purple loosestrife

(Lythrum salicaria)

 

Invasive implications

Purple loosestrife was introduced to North America from Eurasia in the early 1800's, most likely as seeds in ship ballasts (although possibly attached to wool or brought as a medicinal herb). It reached British Columbia in 1915. Purple loosestrife is known for its amazing seed production, with up to three million seeds per plant! This aquatic, perennial weed can swiftly suffocate wetlands, displacing valuable native habitat for both plants and animals. Loosestrife is thought to be responsible for the modification of more wetland habitat than is the current development pressure caused by humans. Despite these detrimental impacts, is often admired for its showy red-purple flowers. Hopefully this site will assist in identification and control of this invasive plant.

   

Identification

Spring

Summer

Winter
  •  difficult to identify
  •  stems emerge rounded,   developing into four-sided

 
  •  tall, erect, perennial herb growing 0.5-2m in a clump
  •  upper plant covered with short hairs
  •  mature plants are bushy having several spikes of clustered flowers
  •  after the first fall frost, leaves turn red for about 2 weeks then fade and gradually fall
  •  rigid stems remain standing through winter/spring and well into the next growing season

Leaves

Stem

Flower
 

 

 
  •  3-10cm long at stem base
  •  lance-shaped, opposite or whorled, heart-shaped base
  •  leaf pairs turn 90 degrees along stem, pointing upwards at a ~30 degree angle
  •  leaves near base of flower spikes are smaller/alternate
  •  firm, four-eight sided stems when mature
  •  smooth or covered with short hairs (glabrous to pubescent)
  •  usually an abundance of stems
  •  10-15mm long
  •  5-7 purple petals (also white, pink, deep purple, red), yellow center
  •  bloom July - September
  •  fruit is a flat, tan-coloured seed
  •  each spike produces up to  120,000 seeds (3 million per plant!)

 

Similar Plants

Can be confused with fireweed, hardhack, swamp loosestrife, blue vervain, winged loosestrife, and others. Hardhack is a common native plant, distinguished by its oval leaves, reddish brown stems, and smaller clusters of pink to deep rose flowers. Fireweed is another native that has large, four-petalled flowers that produce fluffy white seeds. Its alternate, long (up to 20cm), and abundant leaves attach directly to an unbranched stem. Other loosestrife species can usually be distinguished by leaf shape and flower colour/shape.

Hardhack (Spiraea douglasii)

Fireweed (Epilobium angustifolium)

                                                                                           

Key Features

In British Columbia, loosestrife is restricted to the southern parts of the province, common in salt marshes in the Fraser River delta but also thriving in lakes, ditches, and along streams. As mentioned, it reproduces by seed or vegetatively from roots. The woody taproot, branching root systems, and stem fragments are capable of developing into flowering stems - an important consideration for plant removal. Seeds are mainly transported by water. They also disperse via wind and animals, remaining viable for close to 20 years. The phenomenal number of seeds produced by purple loosestrife contributes to its successes as an invader. After gaining a toehold, it overwhelms native vegetation and becomes a monoculture. This weed offers poor ecological benefits with little food value, poor cover and nesting materials, and reduction of habitat diversity. However, it may provide better ecological value in some areas than other weedy species such as reed canary grass. Purple loosestrife also causes economic losses through the blockage of agricultural drainage and may be detrimental to recreational activities along waterways.

 

Control Measures

In recent years, a biological control program was established in Canada to help control the problem of purple loosestrife. Three highly host-specific beetles were approved in Canada (1992) as biocontrol agents; one attacks the root system while the other two are leaf-feeding beetles. Efforts are continuing with goals to establish large colonies of insects that can be used for collection sites to be distributed throughout the range of loosestrife in B.C. There are approximately nine beetle release sites in the lower mainland that are currently being monitored 1-2 times per year. So far there are beetle outbreaks at around half of the sites, with complete defoliation and flower suppression. This is encouraging news, creating opportunities for local stewardship groups to get involved with increasing the number of beetle rearing sites.

Mechanical control is also a popular, economical control method for small clumps of these plants, preventing establishment in new areas. These methods are outlined below. Flooding and burning are alternate mechanical mechanisms but are impracticable for most purposes. As for chemical control, in Canada there are no herbicides registered for use against purple loosestrife in aquatic habitats as of 1996 due to the difficulty of restricting the effects of the herbicide to the target species.

 

Non Chemical Control
  Digging/Grubbing Mowing/Cultivation Cutting
How Use a digging tool or hand pulling to uproot young colonizing plants Use power equipment in pastures and damp meadows to mow Use sharp scissors to remove flowering spikes. Also remove last year's dry seed heads and cut stems at the ground to inhibit growth.
When Late June, July and early August, when it is in flower and before it goes to seed. Once flower petals start to drop from bottom of spike, plant begins to produce seed. Late June, July and early August, when it is in flower and before it goes to seed. Once flower petals start to drop from bottom of spike, plant begins to produce seed. Late June, July and early August, when it is in flower and before it goes to seed. Once flower petals start to drop from bottom of spike, plant begins to produce seed.
Duration

 

 Whenever new colonies identified. Repeatedly over several years Whenever new colonies identified.
Pros/Cons Good for up to 1000 plants in an area less than 1 acre. Can prevent reestablishment and reduce seed banks. Not suitable for natural areas! Unlikely to eradicate plants. Make sure to clean equipment to prevent spread of seeds Good for up to 1000 plants in an area less than 4 acres. Will reduce seed buildup but with moderate impacts.

 

Biological Control
 

H. transversovittatus (root feeding beetle), G. pusilla, G. calmariensis (leaf-feeding beetles)
How Scientists release insects at various sites expecting insect populations to increase once established. Site characteristics including habitat, soil type, size of infestation, and water levels are recorded.
When All year round
Duration This is an ongoing process where established insect populations act to control plant infestations.
Pros/Cons Cost-effective, reduces numbers of target plant, helps prevent population explosions. Eradication is unlikely with this technique.

 

Control Warnings:

Disposal - Make sure to properly discard all plant pieces in thick plastic bags and transport them to a sanitary landfill site or incinerator. Composting is not an appropriate means of disposal as this may result in further distribution. Remember that humans can actually spread invasive plants by taking seeds from one place to another on clothing, tires, equipment, etc.

Chemicals - Although some chemicals are approved for control of invasive plants, extreme caution must be taken as many pesticides are harmful to humans. Permits may be required for chemical use and buffer zones exist beside waterways to protect fish and wildlife. Chemical control is not a long-term solution and therefore should be part of a finite plan and applied sparingly. Please see the following web sites for further information: Provincial: MWLAP Pest Information  Federal: Pest Management Regulatory Agency

Biological - Extreme caution must be taken when introducing one organism to control another. Intensive testing must occur before initiating a safe and effective biological control agent. The three beetles mentioned have undergone this rigorous testing process. Please contact local government or environmental agencies to determine available volunteer opportunities.

 

Additional Resources