Fireweed is a highly invasive and opportunistic weed, native to south-eastern Africa. It quickly colonises overgrazed pastures and disturbed areas.
It is a serious pasture weed of coastal NSW. It is able to grow on most soil types and in all aspects.
It forms a persistent seedbank if not controlled before it flowers and can rapidly take over heavily grazed and neglected pastures, competing strongly with existing pasture plants.
It seeds prolifically and grows to maturity quickly. Density is influenced by groundcover and competition, especially in autumn. Fireweed can quickly establish where ground has been cultivated or disturbed, or where groundcover competition is reduced, for example in drought, or under excessive grazing pressure.
The population of fireweed in infested areas depends on seasonal conditions and varies from year to year.
Fireweed infestations may reduce the total grazing capacity of a property, decrease pasture production, decrease livestock growth rates and incur high control costs.
Fireweed is a daisy-like plant which grows from 10 cm to 60 cm high. It has a variable growth habit and leaf structure, but the most common form of fireweed is a low, heavily branched, annual or short-lived perennial plant.
Leaves
Generally bright green in colour, fleshy and narrow, leaves are 2-7 cm long, alternately arranged on the stem, and have serrated, entire or lobed margins. Broader leaves usually clasp around the stem.
Small, yellow and daisy-like, flowers are 1-2 cm in diameter and arranged in clusters at the end of each branch. They can number up to 200 per plant, and each flower will commonly have 13 petals and 21 bracts forming the ‘cup’ under the flower.
Seeds
Seeds are small (1-3 mm long), light and slender. They are cylindrical in shape and have a downy surface. They are attached to a pappus, consisting of fine, silky, white feathery hairs which aid in dispersal by wind.
Fireweed has a shallow, branched taproot with numerous fibrous roots growing from 10 cm to 20 cm deep.
Fireweed is largely restricted to the south-eastern coast of Australia and has established along the entire NSW coast.
First recorded in the Hunter Valley in 1918, it has spread south into Victoria and occurs as far north as central Queensland. It also occurs on the northern and southern tablelands of NSW and as isolated infestations in parts of inland NSW.
It grows on a wide range of soil types, from high fertility, self-mulching clay soils to low fertility, acid, sandy soils. It is most prolific in well-drained, lighter-textured, acid soils of low to medium fertility. It will not survive in poorly drained or waterlogged situations.
Fireweed does not grow well in shaded areas, preferring open country and areas of bare soil. It will quickly invade roadsides, pastures and open forests, and heavy infestations are common on cultivated or disturbed land.
Fireweed is opportunistic and able to take advantage of changes in the environment. Under favourable seasonal conditions, the plant may behave as a short-lived perennial, with some plants surviving for three years. However, the majority of plants only live for one season.
Germination
On the coastal strip of NSW fireweed is capable of germinating, growing and flowering during most of the year. Most stages of the plant (seedlings to flowering) can be seen at almost any time of the year. However, most seeds germinate in autumn.
Germination depends on a combination of rainfall, light and temperature. Rainfall when the temperature is 15-27°C will produce flushes of new seedlings. Seeds are less likely to germinate if they are buried more than two centimetres below the soil surface. However, some buried seed may remain viable in the soil for up to 10 years.
Once released from the flower head, many seeds can germinate immediately, and then several germinations can occur in one season if conditions are favourable.
Plants grow quickly and can produce flowers 6-10 weeks after emergence, with flowering occurring mainly in spring.
Fireweed seeds profusely, with each flower producing between 50 and 120 seeds of high viability. One plant is capable of producing 5 000 to 30 000 seeds in one season depending on conditions. Most seed germinates quickly, but about 15% of seed has a high level of dormancy.
Fireweed usually begins to die off in the spring. However, in some plants, only the top of the plant dies back, leaving the base and roots intact over the summer. The plant may then quickly regrow from the crown the following autumn.
Spread
The light fluffy seeds of fireweed are easily spread by wind. This is the main method of local spread. Most seed will fall within five metres of the parent plant, but some seed can be spread to greater distances in updrafts and whirlwinds.
Dispersal beyond one kilometre is more likely to be caused by unintentional spread by human activity. Means of spread include livestock, clothing, vehicles, machinery, contaminated hay, silage, grain products, wild animals and feral animals.
Fireweed contains pyrrolizidine alkaloids, which are toxic to livestock and cause liver damage. Young stock, hungry stock or new stock not previously exposed to fireweed are the most at risk of poisoning.
All parts of the plant at all stages of growth are toxic. Hay, silage or grain that is contaminated with fireweed plants or their seeds can also be toxic.
Management
Management should include a variety of methods to ensure the best control of fireweed is achieved.
Management options will vary depending on the situation, with different approaches, for example, for grazing enterprises, environmental areas or small area holdings:
For grazing enterprises, management may include using grazing strategies as well as fertiliser application at appropriate times, upgrading pastures, and strategic herbicide applications;
For environmental areas, hand-pulling individual plants and using spot spraying for herbicide application may be more acceptable.
To ensure that the best control of fireweed is achieved, a thorough and systematic management program needs to be in place. It is also preferable to manage a small area correctly than to poorly manage a large area.
Whatever the situation, once established, fireweed is extremely difficult to eradicate. Therefore follow-up treatment is essential for control to be successful.
For successful fireweed control in the long term, it is essential to maintain a vigorous perennial pasture, using fertiliser applications on existing improved or native pastures and matching grazing pressure to pasture growth to maintain a dense pasture. These measures, and allowing a moderate body of pasture litter, particularly in late summer and autumn, will reduce fireweed seed germination and suppress seedling growth and development.
If density, quality and vigour of pastures have declined then pasture improvement by sowing suitable competitive pasture species may be an option. Without competition, fireweed will be able to quickly germinate following rain in autumn and spring and will grow more rapidly than the remaining pasture species.
In the face of severe ongoing drought, consider reducing overall stocking rates before pastures are overgrazed and the ground laid bare.