Population biology and the control of Common Burdock (Arctium minus (Hill) Bernh.) at the University Field Station, Delta Marsh Norm C. Kenkel and Kelly Graham Department of Botany, University of Manitoba Winnipeg, Manitoba, Canada E-mail: kenkel@umanitoba.ca

Introduction

Common Burdock is an Eurasian weed that has become widely distributed in waste places and on disturbed ground throughout much of North America (Gross et al. 1980). In recent years, common burdock has apparently increased in abundance in the understory of gallery forests adjacent to the south shore of Lake Manitoba, particularly in the region of the University of Manitoba Field Station (Delta Marsh). The success of the species is largely attributable to its production of bur-like fruit that are readily dispersed through attachment to animal fur and human clothing. Common burdock is so abundant in some areas that it is now the dominant understory species. Floristic diversity on the forested ridge has been appreciably reduced as a result.

This report summarizes a study of the population biology and control of common burdock at the University Field Station. The project was funded by Canada Trust and the Friends of the Field Station. The objectives of the study were:

• to develop a minimal-impact program for controlling common burdock at the University of Manitoba Field Station, Delta Marsh.
• to examine the population dynamics of common burdock by examining size distributions and biomass allocations.
• to establish permanent plots for long-term monitoring of individual plants.
• to quantify seed production and germination requirements of the species.

Arctium minus (Hill) Bernh. (Common Burdock)

Common Burdock (also known as bur, or cockle buttons) belongs to the Asteracae family. The name Arctium is derived from the Greek word arktos, meaning bear. The plant is native to Eurasia but has become widely established in North America, where it has become a pernicious weed. A closely related species, A. lappa or Great Burdock, was also introduced to North America from Europe. It is also considered a weed, but is not as common nor as widely distributed as A. minus.

Biology and Ecology

Common Burdock is sometimes referred to as a 'biennial'. Strict biennials are species that in the first year germinate, grow, and produce a large overwintering storage taproot. In the second year, the photosynthate energy stored in the taproot is used to produce a large number of flowering heads, a phenomenon known as 'bolting'. The plant dies in the fall, having produced and dispersed its seed. Like many so-called biennials, common burdock is more correctly termed a 'facultative' biennial (i.e. short-lived perennial). Such species can take a number of years before flowering; like biennials, they die once they have flowered (Silvertown & Lovett Doust 1993). In these so-called semelparous (once-flowering) perennials, flowering is related to plant size rather than age. Examples include common burdock, teasel, wild parsnip, wild carrot and evening primrose (Kelly 1985). Semelparous perennial species are generally poor competitors, depending on site disturbance for survival of the population. Most have large seed pools and employ seeds as a means of colonizing gaps as the opportunity arises. Observations made at the University Field Station suggest that common burdock may remain in the vegetative (non-flowering) phase for up to 4 - 5 years, and possibly longer.

Common burdock produces a basal rosette of large ovate to cordate leaves, and a taproot up to 1 m in length. Flowering plants produce a 1 - 2 m angled stem of alternate leaves topped by globose flowerheads (purple tubular disk-florets with hook-tipped involucral bracts). The spiny burs of the fruit readily attach to animals fur and human clothing, thus assuring long-distance dispersal. Common burdock is said to grow in full sun, and to prefer heavy, moist soils rich in nitrogen. However, at the University Field Station it grows on relatively well-drained, nutrient-poor sand in semi-shaded habitats.

Distribution

Both Common and Great Burdock are native to Europe, where they are commonly found growing in fields and pastures, waste places, and other disturbed sites. Common burdock is naturalized in North America, and now occurs in every province in Canada. It is recognized as a weed of waste places, usually on moist fertile soils (Frankton & Mulligan 1970).

Economic Uses

Common Burdock is a traditional herbal medicine in Europe and parts of North America. The boiled root of year-old plants is used, and recent research has indicated that the plant has diuretic, antiseptic, diaphoretic, hypoglycaemic and choleretic properties. An external poultice of leaves said to be effective against skin irritations, and external antiseptic properties have been demonstrated scientifically (Dwyer & Rattray 1986).

Alternative Methods for Controlling Common Burdock

It was necessary to develop a method for the careful removal of common burdock plants, while minimizing substrate disturbance as well as damage to the native vegetation. The options considered were:

• Selective or 'spot' herbicide application. This method has the advantage of not disturbing the substrate. However, there is the potential for accidentally spraying native vegetation. Furthermore, the use of herbicides is expensive and not environmentally friendly. Finally, there was some concern that herbicides might not kill common burdock, since it has a large, deep taproot.
• Pulling up and/or digging. The main advantages of this approach is that it is environmentally friendly, and it is effective unless the taproot is not completely (or largely) removed. A potential disadvantage is that digging necessarily results in localized substrate disturbance. However, preliminary digging trials indicated that substrate disturbance was minimal, in large part because of the sandy substrate along the forested ridge. This option was therefore implemented.

Population Biology of Common Burdock

Controlling Common Burdock will undoubtedly be a continuing project, at least for a few years. The species likely has a persistent soil seed bank, so that removal of existing plants is no guarantee of immediate species extirpation. Furthermore, it is likely that small seedlings will be missed even by the most careful worker. It is therefore important to obtain more information on the population biology of the species at Delta Marsh, so that an appropriate long-term control program can be implemented. We have collected the following information:

• Determination of the age-size structure of populations. Two major 'age' classes of burdock can be readily distinguished: flowering and non-flowering. In addition, non-flowering individuals can be classified by size (based on leaf size and/or the total number of leaves produced). We have calculated population size distributions to determine the proportion of seedlings that survive to flowering. Permanent plots with marked plants have also been established to follow the fate of individuals over time.
• Seed viability and germination. Seed was collected from one-year old burs to determine seed viability and germination requirements.
• Biomass allocation studies. Allocation to taproot, stem/leaf and reproductive parts (in flowering plants only) was determined. Biomass allocation patterns between non-flowering and flowering plants were compared, and determination of flowering likelihood based on taproot biomass was examined.

Materials and Methods

1. Species Removal

Smaller plants were dug up using a standard garden 'dandelion fork', and larger ones using a long-handled bulb planter. Non-flowering plants were carefully dug up (obtaining as much of the taproot as possible) and scattered on the ground to decay. In this way, nutrients were recycled back into the ecosystem. However, flowering-fruiting individuals (bolting) were removed from the site and burned to prevent further seed spread. At least 5000 individuals were removed during the summer of 1994.

2. Population Size Structure

Plots for determining population size structure were established at various locations. Within each plot, individual plants were randomly selected for measurement. The number of leaves was recorded, and each leaf was measured from the point of blade-petiole attachment to the leaf tip. Each individual was placed into one of three age-size classes: class 1 = all leaves < 20 cm; class 2 = at least one leaf > 20 cm, but not bolting; class 3 = bolting plants, regardless of leaf size. The height of all bolting plants was recorded, and the number of burs counted. Whenever possible, the taproot of each plant was measured from the base of the leaf crown to the tip.

3. Phenology

Throughout the summer months (June - August, 1994), observations on timing of leaf production, bolting and flower and fruit production were made.

4. Species Associations

At each plot location, all plant species within the plot were recorded (nomenclature follows Scoggan 1957), and the surrounding trees and shrubs identified to species. Canopy cover was recorded using three classes: full shade, >70% canopy cover; semi-shade, 30-70% canopy cover, and high light, < 30% canopy cover.

5. Biomass and Reproductive Allocation

Plants were randomly selected for biomass allocation determination in mid-August 1994. Only plants of size classes 2 and 3 (see section 2 above) were selected. Non-flowering plants were divided into above-ground (basal leaves) and below-ground (taproot) parts. For flowering plants, the following divisions were made: taproot, basal leaves, vegetative bolt, and fruit. All material was collected, sorted, and dried to constant mass in a standard drying oven at 85°C.

6. Establishment of Permanent Plots

Six square permanent plots for long-term monitoring were located in various regions of the forested ridge. Sites were chosen to be representative of various site conditions, including proportion of vegetation cover (herbaceous and canopy layer) and proximity to animal pathways. Plot size varied depending on the number and size of common burdock individuals within them. Most plots contained between 15 and 20 individuals.

In June 1994, all burdock individuals >10 cm in height were carefully marked with a stake (placed on the north side of the plant) and given a number. Plants < 10 cm tall were recorded on a map of the plot and labelled as 'seedlings'. Plant measurements were made (as outlined in section 2 above) in June and August of 1994.

7. Germination Experiments

Common Burdock burs from the previous year (1993) were collected from six dead plants on July 8, 1994. Seeds were removed from the involucre and placed in an open petri dish to dry. In August 1994 the following germination trial were undertaken:

(a) Immediate germination in petri dishes:
Petri Dish
1 - 20 seeds placed between 2 paper towels.
2 - 40 seeds placed between 2 paper towels.
3 - 20 seeds placed on top of soil.
4 - 20 seeds placed in soil.

(b) As above, but seed was first refrigerated at 4°C for 24 hours, lowered to -6.5°C for 2 hours, and then returned to 4°C for 15 days prior to the germination trials.

Soil used in the germination experiments was collected from the field station 'garden' and dried for 2 days at 90°C before use. Petri dishes with lids were placed in a growth chamber (light for 15 hours at 22°C, dark for 9 hours at 8°C) and watered as required. Light was from fluorescent tubes measured at 106 µmol/s/m.

Results

1. Species Removal

Common Burdock plants have been removed from the forested ridge in an area extending from the western boundary of the University Field Station to just east of the three cottages (linear distance along the beach about 350m). Plants have not been removed from the area south and west of the Lawrence lab, however. Plants have also been removed from the area of the forested ridge west of the Assiniboine River Diversion road for a distance of approximately 300 m. Due to time constraints, plants along a 300 m section of the forested ridge immediately east of the cottages have yet to be removed.

2. Population Size Structure

Of the 349 individuals collected for measurement, 263 (75.4%) were in the small size class, 64 (18.3%) in the large size class, and 22 (6.3%) were bolting. This suggests that seedling recruitment into the existing population is high, and that a large proportion of plants die before they have a chance to flower.

Summary size information for each of the three classes in presented in Table 1. Plants in the smallest size class produced on average less than two leaves, and had a comparatively small taproot. Larger, non-flowering plants averaged more than 3 leaves per plant, and had a much larger taproot. The mean number of leaves, mean leaf size, and taproot length was greatest for the flowering individuals. These plants averaged almost a meter in height (94.9 ± 44 cm), and produced on average 77.4 ± 56.6 burs per plant.

A summary of the six permanent plots is presented in Table 2. All plants except the seedlings have been marked and will be revisited in the summer of 1995 to determine mortality, plant size and the proportion of plants bolting.

Table 2. Summary of the six permanent plots established to monitor long-term population dynamics in burdock (Arctium minus).

Plot	Location	Size	Habitat	Burdock plant sizes	Associates
1	West side of Wardle residence, UFS (DM)	2 x 2 m	sunny, forest edge	small = 21, large = 10, bolting =3, seedlings (estimated) = 91	Cirsium arvense, Urtica dioica, Humulus lupulus, Convolvulus sepium
2	East of Cottages, UFS (DM)	3 x 3 m	semi-shaded, high canopy forest, no shrubs	small = 50, large = 18, bolting = 2, seedlings (estimated) = 21	Ribes americana, Urtica dioica, Rubus idaeus, Parthenocissus quinquefolia, Acer negundo (seedlings), Sambucus canadensis
3	Along mist net trail, east of cottages, UFS (DM)	2 x 2 m	moderately shaded, low tree canopy, some shrubs present	small = 7, large = 4, bolting = 2, no seedlings	Rubus idaeus, Galium triflorum, Parthenocissus quinquefolia
4	Between plots 2 and 3, east forested ridge, UFS (DM)	3 x 3 m	somewhat shaded, close to beach. Isolated population	small = 10, large = 10, bolting = 4, no seedlings	Artemesia absinthium, Rubus idaeus, Humulus lupulus, Galium triflorum, Poa pratensis, Parthenocissus quinquefolia, Osmorhiza longistylis, Ribes triste
5	west of beach path on Portage Country Club property	2 x 2 m	moderate to full shade, evidence of old (dead) burdock plants	small = 15, large = 6, bolting = 2, seedlings (estimated) = 172	Ribes triste, Urtica dioica, Taraxacum officinale, Fraxinus pennsylvanica (seedlings), Solidago canadensis, Humulus lupulus
6	West of Plot 5	1 x 1 m	highly shaded	small = 48, no others observed	Uritica dioica, Galium triflorum, Lappula deflexa

7. Germination Experiments