Once Upon a Milkweed

A sweat bee (genus Lassioglossum) on swamp milkweed (Asclepias incarnata) is in a precarious position. 

Milkweeds are familiar to many as essential for Monarch butterflies, but there’s much more to their story. A close look at their flowers shows an intricate structure with a tricky way to snag insects – literally.

The flowers of swamp milkweed (Asclepias incarnata), like many other milkweeds, are composed of five reflexed petals, five upright hoods, and five narrow horns around a gynostegium, a central column of fused stamens and pistils. The bases of the hoods hold nectar, and between them are narrow slits bordered by two “guide rails.” Each slit leads to a chamber that contains the reproductive parts of the flower, including the stigma, the part that receives pollen. For that reason, it’s called the stigmatic chamber.

What’s missing from the flowers are anthers shedding dust-like pollen grains. Unlike typical flowers, milkweeds don’t offer individual grains for insects to carry away. Instead, their pollen is packed into waxy sacs called pollinia. Each chamber holds two pollinia connected by a pair of arms and a central gland or corpusculum, Latin for “little body.” The whole structure, called a pollinarium, looks like a small pair of winged maple seeds.

Pollinia are rare. Only orchids also make them. The advantage of packing pollen grains together is that they can be carried as one unit to deliver hundreds of grains to the stigma of another flower. That improves the odds that every ovule in an ovary will be fertilized and develop into a seed. 

[Sidebar: In seed plants, ovules contain egg nuclei and develop into seeds. One or more ovules reside in an ovary, which sits at the base of a pistil, the “female” reproductive part. Above the ovary is a neck-like style and the stigma, the surface that receives pollen. Ovary walls develop into fruits.]

The challenge for milkweeds is to somehow get the pollinia out of the chamber and onto another flower. To accomplish this, milkweeds rely on bees, wasps, flies and butterflies as carriers. The insects visit the flowers to get nectar, and in the process, take the pollinia. And that’s where it gets tricky.

As an insect walks across the waxy surface of a milkweed flower, a leg or other body part can accidentally slip into one of the slits between the hoods. The corpusculum then catches its leg, forcing the insect to pull hard to get it out. Sometimes the insect doesn’t succeed, and it either leaves behind a leg or dies trying to get it loose. But if the insect can manage, it extracts its leg with the pollinarium attached. Then it’s off to another flower and perhaps another slip into a chamber, where the pollinia are deposited and the pollen can reach the stigma.

Pollinarium with dangling yellow pollinia on the tarsus (lowest leg segment) of a digger bee. 
Photo by Allan Smith-Pardo, Bees of the United States, USDA APHIS PPQ, Bugwood.org.

That’s a lot of effort, for both the insect and the plant. The reward for the insect, if it isn’t snagged forever in a milkweed flower, is a source of nectar that is almost pure sucrose, the same as in your sugar bowl. The reward for the plant, as mentioned above, is an abundant and directed source of pollen. No other plants but milkweeds can receive pollinia, so little pollen is lost on plants that can’t use it. Even a different milkweed species is unlikely to accept pollinia from, say, a swamp milkweed, because the size and shape of the receiving chamber may not fit the arriving pollinia. Hybrids are therefore uncommon.

To listen to an ecologist talk about milkweed pollination and why it’s so unusual (and cool!), see this video by Dr. Thomas Rosburg of Drake University for Iowa PBS.

To see milkweed pollination in action, see this video from the Master Gardeners of Northern Virginia (scroll down at the site) or another video from Monarch Butterfly USA. Some of the terms differ, but the process – and the pitfalls – are the same. 

To learn more about swamp milkweed in particular, see this page from Minnesota Wildflowers.

References

Minnesota Wildflowers

Illinois Wildflowers

Milkweed pollination biology. By Eric P. Eldredge, USDA NRCS. November 2015. 

Milkweed pollination: A series of fortunate events. By Chris Helzer in The Prairie Ecologist, January 2021. 

Wyatt, R. and Broyles, S. B. 1994. Ecology and evolution of reproduction in milkweeds. Annual Review of Ecology and Systematics 25: 423-441. https://www.jstor.org/stable/2097319

Plant profile: Jumpseed

Persicaria virginiana, formerly Polygonum virginianum, Antenoron virginianum, Tovara virginiana

 

Jumpseed, also called Virginia knotweed or woodland knotweed, is a perennial herbaceous plant native to the eastern U.S., including southeast and east central Minnesota. It thrives in the damp soils and part shade of deciduous woods and edges, often where there has been some disturbance. These plants were growing along a trail through a woodland.

From July into September, the plants produce long, slender racemes bearing tiny, whitish flowers, each just a few millimeters across when fully open. After pollination by honeybees, bumble bees, leaf-cutting bees, and other bees and wasps, the flowers form small fruits that are deflexed – they angle downward on their short pedicels (flower stalks), a tensioned position that needs only a slight touch to be released. When it is, the fruits “jump” off the plant. Their hooked ends, formed by remnants of their styles, can latch on to fur or clothing and help the seeds travel farther from their parents.

Jumpseed flowers in mid to late summer, producing slender racemes up to 16 inches (40 cm) long. The small flowers
have four sepals, but no petals. Fruits have hooked tips that can latch on to passing animals, including humans. 

Jumpseed also spreads by rhizomes, underground stems that form patches of plants. Where the plants aren’t desired, this can be a problem, because they tend to persist even after pulling. The same is true for eastern jumpseed, an introduced plant. Once considered a variety P. virginiana called filiformis but now recognized as a separate species, Persicaria filiformis, it is beginning to develop a reputation as invasive because of its rhizomatous habit. It differs from P. virginiana in having pink to red flowers and often variegated leaves, characteristics that make it popular in the horticultural trade. Several cultivars of eastern jumpseed, such as ‘Painter’s Palette,’ ‘Lance Corporal,' and ‘Batwings,’ are offered for sale from some nurseries.

The last two cultivar names come from colorations on jumpseed’s leaves. In spring, the leaves are marked with maroon or dark green chevrons, upside down V’s that resemble military insignia or wings. The chevrons on Virginia jumpseed disappear by the time the plants flower, but those on eastern jumpseed leaves may persist.

Jumpseed has alternate leaves and hairy ocreas.
Notice the swollen nodes. 

Jumpseeds belong to the buckwheat or knotweed family, Polygonaceae (poly-gon-AY-see-ee). A typical trait of the family is a thin sheath called an ocrea (or ochrea) above each node. In formal terms, the sheath is described as scarious, meaning it is dry and membranous. The ocrea is formed from stipules that, sometime in the long history of knotweeds, fused to form a tube around the stem. 

Another trait that identifies this family is its knobby or swollen nodes. They resemble knees, if you use your imagination. Polygonaceae means “many knees.”

References

Minnesota Wildflowers

Illinois Wildflowers

Maryland Invasive Species Council (Persicaria filiformis)

John Philip Baumgardt. How to Identify Flowering Plant Families: A Practical Guide for Horticulturists and Plant Lovers. Timber Press, 1982. 

Plant Profile: Michigan Lily

 

Michigan lily flowering in mid-July in Hennepin County, Minnesota.

This lone Michigan lily (Lilium michiganense), barely out of reach of the ditch mower and surrounded by invasive reed canary grass (Phalaris arundinacea), grows at the edge of a wetland. That’s typical for this native perennial, which is also found along streambanks and shores and in wet meadows, prairies, bogs and woodland edges and openings.

Michigan lily looks much like Turk’s cap lily (Lilium superbum), another native that grows in the same habitats farther south. In fact, some references call Lilium michiganense Turk’s cap lily, a mix-up that shows why scientific names are helpful. They may be tongue-twisters, but unlike common names, scientific names are usually the same no matter where you are (or what you’re reading), so there’s less confusion.

According to most references, this one is almost certainly Michigan lily. It’s 3–4 feet tall, well within the 3– to 6–foot height typical of this species. Turk’s cap lily tends to grow taller, usually 5–7 feet.

Their flowers also differ, but the differences are subtle. Both species have umbels of nodding flowers with orange-red, dark-spotted tepals (similar petals and sepals) that are reflexed, bending back toward the base of the flower. Large stamens and a long pistil emerge from the center of the flowers and hang downward.

Umbels of Michigan lily flowers (left) and sets of whorled leaves on the stem (right).

In Michigan lilies the tips of the tepals are said to reach the base of the flower, but not much farther. In contrast, the tepals of Turk’s cap lily reach so far back they go beyond the base of the flower and may touch each other. In addition, their anthers, the pollen-bearing tips of the stamens, differ in length. According to several references, those of Michigan lily are never more than ½ inch long, whereas those of Turk’s cap lily are at least ½ inch long or longer.

If you don’t have a ruler handy, there are other differences to look for. If flower buds are present, look at their shapes. Michigan lily buds are more or less round in cross section, whereas Turk's cap lily buds are triangular. Open flowers also display differences. The pistil of Michigan lily is orange-red, whereas the pistil of Turk’s cap lily is greenish white to whitish orange. Looking deep into the flower, you’ll sometimes see a green, star-shaped center in Turk’s cap lily, but not in Michigan lily. 

Another look-alike is the introduced tiger lily, Lilium lancifolium, a garden favorite. It differs from both Michigan and Turk’s cap lilies in that it has alternate, not whorled, leaves, and small bulbs in the leaf axils.

Growing near this Michigan lily was yet another look-alike, Tawny day lily, Hemerocallis fulva. Also called ditch lily for its common habitat, it has orange-red flowers that open upward and have streaked but not spotted tepals. Day lilies have strap-shaped basal leaves but no leafy stems.

Tawny day lily flowers on leafless stems amid strap-shaped basal leaves. The flowers open upward, their red-orange tepals streaked but not spotted.

Michigan lily ranges throughout the upper Midwest and the Great Lakes region and less commonly farther east and south. In this region, it flowers in July. Pollinators are thought to be hummingbirds, butterflies and moths.

Michigan lily's range in North America (left) and the Upper Midwest (right). Maps from USDA Plants Database.

References

Minnesota Wildflowers - Michigan Lily

Friends of Eloise Butler Wildflower Garden - Turk's Cap Lily

Illinois Wildflowers - Michigan Lily

Illinois Wildflowers - Turk's Cap Lily

Flora of North America – Michigan Lily

Flora of North America – Turk’s Cap Lily

USDA, NRCS. 2023. The PLANTS Database (http://plants.usda.gov, 07/18/2023). National Plant Data Team, Greensboro, NC USA.

Plant profile: Cow parsnip

Caution: The sap of cow parsnip and other members of the carrot family can cause skin rashes and blisters. See below for more information.

Cow parsnip, Heracleum maximum, is a big plant with a big name. This 4- to 8-foot-tall native plant of damp meadows and fields, streamsides, ditches, and low, open woods is named for Heracles or Hercules, the mythic Greek hero of superhuman strength. Even the species name, maximum, hints at its size. Few herbaceous plants are as robust.

A biennial or short-lived perennial in the carrot family, cow parsnip has alternate, divided leaves up to 2 feet across, with smaller, undivided leaves higher on the stem. The leaflets are irregularly lobed, coarsely toothed, and hairy. The petioles of lower and middle leaves are 3 to 10 inches long with sheaths where they meet the stem. Stems are stout, hollow, ridged, and hairy.

Lower leaves of cow parsnip are divided into three lobed, toothed leaflets. This leaf blade (left) is about 18 inches long. Petioles are long with sheaths where they meet the stem (right).

Cow parsnip flowers from late May into early July. Small, white, 5-petaled flowers are in flat-topped, compound umbels, clusters that resemble a collection of little umbrellas. (See Flower Parts for Plant ID to learn more about types of flowers and flower clusters, called inflorescences.) The flowers are pollinated by honeybees and many kinds of native bees and flies, the variety so great that some consider cow parsnip a pollinator magnet.

Cow parsnip flowers are arranged in compound umbels. Each spoke of the umbel extends to another, smaller umbel, called an umbellet. The tiny, white flowers have five petals. 

Fruits are flattened, up to 1/3 inch long and 1/4 inch wide, and brown with dark, vertical lines when they mature in late summer or early fall. They are winged on their edges and so can be dispersed a limited distance by wind. Eventually the fruits split apart and release two seeds. Cow parsnip reproduces only by seed.

These immature fruits of cow parsnip will turn brown when they mature in late summer.

Although the plant is native here and in much of North America, in some states it’s introduced and invasive. In North Carolina, for example, cow parsnip is described as aggressive and insidious and is classified as a noxious weed.

Be Careful

Cow parsnip is rarely considered an ecological problem here, but it should be handled with care. The leaves and stems contain furanocoumarins, compounds that sensitize skin to ultraviolet light. If skin is exposed to plant sap and then sunlight, blistering rashes may result. The condition, called phytophotodermatitis, can take weeks to heal. Always wear gloves, long sleeves and pants when handling this plant, and wash off and cover skin if exposed to the sap. The sap can also damage the eyes, so it’s best to wear eye protection if the plant will be disturbed.

Watch Out for a Look-alike

Giant hogweed (Heracleum mantegazzianum), a similar but much larger plant, is introduced and invasive. So far, this 10- to 15-foot-tall plant hasn’t been found in Minnesota, but it is present in Wisconsin, and it’s known to be especially hazardous. The Minnesota Department of Agriculture includes giant hogweed on its list of noxious weeds that should be eradicated if found. Report the plant’s location using Report a Pest or EDDMapS and handle it using extreme caution.

Invasive Plants in the Carrot Family

Cow parsnip isn’t one of Minnesota’s “bad carrots,” but several other species are, including not only giant hogweed but also wild parsnip, burnet saxifrage, Queen Anne’s lace, and several others. A guide to identifying these plants is available in the Downloads tab.

 

References

Friends of Eloise Butler Wildflower Garden

Minnesota Wildflowers

BWSR Featured Plant: Cow Parsnip

Illinois Wildflowers

USDA, NRCS. 2023. The PLANTS Database (http://plants.usda.gov, 06/21/2023). National Plant Data Team, Greensboro, NC USA.

Giant hogweed – Minnesota Department of Agriculture

Giant hogweed (Heracleum mantegazzianum) – Minnesota Department of Natural Resources

Plant Profile: Starflower

Starflower blooming in late May in a mixed coniferous-deciduous forest in north-central Minnesota.

Starflower (Trientalis borealis, aka Lysimachia borealis) is a spring-blooming, perennial wildflower of coniferous and deciduous forests. In early spring, stems emerge from overwintering tubers and grow 4–8 inches tall, their slender stems bearing six to eight lance-shaped leaves of unequal size. In May and June, one, two, or rarely three flowers grow from the leaf axils. Each flower is about ½ inch wide and typically has seven white, pointed petals and orange anthers that later turn brown.

The flowers are self-incompatible, so they can’t pollinate themselves. To form seeds, they must receive pollen from another patch of starflowers, delivered primarily by native mining bees (andrenid bees), sweat bees (halictid bees) and hover flies (syrphid flies). If the bees are present, if the patches are close enough for the bees to transit, and if pollination is successful, small seed capsules eventually form at the tips of the stems.

That’s a lot of ifs and little assurance of a next generation. Starflower doesn’t depend only on seeds for reproduction, however. In fact, very little of its energy is dedicated to flowering and seed set. Most of its reproductive effort is spent on rhizomes, underground stems that extend the plant’s reach and give rise to new plants. It’s a faster way of reproducing, and in a stable environment, it’s more reliable. The downside is that the parent plant and its vegetative offspring are genetically identical, so if the environment changes, the plants may not have what it takes for a population to survive.

If conditions remain favorable, though, the rhizomes grow and form patches of new plants. By midsummer, tubers begin forming at their tips. Aided by the cool nights of late summer and fall, they fill with starch to fuel next year’s growth. Rhizome connections then wither and the leaves yellow and fall. Bare stems topped with capsules are all that remain above ground, while tubers below ground carry their incipient roots and shoots through winter, ready to resume growth in spring. Starflower seeds also overwinter, but they don’t germinate until fall of the second year.

Starflower range in North America (left ) and the Upper Midwest (right). Maps from USDA Plants Database.

As with many plants, Starflower is facing challenges brought by climate change. The cool nights needed for maximum tuber development are warmer now, and researchers have found that flowering and seed set lessen toward the southern edge of the plant’s range. These changes raise questions and concerns about whether the species can adapt, because in some places, it isn't. Starflower is state-listed as endangered in Georgia and state-listed as threatened in Illinois.

Populations in Minnesota and other northern locations are responding to warmer May temperatures by flowering earlier. That may or may not be beneficial, but so far, starflower seems to be holding its own here. The species name borealis, meaning “of the north,” may be truer than ever.

References

Minnesota Wildflowers

Illinois Wildflowers

USDA, NRCS. 2023. The PLANTS Database (http://plants.usda.gov, 06/03/2023). National Plant Data Team, Greensboro, NC USA.

Roger C. Anderson. June 1970. The role of daylength and temperature in tuber formation and rhizome growth of Trientalis borealis Raf. Botanical Gazette, Volume 131, Number 2, pp. 122-128.

Roger C. Anderson and Orie L. Loucks. July 1973. Aspects of the biology of Trientalis borealis Raf. Ecology, Volume 54, Issue 4, pp. 798-808.

Roger C. Anderson and Michael H. Beare. March 1983. Breeding system and pollination ecology of Trientalis borealis (Primulaceae). American Journal of Botany, Volume 70, Issue 3, pp. 408-415.

Emily Dangremond. No date. Climate change and starflower in the Midwest. Illinois Native Plant Society.

Emily Dangremond, Christopher H. Hill, Shahd Louaibi, and Ivette Muñoz. 2021. Phenological responsiveness and fecundity decline near the southern range limit of Trientalis borealis (Primulaceae). Plant Ecology, Volume 223, pp. 41-51.  

Linda G. Chafin. 2020. Trientalis borealis Raf. Georgia Biodiversity Portal, Georgia Department of Natural Resources/Wildlife Resources Division.

What Are Catkins -- and Why Does "Gesundheit" Come to Mind?

Catkins of quaking aspen, Populus tremuloides, began emerging in March in southern Minnesota. This photo was taken in mid-April.

Catkins, also called aments, are cylindrical, sometimes pendant clusters of inconspicuous flowers. They are typical of willows, aspens, poplars, birches, alders, hazelnuts and ironwood trees and shrubs. All these plants bloom in spring, often before leaves emerge, and most are wind pollinated. Willows are also insect pollinated and can be an important source of pollen and nectar for early-emerging insects, including those that later pollinate crops (1).

Catkins contain either male (pollen producing) or female (seed producing) flowers on the same or different plants. Ironwood (Ostrya virginiana), hazelnuts (Corylus spp.), birches (Betula spp.) and alders (Alnus spp.) have male and female catkins on the same plants, so they’re said to be monoecious (mon-EE-shus), which means “one house.”

In contrast, aspens and poplars (Populus spp.) have male and female catkins on different plants, so they’re dioecious (di-EE-shus), meaning “two houses.” Willows (Salix spp.) are also dioecious.

The emergence of catkins and the release of pollen marks not only the beginning of spring but also the start of allergy season. Wind-pollinated plants tend to produce abundant pollen because the grains could land anywhere – perhaps on a female flower of the same species, but maybe on those of a different species or even on no plant at all. Such as on you.  

Flurries of pollen may add to the misery for allergy sufferers, but for aspens, willows, and similar plants, they’re an insurance policy. The possibility of a next generation literally blows in the wind, so the more pollen, the better. "Gesundheit" for one, then, is good fortune for the other. 

Reference

1) Ostaff, D. P., Mosseler, A., Johns, R. C., Javorek, S., Klymko, J. and Ascher, J. S. 2015. Willows (Salix spp.) as pollen and nectar sources for sustaining fruit and berry pollinating insects. Can. J. Plant Sci. 95: 505[1]516. DOI:10.4141/CJPS-2014-339.

New Invasive Plants to Watch For

The Minnesota Department of Agriculture has added several new plants to its 2023 noxious weed list. This post focuses primarily on additions to the Prohibited-Eradicate category, but three other categories – Prohibited-Control, Restricted and Specially Regulated – are covered at the end of the post.

Prohibited-Eradicate: The Early-Detection List

The plants in this group either aren’t here yet or are present in low numbers. They're in the early-detection stage, when ideally, they can be found and removed before they become widespread. As the name of the group suggests, these plants should be eradicated by destroying all above- and below-ground parts. Transporting, propagating or selling them is illegal.

Three plants are new to the list this year: Johnsongrass, pale swallow-wort and red hailstone.

Johnsongrass
Sorghum halepense

USDA NRCS Plants Database (see references). Johnsongrass is
confirmed in counties shaded blue. Lakes and rivers are also
shaded blue.

This perennial grass was introduced to the U.S. in the 1800s as a forage crop. It is now found in many
habitats, including pastures, roadsides, ditches, old fields and wetlands. It reproduces by seeds and rhizomes and can spread aggressively to form dense mats that exclude other plants. It will not tolerate drought or extreme cold, so where winters are severe, Johnsongrass may be a facultative (optional) annual. Stressed plants can produce toxic levels of cyanide.

Johnsongrass grows 8-12 feet tall when flowering. Leaves are alternate, hairless and up to 2 feet long with white midribs. Ligules are 3-4 mm long, membranous and sometimes toothed. Johnsongrass flowers from mid-summer to fall in loose, purplish panicles.

Leaves of Amur silvergrass (Miscanthus sacchariflorus), another introduced plant, also have white midribs, but this plant is shorter at 6-8 feet. Its ligules are a hairy fringe, and its panicles are silvery and silky in fall.

Clockwise from left: Johnsongrass stem and dense stand by Chris Evans, University of Illinois, Bugwood.org; Johnsongrass rhizomes and panicle by Steve Dewey, Utah State University, Bugwood,org. 

Pale swallow-wort
Cynanchum rossicum (aka Vincetoxicum rossicum)

EDDMapS (see references). Pale swallow-wort is confirmed in
counties shaded green.The plant has also been confirmed in Ontario.

Pale swallow-wort, also called European swallow-wort, is a perennial, twining vine that was imported
to the U.S. in the late 1800s, probably as an ornamental. It thrives in disturbed sites but can grow in a variety of habitats, including fields, pastures and woodland edges and understories. Fast-growing and shade tolerant, it can quickly overrun and outcompete other plants. Because swallow-wort is in the milkweed family, female monarchs will sometimes lay their eggs on it if common milkweed is unavailable. There is concern, however, that swallow-wort is toxic to monarch larvae.

The vine has opposite, shiny, oval or heart-shaped leaves with pointed tips. Stems grow to 7 feet long, wrapping around other plants or structures for support or clambering over the ground. Sap is clear, not white. Clusters of pink to reddish-brown, star-shaped flowers bloom in June and July. Pods are slender, smooth and 2-3 inches long.

Black swallow-wort (Cynanchum nigrum) is also on the Prohibited-Eradicate list. It looks like pale swallow-wort but has dark purple flowers.

Clockwise from left: Pale swallow-wort vines by Rob Routledge, Sault College, Bugwood.org; Pods and seeds by Leslie J. Mehrhoff, University of Connecticut, Bugwood.org. Leaves and flowers by Rob Routledge, Sault College, Bugwood.org.

 

Red hailstone
Thladiantha dubia

EDDMapS (see references). Red hailstone is confirmed in 
counties shaded green. 

Also called golden creeper or tuber gourd, red hailstone is a perennial vine introduced to North America
in the late 1800s or early 1900s as an ornamental. This adaptable plant grows in abandoned fields, roadsides, gardens, crop fields, railroad corridors and natural areas. Red hailstone can quickly overgrow and smother other plants, including crops.

Vines grow up to 20 feet long with tendrils that grasp other plants or structures for support. Leaves are alternate and heart-shaped. Stems, leaves and petioles (leaf stalks) are hairy. Yellow, tubular flowers bloom from July to September on separate male (pollen-producing) and female (seed-producing) plants.

So far, all plants found in Minnesota are male, so the vines aren’t spreading by seed. Instead, they reproduce vegetatively by small tubers carried along waterways; many of the mapped infestations are along rivers or streams. Where both male and female plants grow, 2-inch oblong fruits may form on female vines. The fruits turn red when mature, thus the name red hailstone.

Before it flowers, red hailstone resembles other tendril-bearing vines in the same family. Wild cucumber (Echinocystis lobata) is a hairless vine with five-lobed leaves and white flowers that bloom in late summer. Bur cucumber (Sicyos angulatus) is hairy but also has five-lobed leaves. Its flowers are white or greenish-white. Neither cucumber vine has red fruits.

Clockwise from left: Red hailstone leaves and infestation by Katy Chayka, Minnesota Wildflowers; Red hailstone flowering plants by Peter Dzuik, Minnesota Wildflowers.

 

Clockwise from left: Red hailstone flowers by Peter Dzuik, Minnesota Wildflowers; male flower closeup by Katy Chayka, Minnesota Wildflowers; rhizomes and tuber by Katy Chayka, Minnesota Wildflowers.

Other Categories of Noxious Weeds

The MDA defines three other categories of noxious weeds. Two were expanded in 2023.

·         Prohibited-Control: Plants in this category are already established here, so eradication isn’t practical. Management aims at preventing them from reproducing by seed or vegetative organs, such as rhizomes, tubers or stem fragments that can take root. Transportation of all propagating parts is illegal except as allowed by state law, and the plants may not be propagated or sold in the state. Sixteen species are on the list, including three kinds of knotweed featured in a previous post, Are Psyllids the Solution to Invasive Knotweeds? No new species were added in 2023. 

·        Restricted: These plants are widespread in Minnesota. The only practical way to manage them is to restrict their importation, sale and transportation in the state, except as allowed by state law. Two plants are new to the list this year: lesser celandine (Ficaria verna), an aggressive spring ephemeral and garden escapee, and salt cedar (Tamarix ramosissima), also called tamarisk, a shrub first introduced in the West for landscape use, windbreaks and erosion control. 

·        Specially Regulated: These are native or nonnative weeds that are economically valuable but potentially harmful if not controlled. Three plants are new to the list this year. Amur corktree (Phellodendron amurense) now must be removed wherever females have been planted or escaped, or their fruits and seeds must be prevented from spreading. Only male cultivars are legal to sell. Production of Callery pear (Pyrus calleryana) is being phased out over the next three years, after which the tree will be moved to the Restricted category. Tatarian maple (Acer tataricum) and its cultivars can be sold only if a label is attached advising that they should be planted only where the seedlings can be controlled, and ideally at least 100 yards away from any natural area.

 

References

EDDMapS. 2023. Early Detection & Distribution Mapping System. The University of Georgia - Center for Invasive Species and Ecosystem Health. Available online at http://www.eddmaps.org/; last accessed March 20, 2023.

USDA, NRCS. 2023. The PLANTS Database (http://plants.usda.gov, 03/23/2023). National Plant Data Team, Greensboro, NC USA.

Johnsongrass

Wisconsin Department of Natural Resources

Minnesota Department of Agriculture

USDA Fire Effects Information System

University of Missouri

Pale swallow-wort

Minnesota Department of Agriculture

USDA Forest Service

Wisconsin Department of Natural Resources

Michigan Department of Natural Resources

Red hailstone

Minnesota Department of Agriculture

University of Minnesota Extension Service

Minnesota Wildflowers