Controlling Fireweed

Fireweed is an opportunistic weed that will germinate mainly in bare of uncompetitive pasture. The biggest problem for the southern tablelands is that fireweed germinates on autumn rains, right when our pastures have come through summer and are generally lacking any competitiveness. Try not to overgraze the summer feed, leaving some ground cover to provide early shading, minimise bare ground and allow pasture plants to recover quickly.

Most seedlings emerge in flushes during autumn and then to a lesser extent in late winter/spring, though the weed is capable of germinating and flowering throughout the year.

Look for seedlings 2 weeks after rainfall events in autumn in bared areas. If populations are detected, act early. For large-scale and dense infestations, herbicides can be effective in killing plants before flowering but vigorous pasture competition will be needed to prevent plants emerging in future.

To maximise pasture competitiveness:

• rotate grazing between paddocks and avoid high stocking rates in heavily infested pastures at times prior to and when fireweed is germinating, particularly in late summer and autumn.
• Use timely fertiliser applications with the right products at the start of the active growth period of the pastures can help suppress fireweed.
• Ensure a healthy pasture by soil testing and making sure your soil health is at its optimum and that lime and fertilisers being applied are correct for your pasture.

The 3 dimensions to success
Have a plan, in which you use several methods of control, and be dedicated to achieve success.

• Broadacre selective herbicide
• Grazing management
• Pasture competition

Pasture competition
A competitive pasture is a key requirement of effective long term fireweed control. However, if re-establishing pasture, it is vital that this is managed carefully, so that the fireweed problem is not made worse when disturbing soil and weakening or killing existing pasture.

A central principle of any fireweed control programme must be maintenance of a vigorous, competitive pasture. Any factors that open up pastures, such as drought, overgrazing, uncompetitive pasture species, cultivation, spraying out a pasture with a non-selective herbicide, and areas bared by trampling (e.g. livestock camps and those around watering or feeding places), favour the development of fireweed. Managing your existing pasture well to ensure a dense cover will go a long way towards minimising the growth of fireweed on your land. A dense pasture during early autumn to winter and spring (including maintaining a moderate level of leaf litter into the winter) is likely to provide the best form of control. This may be achieved by growing early winter pasture species, by allowing standover of summer pasture feed, or by using winter-summer pasture combinations.

Grazing Management

Grazing management may involve:

• Grazing strategies that ensure pasture cover is maximised and fireweed has less chance of establishing and spreading
• Using livestock that are able to consume fireweed to control established fireweed populations.

Landholders running cattle on their property can adapt their grazing strategy to allow pasture to become more competitive, and minimise the chance of fireweed establishing dense populations. A simple approach involves avoiding a high stocking rate (i.e. not grazing your pastures with too many animals), rotating your grazing between paddocks, and reducing grazing pressure at certain times in impacted paddocks. Removing livestock is particularly helpful prior to fireweed germination to increase pasture competition, and when pasture plants are setting seed (often in summer) to fill in the pasture gaps with new seedlings. Experience shows that grazing with sheep or goats may also be of some use in fireweed control, since they readily eat fireweed. A recent survey of farmers found this to be the most successful fireweed control method. Farmers who used the method suggested that sheep or goats will control most but not all fireweed plants.

Herbicides

It is crucial to commence fireweed control with herbicides well before the first flowers appear and the plant becomes more highly visible in the paddock. Look for seedlings 2 weeks after rainfall events in autumn in bared areas, and if populations are detected, act early to control to cut costs and increase the likelihood of success.

Using the non-selective glyphosate herbicide through a wick-wiper can be effective as the herbicide is only wiped on to the taller weeds and not the shorter grazed pasture. Even with older more resilient flowering plants, two passes of an applicator using glyphosate can give up to a 95% kill of fireweed. large outbreaks of fireweed may best be sprayed with a registered selective herbicide using a vehicle mounted ‘boom’ applicator. Small outbreaks can be ‘spot sprayed’ using a backpack or handheld spray applicator. The selective herbicide bromoxynil is effective on young plants, and is suitable in pasture situations as it will not affect most desirable pasture species. However, when applied to larger, more advanced plants, the effectiveness of bromoxynil drops.

A general herbicide strategy is to apply a selective herbicide with a boom spray during autumn, when the highest proportion of fireweed seedlings will be present in the pasture. This may mean waiting for a second flush of germination where further rain is expected to make the most of your single herbicide treatment. Selective herbicides registered for boom spray application for fireweed include bromoxynil (e.g. Bromicide 200) and a mixture of bromoxynil and diflufenican (e.g. Jaguar). Jaguar has a shorter 2 week withholding period for livestock, compared with 8 weeks for Bromicide. A permit has also been issued to allow off-label use of 2,4- D amine (e.g. Amicide 625) to control fireweed. Trials suggest a mix of bromoxynil and 2,4-D amine may offer the best fireweed control. Follow-up spot spraying of more mature or flowering plants may be undertaken later in the season (for example in spring) using herbicides registered for that purpose, including triclopyr/picloram/aminopyralid (Grazon Extra) and fluroxypyr/aminopyralid (Hotshot). However, late spraying is not ideal for the reasons already described, and these less selective herbicides may also result in greater damage to desirable legume plants where they are present in the pasture.

Herbicide application has been a key aspect of some successful fireweed control programmes. However, herbicides do not generally kill fireweed seeds if applied after flowering. Herbicides are short term solutions and pasture improvement should be the long term goal. Herbicide resistance in weeds is also a potential problem from repeated broad scale applications of a particular herbicide group.

Bringing the control methods together

• Plan your approach to fireweed control.
• Use a combination of both ‘weed removing’ and ‘pasture improving’ techniques.
• Maintain a thick competitive pasture and avoid creating bare ground to suppress emergence.
• Target seedlings with herbicides.
• Pull mature plants or slash before flowering.
• Enlist the help of sheep or goats.
• Stick at it!