Article by Sandy Benson

Many weed species in the Blue Mountains are ‘fire-responsive’. Post-fire conditions make it easy for weeds to establish due to favourable conditions, they germinate prolifically and can spread vigorously within the first few seasons.

Weed species gradually establish long-term soil seed banks that are triggered to germinate en masse by fire. In the absence of targeted weed control, weed species rapidly spread and can form a dense ‘carpet’, outcompeting native species.

However, this can be to your advantage. It’s a great time to treat weed infestations as they are more accessible than ever before. Usually they are the first to emerge, easy to spot but also easy to access. If you control emergent weeds before they set seed, you’ll be able to get on top of these weedy patches much more quickly. Timely post-fire management action (usually within 18 months) is necessary for control.

Opportunities

Improved access post-fire provides an excellent opportunity to control weeds that are not usually easily accessible. This certainly applies to dense riparian vegetation and our
Blue Mountains Swamp communities, where the dense vegetation impedes access to established weeds, or wherever the foliage of established weeds is beyond the reach of
physical or chemical methods. Unless burnt, weeds in these locations usually escape control efforts. Post-fire, the sparse vegetation allows the foliage of resprouting taller weed species to be easily located and within range of control.

Weed Species proliferation post fire

There are several ways weed species can proliferate after fire:

• Weed seed bank explosion in usually unaffected areas
• Kill or reduce the number of established mature plants but post-fire conditions are ideal for the seed bank to explode
• Resprout from base of mature plants
• Burn or char the weed species, then plant sends up numerous suckers

Each of these reproductive pathways requires a different weed management strategy

Weed seed bank explosion in usually unaffected areas

Weed seeds can remain dormant in the soil for many years.The fruits of weeds are attractive to a wide range of animals that can spread seeds such as foxes, rabbits and bird species. Seeds can also be spread by dispersal from the parent plant or by wind or water. This may increase the number of weeds present at a site in the short term. But over time and with weed treatment it can deplete the weed soil seed bank, and be replaced by native species and a healthier natural state in the long term.

Likely pathways of weed seedlings germinating in areas previously clear of weed species are post-fire flooding, wind and bird distributed seeds from neighbouring unburnt areas. Post-fire these seedlings once mature, set seed and are likely
to spread dramatically.

An opportunity exists where, a weed seed bank explosion can lead to a weed seed bank depletion. Fire can result in one-off increases in weed densities, which after subsequent
fires rapidly decline if weed treatment is quick and consistent. If weed seedlings are not controlled, they will outcompete native seedlings, exhausting the native soil seedbank.

Consistent follow-up seedling control is necessary in areas of low fire intensity, near water sources, and where seeds have dispersed into the burnt area from unburnt sections of the population. This can happen particularly in the Blue Mountains
where seed from unburnt areas can carry down a slope and be deposited on burnt ground. The removal of these seedlings before they mature and set seed is a high priority.

Weeds commonly displaying this type of fire response in the Blue Mountains include:

• Gorse Ulex europaeus
• Cape Broom Genista monspessulana
• Scotch/English Broom Cytisus scoparius
• Perennial grass Ehrharta calycina
• African Lovegrass Eragrostis curvula
• Coolatai Grass Hyparrhenia hirta
• Patersons Curse Echium plantagineum
• Cotoneaster Cotoneaster spp. (franchetii, pannosus, lacteus, glaucophyllus, horizontalis)

Kill mature plants/seed bank explosion

Depending on fire intensity and thickness of the weed species trunk or stem, fire can kill adult plants with little or no re sprouting post-fire. However, weed seeds are long lived, remain in the soil and can be triggered to germinate by fire. Many of the Blue Mountains most invasive and pervasive woody weeds fall into this group.

Trees and shrubs – some trees and shrubs are killed by fire and do not sucker or resprout post-fire, these plants rely solely on seed to regenerate. Many germinate straight after fires. The removal of seedlings and juveniles before they mature and set seed is a high priority.

Trees commonly displaying this type of fire response in the Blue Mountains include:

• Radiata Pine Pinus radiata
• Cherry Laurel Prunus laurocerasus
• Holly Ilex aquifolium
• African Olive Olea europaea ssp. cuspidata

Woody Weeds – A hot fire will kill mature wood weeds but encourage mass germination of seeds, occurring in denser, more vigorous patches. Once these become established,
they quickly produce large amounts of seed. Mass germination can reduce the weed soil seed bank over time, but only through sufficient follow-up weed treatments
over many years, otherwise a denser infestation is likely to result.

Post-fire look for:

• Cape Broom Genista monspessulana
• Scotch/English Broom Cytisus scoparius
• Gorse Ulex europaeus
• Lantana Lantana spp.
• Cotoneaster Cotoneaster spp. (franchetii, pannosus, lacteus, glaucophyllus, horizontalis)

Climber, scrambler or creeper – Usually fire kills adult climbers, scramblers and creepers, but triggers germination in the soil weed seed bank. These seeds dependent on the species can return in denser infestations, particularly with climbers that produce many seeds in their fleshy fruits. These seeds are usually favoured by birds so may be found close to the parent plant or in a new location.

In the Blue Mountains look for:

• Balloon Vine Cardiospermum grandiflorum
• Banana passionfruit Passiflora mollissima

Resprout from base of mature plants

Some weeds resprout from the base of burnt mature plants from regenerative buds protected underground or beneath layers of bark. There is a short-term decrease and
reduction in biomass and/or densities as mature plants are temporarily ‘weakened’ post fire. There is an opportunity to achieve better results if prompt weed treatment is
undertaken on fire weakened weeds.

Trees and shrubs – Weed trees may resprout post-fire, but not for all species.
• African Olive Olea europaea ssp. Cuspidate
• Lantana Lantana spp.

Woody Weeds – Gorse evolved as a fire-climax plant, readily catching fire and burning to ground level but regenerating from the base after the fire. In the Blue Mountains look for:
• Gorse Ulex europaeus

Climber, scrambler or creeper – When burnt these species may receive a boost post-fire with vigorous resprouting and/or seedling regeneration into fertile, sunny sites.
Response is strongly dependent on fire severity.

Fire generally kills Blackberry’s seasonal canes but the root crown usually survives and regrowth can be quite vigorous after fire. Post-fire environments provide a unique opportunity for control as all foliage is accessible.

Climbing, scrambling and creeper weeds commonly displaying this type of fire response in the Blue Mountains include:

• Blackberry Rubus fruticosus spp
• Balloon Vine Cardiospermum grandiflorum
• Cats Claw Creeper Dolichandra unguis-cati syn. Macfadyena unguis-cati
• Japanese Honeysuckle Lonicera japonica

Bulbs, corms, tubers or rhizomes – Weeds species with a bulb, corm, rhizome or tuber are unlikely to be burnt by fire as the corms or tubers are protected from the heat by being located underground. Many species resprout vigorously post-fire and can invade
bare ground.

The removal of corms or tubers is a high priority. Those with long strappy leaves can be treated by wiping (wiping herbicide along the strappy leaves with a herbicide wiper).

Weeds commonly displaying this type of fire response in the Blue Mountains include:

• Montbretia Montbretia Crocosmia x crocosmiiflora
• Watsonia Watsonia meriana var. bulbillifera
• Turkey Rhubarb Acetosa sagittata
• Morning-glory Ipomaea purpurea
• Madeira Vine Anredera cordifolia

Suckering

Weed species can sucker post-fire, meaning its principal means of propagation is suckering from the roots vegetatively. Trees under stress post-fire can send up numerous suckers as a defensive mechanism. This can lead to dense stands forming and a monoculture of the same species, excluding all native species from that site.

Weeds commonly displaying this fire response include suckering trees, suckering woody weeds and brambles.

Trees and shrubs – Mature Camphor Laurel trees have been documented suckering profusely after being burnt.

Weed management should be undertaken by killing the parent tree and suckering plants by treating each with herbicide. Weed trees commonly displaying this type
of fire response in the Blue Mountains are:

• Tree of Heaven Ailanthus altissima
• Weeping Willow Salix babylonica
• Small and large leaf Privet Ligustrum spp.
• Sycamore Platanus orientalis
• Camphor laurel Cinnamomum camphora
• Black Locust/Robinia Robinia pseudoacacia

Wood Weeds – There are a limited number of wood weeds that sucker or resprout post-fire in the Blue Mountains. These should be treated with herbicide as a high priority. Look for:

• Spanish Heath Erica lusitanica
• Cotoneaster Cotoneaster spp. (franchetii, pannosus, lacteus, glaucophyllus, horizontalis)

Are weeds good after fire?

Weeds can act as a buffer to reduce erosion and as a cover crop for native seedlings and animals post fire while natural native regeneration occurs.

Erosion risk increases after a fire due to the lack of groundcover to stabilise the soil which slows down the speed of runoff. Weeds are usually the first plants to emerge
after fire. If left for 3 to 6 months they can act as a ground layer, the plant roots stabilise the soil, and stems and leaves slow the water to give it time to percolate into the soil profile.

At the early stages any vegetation cover, including weeds can protect native seedlings. Weed seedlings grow quickly and can perform several jobs; protecting native seedlings
from erosion, drying out, returning nutrients to the soil and to provide food and shelter for insects and animals.

Weeds can be helpful up until a point, then they can be the bushlands worst enemy if left too long.

Assess burnt areas for weeds and the best control methods for the species. Control and target weeds before seed set, but limiting trampling as much as possible while bushland is still fragile.

When to treat weeds?

The months following a bushfire are among the best times to control weed species. However, it is very important to remember to leave burnt areas alone for the first 3-6 months to allow the soil to recover and native seedlings to establish, as over enthusiastic weed control can cause damage.

Post fire, soil forms a crust (soil sealing) that protects and reduces the loss of soil, organic matter and seedbank from rain events and erosion. The crust is formed through a combination of elements; when rain hits the soil, it dislocates the silt and clay making way for moss, lichen, algae or fungi, and cyanobacteria to enter which then
forms a surface crust.

The combined protective cover elements such as the soil crust and seedlings can protect the soil throughout the first-year post fire. Natural regeneration is the priority.
Monitoring the site for weed growth indicators such as fresh new growth and flowering should be used as the cue for treatment. Take care to prevent any off-target damage
to native plants.