Biology and Ecology

Taxonomy. Hieracium is a large genus with about 260 plant species native to Europe and approximately 25 species native to North America. Twenty-one non-native Hieracium species are known to occur in North America, four of which have been found in Montana. They are H. aurantiacum (orange hawkweed), H. caespitosum (=Hieracium pratense, meadow hawkweed), H. floribundum (yellow devil hawkweed), and H. piloselloides (tall hawkweed). The latter three species all have yellow flowers and are hard to distinguish from each other due to hybridization. As of 2006, only H. aurantiacum has been reported in Wyoming. Hieracium is from the Greek root hierac, “a hawk”. The ancient Greeks attributed the keen eyesight of hawks to their eating the milky sap of Hieracium species. Other common names include devil's paintbrush, fox and cubs, king devil, and missionary weed.

Hawkweed species interbreed freely and have long been known for their complex, convoluted, and confusing classification. Detailed descriptions of Hieracium pilosella have listed 624 subspecies. Historically, Hieracium is one of the genera Gregor Mendel used to verify the laws of inheritance he developed from crossing varieties of pea plants (Pisum). Unfortunately for Mendel, all his painstaking emasculations and crossing experiments on Hieracium species he used led to results in direct contradiction to his laws because of the apomictic development of seeds. Embryos of Hieracium developed into seeds without pollen fertilization (parthenogenesis) and resulting plants were clones of the mother plant and not the combination of DNA from pollen and ovule.

Figure 2. Orange hawkweed scape, rosette, and seed schematic.

Identification. The invasive hawkweeds are creeping perennial forbs with fibrous roots and milky sap. They produce basal rosettes with persistent narrow leaves four to six inches (10 to 15 cm) long (see Figure 2). The dorsal leaf surface is dark green, the ventral leaf surface is light green, and the margins are entire or minutely toothed. Orange hawkweed leaves are bristly with short hairs and meadow hawkweed leaves are smooth. The leaves are broadest near the tip tapering to a somewhat pointed base - oblanceolate (see Figure 2). Up to 30 flowering stems with short stiff and stellate (branched, or star-shaped) hairs can grow from each rosette. Flowering stems can be leafless, or there can be one to three clasping leaves alternately arranged on the lower half of the flowering stems, the upper leaves smaller than the lower leaves. Each stem terminates with one to 50 bright orange (orange hawkweed) or yellow (meadow hawkweed) dandelion-like flower heads arranged in umbelliform (where there are many flower heads) inflorescences (see Figure 2). There are 12 to 30 ray-like (ligulate) flowers per head that produce columnar seeds with a tufted, tawny papus of persistent bristles on the flattened end of the seed. Up to 12 leafy, creeping aboveground stems (stolons) grow four to 12 inches (10 to 30 cm) from the flowering plants and can establish new rosettes (similar to strawberry plants).

Native Hieracium species can be distinguished from the non-indigenous hawkweeds by their leafy-branched flowering stems, absence of stolons, and flower heads arranged in open panicles. Hieracium canadense (=H. umbellatum, Canadian hawkweed), reported from north and central Montana, is yellow flowered and has small and early deciduous basal leaves. The leaves along the flowering stem are larger in the middle of the stem and reduced above. Hieracium albiflorum (white hawkweed), reported from western and central Montana, has white flowers. Hieracium gracile (slender hawkweed), reported from west and central Montana, has yellow flowers but the rosette leaves are hairless and less than four inches long (<10 cm). Hieracium albertinum, H. scouleri, and H. cynoglossoides all have long hairs on the leaves and flowering stems. None of the native hawkweeds are considered invasive.

Roots. The hawkweeds have shallow, fibrous rooting systems. Rooting depth ranges from 1.5 to 4.5 inches (4-14 cm) and roots spread only about 4 inches (10 cm) on average with only a few small branches, many with no root hairs. Mycorrhizal associations (symbiotic fungi) have been reported from populations in Europe with up to 92% vesicular-arbuscular mycorrhiza infection in roots of Hieracium pilosella. In this association, the fungi gain sugars from the plants and the plants gain soil nutrients from the fungi. Radio labeled carbon studies showed a significant transfer of 14C from Festuca ovina to H. pilosella via mycorrhiza.

Meadow hawkweeds also grow shallow rhizomes from axillary buds at the base of rosettes, and new rosettes can grow from adventitious buds located on the fibrous roots. Underground rhizomes and adventitious buds also serve as over-winter structures from which plants re-grow each year.

The phytotoxic chemicals chlorogenic acid, caffeic acid, umbelliferone, and apigenin-glucoside have been isolated from the roots of hawkweeds and are exuded into the soil by hawkweed plants. These chemicals may suppress the growth of competing plants, and may be antibacterial. Antibacterial effects of extracts of H. pilosella have been used as an antibiotic against brucellosis.

Rosettes. Rosettes are the vegetative phase of hawkweed populations and consist of five to eight low-lying hairy basal leaves. Among other functions, the hairs on the leaves prevent desiccation and herbicide uptake. Rosette leaves begin growth in April and usually grow upward at an angle to the ground except when growing under grazing pressure or harsh conditions, when they lie flat on the ground (see Figure 3). Rosettes form from seed, and from buds on stolons (orange and meadow hawkweeds), rhizomes, and roots (in the case of meadow hawkweed). In dense patches, rosettes tend to produce stolons in the middle of the patch and flowering stems on the perimeters. Under ideal conditions, one orange hawkweed plant can spread by stolons and infest an area two to three feet in diameter in its first year of growth. Rosettes are intolerant of shading by competitor plants. Hawkweed rosettes contain a number of phenolic chemicals with phytotoxic properties, including chlorogenic acid, caffeic acid, and umbelliferone, in the green leaves and in water extracts of dead leaves. Leachate extracted from hawkweed leaves suppress the germination and growth of some plants.

Figure 3. Hawkweed rosettes form early in spring.

Flowers. The initiation of flowering begins in late May and peaks in mid-June. A second flowering period in September has been reported. Hawkweed rosettes are monocarpic perennials. They can survive many years (half-lives of up to nine years) but die within a few months after flowering. A single rosette is capable of producing up to 30 flowering stems. Often the stems are bare of leaves although they may have one to three small clasping leaves. The stems are one to 36 inches tall and can produce up to 50 flower heads. The stems have black hair all along them and produce a milky sap.

Each flower head ranges from one-half to three-quarters of an inch in diameter, and consists of many tiny orange florets, each capable of producing one seed (see Figure 4). Flowers open in bright sun and are pollinated by a number of insects including bees, flies, moths, butterflies, and beetles. Although pollen of an individual floret develops before the style is receptive, self pollination is facilitated at the end of floral development when the stigmata recurves and contacts their own pollen-coated style. This increases pollination either by insect or by self pollination. In addition, Hawkweed plants can produce seeds without pollination. Each flower head can produce between 12 and 30 seeds.

Figure 4. Hawkweed flowers showing vibrant color and square petals.

Seeds. The seed of Hawkweeds is an achene. Seed production can be as high as 13,000 achenes/m2 in ungrazed grasslands and 500 achenes/m2 under moderate grazing by rabbits. Intensive grazing can prevent all seed production. The seeds are black and have a pappus attached to one end that facilitates wind dispersal (see Figure 5). In addition, each seed is ribbed, and the ribs have minute barbs that enable them to stick to hair, fur, clothing and vehicles. Seeds can be dispersed by wind, water, or "hitch-hiking", and are often moved in contaminated soil associated with transplanting new plants into gardens and flowerbeds. Seeds remain viable in soil for up to seven years and can germinate as soon as they are released from the parent plant.

Most (99%) of new plants in a population are derived from vegetative reproduction rather than from seeds. Also, most seeds (80%) are dispersed within a population colony and less than 1% is found at a distance farther than 30 feet (10 m) from the parent patch regardless of the papus that aids in wind dispersal. Ninety percent of seeds are produced along the periphery of a hawkweed patch.

Many newly shed achenes germinate readily but there is some evidence of dormancy. Storage of achenes at 41oF (5oC) for 3, 6, and 12 months increased germination from 47 to 83, 72, and 81% respectively. Seeds had 25% germination after storage at 72 oF (22 oC) for six years.

Life history. The hawkweeds have four general life history stages: seeds, seedlings, rosettes, and flowering plants. The seed stage is important for establishment of new populations at long distances from the parent population and regeneration of the population after an event that kills the parent population, like an herbicide application. The seed stage is adapted to survive long periods of time and harsh conditions.

The seedling stage is the most vulnerable life history stage. In New Zealand, distinct flushes of germination occurred at the end of five to eight day moisture intervals, but seedlings only survived a few weeks. Experiments show seedling emergence is greatest in September and July and tends to follow soil disturbance in summer or early autumn. Seedlings that germinated in the summer grew quickly and reached adult size in eight to ten weeks. Most seedlings that germinate outside the parent colony die of desiccation or winterkill. Of 1,500 H. Pilosella seedling sown in July in experimental plots in Nova Scotia, 102 seedlings were recorded but only 18 were well established by October, the rest were killed by drought, high temperatures, and browsing. Over-wintering rosettes can be produced in as little as 47 days after germination. Only about 1% of rosettes in a colony are derived from seed, the rest are derived from stolons. Seedlings that germinate in the spring are most likely to survive and transition to rosettes.

Hawkweed populations are made up mostly of rosettes with densities of up to 3,200 plants per square yard (3,500/m2). Population expansion within established patches is predominantly vegetatively from stolons or rhizomes that initiate from rosettes. The transition from the rosette stage to the flowering stage is density dependent. Most flowering plants develop at the edge of a patch where rosette densities are lower than at the center of the patch where densities are greatest.

Hawkweeds are capable completing their lifecycles quickly. Seedlings emerging in March can produce flowers by mid-June and viable seeds by early August. As perennial hemicryptophytes, hawkweeds have their over-wintering buds at the soil surface.

Spread. Patch expansion of hawkweeds is primarily via stolons or rhizomes. Long-distance spread is primarily via seeds that can be carried by the wind, water, on the fur or in the gut of wildlife and livestock, and in soil. Seedlings have been reported in the excreta of birds. The buoyancy of the achenes in water is less than 18 hours. The rhizomes and stolons are spread during cultivation and under intensive grazing. Hawkweeds were first cultivated in the United States as garden plants because of their bright flowers. Hawkweed seeds are still marketed in flower packets and sold as garden flowers.

Habitat. Meadow hawkweed is a regular component of the native flora of the foothills of northern, central, and eastern Europe. Orange hawkweed originates from mountainous meadows and hillsides of northern and central Europe, and is cultivated elsewhere. The sub-arctic, temperate, and montane distribution of mouse-eared hawkweed indicates intolerance of high temperatures. In England, hawkweeds thrive in areas of low rainfall, dry over-grazed pastures, and steep slopes, and are seldom found in shaded habitats. In Europe they are mostly found in small patches in disturbed areas, and are considered ruderal species of pastures, roadside cutbanks, and meadows. Sites most susceptible to invasion are roadsides, moist mountain meadows, forest meadows and clearings, permanent pastures, hayfields, cleared timber units, and abandoned farmland. Soils that are well-drained, coarse-textured, and moderately low in organic matter are most likely to support hawkweeds, but they can tolerate a full range of conditions from gravelly to acidic soils, full sun to partial shade, and frost and snow cover. Hawkweeds are found in habitats that support oxeye daisy (Chrysanthemum leucanthemum), sulfur cinquefoil (Pontenilla recta), spotted knapweed (Centaurea bieberstenii), grey goldenrod (Sloidago nemoralis), wild carrot (Daucus carota), and dandelion (Taraxicum spp.).

Economic Impacts. Hawkweeds can form dense mats and choke out all other native vegetation in wildland settings. They cannot compete with crops due to herbicide use and many crops have a higher competitive ability. Hawkweeds are palatable species, fairly high in nutrients, so many grazing animals will eat them. However, they can become mono-cultures and choke out other desirable forage species. Infestations can spread to threaten lawns and gardens.

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Last Modified: 10/11/2007