Identification and Control of Purple Loosestrife
(Lythrum salicaria L.)
W-1132(Revised) June 2004
Rodney G. Lym, Professor,
Department of Plant Sciences
North Dakota State University
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Purple loosestrife, a beautiful garden
plant with an aggressive nature, was first introduced into North America in
the early 1800s. The plant was sold in North Dakota by its genus name Lythrum
for at least 50 years. Lythrum plants were brought to North Dakota for flower
gardens because of their striking color, ease of growth, winter hardiness, and
lack of insect or disease problems (Figure 1). The garden varieties of purple
loosestrife were sold by many cultivar names including Morden Pink, Dropmore
Purple, and Morden Gleam. These garden cultivars were thought to be sterile
but have now been shown to cross-pollinate with the wild Lythrum type and sometimes
with other Lythrum cultivars.
Figure 1. Many landscapes
and gardens in North Dakota use Lythrum as a highlight of the planting.
(Click here for a 74KB color photo.)
Known purple loosestrife infestations in North Dakota
are small and generally found in or downstream of urban areas. These infestations
can be traced to escapes from public or private horticultural plantings, often
from seed that finds its way to streams and rivers through storm drains. Purple
loosestrife was added to the North Dakota Noxious Weed List in 1996 and state
law requires all plants to be removed to prevent this plant from becoming a
major weed problem in the wetlands of the state.
The most destructive impact of purple loosestrife invasions
is on the ecology of aquatic sites. Purple loosestrife forms dense monotypic
stands as it displaces native wetland plants (Figure 2). Under optimum conditions,
a small isolated group of purple loosestrife plants can spread to cover aquatic
sites in just one growing season (Figure 3). When purple loosestrife replaces
native vegetation it also can displace wildlife. For example, songbirds do not
consume the small hard seed. Muskrats use cattails to build their homes, and
they show a preference for cattail over purple loosestrife for food. Waterfowl,
especially ducks, avoid wetlands that have become dominated with purple loosestrife.
In addition, overall waterfowl production decreases as suitable nesting habitat
is eliminated. The plant's growth is generally too compact to offer cover, and
cover may be as crucial to wildlife as food.
Figure 2. Purple loosestrife
invades wetland areas and displaces native plants, such as cattails shown
here. (Click here for a 62KB color
photo.)
Figure 3.When Lythrum
reverts to the weedy purple loosestrife, it can invade wetlands and rapidly
spread. (Click here for a 117KB
color photo.)
Purple loosestrife has been heavily utilized in North
Dakota flower gardens, park plantings, and golf courses. Research has found
that all varieties of lythrum produce seed, which is a source of infestation
to aquatic sites, including rivers, lakes, sloughs, dams, dugouts, bogs, swamps,
irrigation ditches, streams (perennial or semi-permanent) and other water courses,
or wet sites. To prevent spread into aquatic sites, all purple loosestrife plantings
are required by state law to be removed.
Description
Purple loosestrife is a rhizomatous perennial forb introduced
to North America from Eurasia and Africa. Wild infestations are associated with
moist or marshy sites. The stems are erect (1.5 to 8 or more feet tall), four
to six angled, and can be smooth or pubescent with few branches. Leaves are
simple (0.75 to 4 inches long, 0.2 to 0.5 inches wide), entire, and can be opposite
or whorled.
The most identifiable characteristic of purple loosestrife
is the striking rose to purple colored flowers (Figure 4). The flowers are arranged
on a spike, which can be a few inches to 3 feet long. Each flower has five to
seven petals arising from a cylindrical green tube. The plant usually flowers
from early July to mid-September in North Dakota. The seed capsule is two celled
and contains many very small seeds (1 mm long or less). The roots become thick
and woody in mature plants. The aerial shoots die in the fall and new shoots
arise the following spring from buds at the top of the root crown. Although
the root crown expands and produces more shoots each year, the maximum growth
of the root crown diameter is limited to about 20 inches.
Figure 4. Purple loosestrife
flowers are very striking purple arranged on a spike. (Click
here for a 30KB color photo.)
Spread of purple loosestrife is primarily by seed, but
the plant can also spread vegetatively from stem cuttings. Research at NDSU
has shown that seed viability of purple loosestrife growing in North Dakota
wetlands ranged from 50 to 100 percent. With approximately 2.7 million seeds
produced per plant, purple loosestrife has the potential to spread rapidly once
established in an area.
Control
Several methods are available for purple loosestrife
control, including mechanical, biological, and chemical. The size and location
of a specific infestation will determine the best control methods. In general,
small infestations of a few plants can be controlled by digging, especially
when plants are only a few years old. Larger infestations require treatment
with herbicides and/or biological control agents.
Mechanical control
Small infestations can be controlled by removing all
roots and underground stems. It is difficult to remove all of the roots in a
single digging, so monitor the area for several growing seasons to ensure that
purple loosestrife has not regrown from roots or seed. This method is most useful
on garden plantings or young infestations.
Dispose of plants and roots by drying and burning or
by composting in an enclosed area. Take care to prevent further seed spread
from clothing or equipment during the removal process. Removal of all plant
material is important. Small segments of purple loosestrife stems can become
rooted and reestablish the infestation.
Chemical control
Herbicides can be used to control purple loosestrife
in areas too large to be controlled by digging. Also, herbicides can be applied
to individual plants selectively in landscape situations to prevent killing
desirable plants. Infestations growing along streams or in marshy areas may
require specialized equipment and application by trained professionals.
Rodeo or Roundup, various other trade names
(glyphosate)
Glyphosate will provide good control of purple loosestrife
when applied from July to early September. Many formulations of glyphosate
are sold but only those labeled for aquatic use can be applied in or near
water. For example, the Rodeo and Glypro formulations of glyphosate can
be used in water. With the Rodeo or Glypro formulations, a nonionic surfactant
approved for aquatic sites at 0.25% vol/vol must be added to the spray solution.
Roundup and similar glyphosate formulations can be used to remove purple
loosestrife from large plantings or infestations away from water. Glyphosate
has no soil residual so it could be used to remove purple loosestrife located
within an ornamental planting without having to dig in the flower bed. Best
results have been obtained when glyphosate is applied as a 1 to 1.5% concentration
(1 to 1.5 gallons glyphosate per 100 gallons of water) or (1.3 to 1.9 fl.
oz./gallon of water) at bloom or shortly thereafter.
A variety of sprayers, including backpack sprayers
and boat-mounted sprayers, can be used to control purple loosestrife in aquatic
sites. Wick application is also effective but is labor intensive. Spray dye
added to the tank may be useful to ensure uniform application to purple loosestrife
with minimal herbicide applied to desirable plants.
Eliminating the entire vegetative cover will promote
purple loosestrife seed germination, which can result in an increase in plant
density rather than control. Since glyphosate does not provide residual control,
treated areas will need to be monitored for regrowth from the roots or seedlings
for several years. A 2,4-D formulation labeled for use near water applied
as a 2% solution (2 gallons 2,4-D per 100 gallons of water) or (2.6 fl. oz./gallon
of water) will prevent seedling establishment when applied in early fall or
spring before the plants can establish perennial characteristics.
Garlon 3A (triclopyr)
Garlon is a selective broadleaf herbicide that will
not kill cattail or other desirable monocot species. Important: Only Garlon
3A formulation is labeled for use in wetland sites. Minimize overspray to
open water. See label for precautions for use near potable water intakes.
Garlon will provide good to excellent purple loosestrife control when
applied in the pre to early flower or late flower growth stages. Garlon should
be applied as a 1 to 2% solution (1 to 2 gallons Garlon per 100 gallons of
water or 1.3 to 2.6 fl. oz./gallon of water) and will provide some residual
seedling control. Garlon can be applied in dryland sites but should not be
used in landscapes or flower beds because soil residual of the herbicide may
prevent establishment of other horticultural plants.
Regardless of the herbicide applied, the infested areas
should be monitored to ensure that purple loosestrife does not reinfest from
root or seed. Also, areas downstream from river or creek infestations and
on all sides of a lake or pond infestation should be monitored for purple
loosestrife seedlings.
Biological control
Three biocontrol insect species were first released in
North Dakota in 1997 and include:
- Galerucella pusilla -- a leaf-feeding beetle
- Galerucella calmariensis -- a leaf-feeding
beetle
- Hylobius transversovittatus -- a root-mining
weevil
Of these insects, the two Galerucella spp. leaf
feeding beetles have been most successful. These insects overwinter as adults
and lay eggs in early June in North Dakota. The adults and especially the larvae
feed on the leaves and flowers of purple loosestrife (Figure 5). Following several
summers of heavy feeding, purple loosestrife infestations have been greatly
reduced. However, since the largest infestations in North Dakota are in urban
areas, mosquito control programs have kept these insects from becoming well
established. Purple loosestrife infestations in North Dakota are generally small
and isolated and should be controlled by chemical and/or mechanical methods.
Figure 5. Purple loosestrife
biocontrol agents are much smaller than the more familiar Aphthona
spp. flea beetles used for leafy spurge control and have a much different
life cycle. Galerucella spp. adults feed on purple loosestrife leaves
beginning in early June, then lay eggs underneath the plant leaves and cover
them with frass. The eggs hatch in late-June and July and larvae feed on both
leaves and flowers before pupating and over-wintering as adults. (Click
here for a 30KB color photo.)
Possible replacements for horticultural plantings
If you currently have a cultivar of purple loosestrife
growing in your garden or in a public planting, state law requires the plants
be removed. Removal of purple loosestrife is the only way to prevent the plant's
spread into North Dakota wetlands (Figure 6).
Figure 6. Don't let North
Dakota wildlands become purple. Help stop the spread. (Click
here for a 90KB color photo.)
Several perennial plants that produce flowers on spikes could serve as replacement
plants for purple loosestrife and include:
- Blazing Star, Gay Feather (Liatris
spp.) -- Plants grow 2 to 3 feet tall and have flower spikes in white, pink,
or purple from July through September. Good cut flower. They are drought tolerant
and grow best in full sun. Some species of Liatris are native. Likely the
best overall replacement plant.
- Bugbane or Black Snakeroot (Cimicifuga
racemosa) -- Grows 3 to 5 feet tall with 4 to 6 foot ivory-white flower
spikes in late summer. Prefers moist site with high organic matter in full
sun or partial shade.
- Catmint (Nepeta X faassenii)
-- Hardy border plant with lavender-blue flowers from early summer to fall.
Most cultivars are short growing but `Blue Hills Giant' will grow up to 3
feet.
- Cardinal flower (Lobelia cardinalis)
-- Scarlet red flowers from early to late summer. Grows 2 to 3 three feet
tall and prefers partial shade and moist growing conditions. Excellent choice
for near ponds or streams. Native to North America.
- Gas Plant (Dictamnus albus)
-- Long-lived perennial with lemon scented foliage. Plant grows 2 to 4 feet
tall with dark glossy foliage and 30 inch spikes of white or pink flowers
produced in early to mid June. Prefers full sun, plants can cause skin irritation.
- Lupine (Lupinus spp.) -- Plant
grows 3 to 4 feet tall with up to 2-foot flower spikes from spring to early
summer. Best in full sun. Available in a variety of colors including blue,
pink, yellow, and white. Lupine is a short-lived perennial and will need to
be replaced.
- Penstemon, beard-tongue (Penstemon
spp.) -- Bell shaped flowers. Grows about 3 feet tall and wide in full sun
with good drainage. P. strictus, Rocky Mountain penstemon, has lavender-blue
flowers. Prairie Dusk and Prairie Fire also do well.
- Russian sage (Perovskia artriplicifolia)
-- Grows to 2 to 5 feet tall and wide with feathery spires of purple flowers
from mid-summer to frost. Aromatic foliage, gray-green sage color. Grows best
in full sun. Drought tolerant.
- Salvia (Salvia X superba, S. X
sylvestris) -- Drought resistant, hardy perennial. Plant grows 2 to 3
feet tall with blue to violet flowers on spikes in June and again in August.
`May Night' and `East Friesland' are good choices for North Dakota gardens.
- Spike speedwell (Veronica spicata)
-- Shorter growing (18 inches) than others listed with dense blue, white,
or pink flowers on a spike. Best in well drained soil with full sun. Drought
tolerant. The cultivar `Sunny Border Blue' was an award winner.
Plants suitable to replace purple loosestrife that
was grown for a tall background border but have a different growth form and
do not have flowers borne on a spike:
- Blanket Flower (Gaillardia aristata)
-- Yellow flowers with red centers. Blooms from June through September and
grows 2 to 3 feet tall and wide. Will tolerate heat and survives in poor soils.
- Heliopsis (Heliopsis spp.)
-- Resembles sunflowers and used for background border. Grows 3 to 4 feet
tall and flowers in mid-summer. Best in full sun. Native to North America.
- Purple cone flower (Echinacea
purpurea) -- Drought tolerant, a daisy like flower which blooms from mid-summer
to frost with white to purple or red flowers.
- Rudbeckia (Rudbeckia spp.)
-- Black-eyed Susan is a short growing example. Some plants grow over 6 feet,
such as Autumn Sun coneflower. Yellow daisy like flowers. Native to North
America.
For more information on this and other topics, see:
www.ag.ndsu.nodak.edu
W-1132 (Revised), June 2004
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