several feet away from parent plants through mechanical adaptations that collectively are called “explosive dehiscence.” These dispersal mechanisms help to explain the rapid spread and persistence of these non-native weed invaders. Each technique merits a closer look.

Bittercress (Cardamine spp.)

Bittercress species are winter annual relatives of mustard that are common contaminants of both container production systems and shady, moist landscapes. Bittercress plants serve as refuge for populations of plant-feeding whiteflies, broad mites and spider mites, as well as soybean cyst nematodes. They can be alternate hosts for diseases including tomato spotted wilt virus and sclerotinia blight. Small, white flowers have four petals and form in clusters at the ends of flowering stalks that grow out of a basal rosette of leaves.

Immature seedling leaves are kidney-shaped. As bittercress plants mature, fully expanded, compound leaves often display leaflets with deeply lobed margins. Once pollinated, seeds develop in linear rows within 1/2- to 1-inch-long, slender fruits called siliques.

When mature, bittercress seeds are slung 9 feet or more from parent plants by miniature catapult action. Fresh seeds germinate without dormancy requirements and can generate prolific seedling densities in adjacent production areas and nursery containers.

Woodsorrel (Oxalis spp.)

Woodsorrel plants tolerate sun, shade and soil type, nutrient status and moisture level to ensure that they’re persistent pests in greenhouse and nursery production and in landscape settings. Though not related to leguminous clovers, leaves of perennial woodsorrel species look very similar in that both possess trifoliate leaves with heart-shaped leaflets on stems that arise from underground rhizomes.

Flowers are typically simple, yellow and have five petals. Pollinated flowers develop into 1/4- to 1/2-inch-long seedpods that are slightly star-shaped when viewed from above. When mature, Oxalis seeds are explosively propelled 12 feet or more from parent plants by means of spiral torsion of the ovary wall. Oxalis seeds do not have requirements for breaking dormancy and germinate readily in container media and soil.

Mulberry weed (Fatuoa villosa)

Plants have leaves and stems that grow erect and may be easily confused with seedling mulberry trees (Morus spp.). In both species, leaves alternate on the stem, have toothed margins and appear roughly triangular. A key field diagnostic test for mulberry weed is to look for small hairs on leaves and stems. By contrast, mulberry tree seedlings do not possess hairlike trichomes. Non-showy, pale-lavender mulberry weed flowers, which develop in late spring and persist through fall, have four tiny, curved petals. Flowers are held upright in a tightly packed cluster that extends out of the leaf axils where leaf petioles meet the hairy stems.

In 2003, Gina Penny and Joe Neal reported from North Carolina that mulberry weed seeds are capable of germinating across a wide range of temperatures. Because seedlings can also flower and set fruit as early as 12 days after reaching the two-leaf growth stage, multiple generations of seed-bearing plants are produced each year. Because seeds need light to germinate, tilling after mulberry weed sets seed will increase seedling densities. In Tennessee, mulberry weed continues to grow and set seed until killed by frost. When mature, seeds are forcibly ejected 4 feet or more from parent plants by increasing hydrostatic pressure in opposing ovary walls.

Longstalked phyllanthus and chamberbitter, or leafflower (P. tenellus and P. urinaria)

These warm-season weeds are increasingly encountered in container production systems, managed landscapes and turf in the Southeast. Root systems of both phyllanthus species are densely fibrous. Smooth, margined leaves appear similar to those of mimosa seedlings. While leaves of both species are 1/4- to 1/2-inch oblong ovals arranged alternately in two rows along 1- to 5-inch-long branchlets, leaves of P. urinaria are slightly wider, thicker and are often more closely spaced along a generally shorter branchlet. Flowers of both species are greenish-white and inconspicuous. They first appear in summer and persist until fall. In Tennessee, plants are killed by frost. Unlike other euphorbia species including spurges, Phyllanthus stems and leaf petioles do not exude milky sap when broken. Plants of both species produce hundreds of three-chambered, 1/10-inch-wide fruits that each yield six seeds. The pumpkin-shaped fruits are initially green and mature to pinkish-red.

In the field, key diagnostic differences between these two weed species can be easily observed in their flowering and branching habits. Flowers and fruit of longstalked phyllanthus develop at the ends of 1/4- to 1/2-inch-long stalks arising at each leaf axil along the underside of stems, while chamberbitter flowers and fruits are held sessile (without stalks) beneath the stem. In contrast to longstalked phyllanthus that typically grows with a relatively unbranched, 2- to 3-foot-tall erect stem, chamberbitter stems are heavily branched, yielding a mounded plant that seldom exceeds 2 feet tall. As phyllanthus fruits dry, ripe seeds are propelled 4 feet or more from the seedpod by a combined torsion and sling apparatus. Like mulberry weed, phyllanthus seeds are capable of germinating across a wide range of temperatures but germination requires both light and moisture.

Management and control strategies

If you do not yet have populations of these weed species, continue your efforts to keep them out. Sanitation is key for preventing weedy plant establishment in production areas and landscapes. Carefully inspect new plants and liners before introducing them into production areas and landscape beds. Don’t forget to inspect hanging baskets in greenhouses.

Simple ballistics shows that the higher fruits are placed above the ground, the farther seeds will be thrown when seedpods explosively dehisce. Do not allow fruits of these and other weeds to mature.

Individual plants can produce prolific seed set and yield several generations in a single growing season. A persistent weed seed bank is easy to establish, then difficult to eradicate without aggressive and expensive sanitation management. Similarly, re-growth of Oxalis from rhizomes beneath the soil or container medium is very hard to control. Pre-emergence herbicides offer growers the best chemical option for neutralizing the spread of explosively dehiscent weeds through production areas. Phyllanthus species, in particular, may not be completely controlled by many of the pre-emergent herbicides labeled for nursery crops. While hand-weeding is expensive, pulling immature seedlings at an early stage is well worth the labor and time investments to prevent seed set.

- Bill Klingeman and Greg Armel

Bill Klingeman is associate professor of urban landscape and nursery research, and Greg Armel is assistant professor in horticultural weed extension and research at University of Tennessee Plant Sciences Department, (865) 974-7324; plantsciences.utk.edu.

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July 2008