Taming Small flowered Mallow ( Marshmallow)

Author, John Stuchbery
Agency JSA Independent

Introduction

Small flowered mallow or marshmallow (Malva parviflora) has increased in prevalence in grain growing regions in recent years. There have been several contributing factors to its increased prevalence including: more intensive weed control removing competition and enabling marshmallow to dominate; a reduction of grazing in cropping systems; less cultivation particularly with full cut shears and few cost effective herbicide options (Taylor 2002).

Biology

The following information on biology of marshmallow is drawn from Michael et al (2009). Marshmallow is adapted to the southern cropping zone and can germinate and emerge throughout the year. Maximum emergence occurs when seeds are buried 0.5 to 2 cm deep, with emergence decreasing when seed is buried deeper up to a depth of 8 cm. Emergence is greater under no till systems than systems which include cultivation. Seeds can germinate over a wide range of temperatures (3.3 – 37 degrees C) but optimum temperatures for germination are between 15 and 20 degrees C, which are typical of a winter rainfall Mediterranean type of environment.

Marshmallow seeds have a physical dormancy which inhibits imbibition of water. This dormancy is broken down by fluctuations in temperature over summer, however dormancy is maintained in a percentage of seeds for a long period of time. Seeds also have a physiological dormancy for a short period after maturity when they won’t germinate. Dormant seeds remain viable for many years if physical dormancy is retained and it is thought that the seed bank could persist for decades.

Seeds can pass through animals although testing in sheep showed that 82% of seeds were killed. There is low risk of spreading seed though livestock if they graze immature plants but there is potential for some spread of seed if mature plants that have set seed are grazed. Sheep ingesting a high proportion of marshmallow in their diet can become ill.

Implications for Management

Marshmallow is particularly difficult to control with herbicides when well established or when under stress, which is often the case during summer. One of the difficulties is that marshmallow plants have a natural resistance to glyphosate (Michael 2003).

In the cropping areas in Victoria, late summer and early autumn germinations of marshmallow cause the most issues for management. Sometimes late summer and early autumn knockdowns do not adequately control marshmallow because they do not contain appropriate products, or paddocks are not sprayed at all, resulting in there being very large marshmallow plants present by the time crops are to be sown. These plants can be difficult to control with the non selective herbicides applied prior to sowing and often reshoot and grow so that there are large marshmallow plants present in young crops. Late summer and early autumn germinating marshmallow are also problematic in chemical fallows as plants are typically very large and well established by the time the paddock is to be sprayed off for fallow in July or August.

Marshmallow plants germinating in crop have been less of a problem as seedling marshmallow is suppressed by a range of selective pre and post emergent herbicides. Marshmallow plants surviving in crop need to be controlled as they cause yield loss through competition with the crop if populations are high and they lead to contamination of harvested seed and the subsequent spread of marshmallow around the farm.

Control with Non Selective Herbicides

Control of marshmallow has been the focus of investigation in the southern and western cropping regions over the last decade. The following conclusions on control with non selective herbicides are drawn from a project conducted by John Stuchbery & Associates and funded by the GRDC titled ‘Summer Weed Control’ (2008), commercial experience and previous research. The supporting data is from the GRDC project conducted by John Stuchbery & Associates. In this work, various herbicide mixes were applied to marshmallow ranged in size from 10 to 50 cm in diameter but was mostly 35 cm in diameter and was stressed at the time of application.

Timing

Early control of actively growing marshmallow is essential.  This means targeting summer and autumn marshmallow plants while they are small. Commercially acceptable control of marshmallow greater than 10cm diameter is difficult, especially if plants are stressed.

Ammonium sulphate

Ammonium sulphate improves the level of control achieved by glyphosate mixes and should always be added to the herbicide mix regardless of water quality.

Glyphosate Rates

Glyphosate 450 rates should be at least 1.5L/ha for robust control of established marshmallow plants. Improvement in control at higher rates is marginal (Table 1).

Table 1: Effect of increasing glyphosate rate with Goal on large stressed marshmallow, Gooroc April 2007

¹ All treatments applied with 2 % liquid Ammonium Sulphate + 0.2 % LI700 as adjuvants unless otherwise stated.

² Approximate cost, GST inclusive May 2010 includes adjuvants with Ammonium sulphate in crystalline form.

³ EWRC Weed Control Scores: 1 = 100% weed control and 9 = no control, 4 = commercially acceptable.

Group G Spike

Addition of a Group G spike is essential. Hammer tends to have a faster brown out than Goal /Striker but the final level of control is often similar. This is demonstrated in the results listed in Table 2 where the levels of control achieved by the glyphosate Goal and glyphosate Hammer mixes were similar 41 days after the herbicides were applied. Hammer may have slight advantage when marshmallow plants are stressed and when grazing is required (Goal does not have a grazing withholding period) but plants treated with Hammer have a tendency to regrow when follow up rain occurs or soil moisture conditions are favourable.

Table 2: Comparison of Goal and Hammer, Gooroc April 2007, on large stressed marshmallow

1 All treatments applied with 2 % liquid Ammonium Sulphate + 0.2 % LI700 as adjuvants unless otherwise stated.

2 Approximate cost, GST inclusive May 2010 includes adjuvants with Ammonium sulphate in crystalline form.

3 EWRC Weed Control Scores: 1 = 100% weed control and 9 = no control, 4 = commercially acceptable.

Adding Phenoxy Herbicide

Inclusion of a phenoxy herbicide (2,4-D Ester or Surpass) improves the levels of control but plant back periods for pulses and canola need to be considered if these mixes are being used prior to cropping. Table 3 shows the significant improvement in marshmallow control when LV Ester 600 is added to glyphosate plus Group G mixes in a trial at Gooroc in 2007. Surpass is as effective as ester in mixes with glyphosate and Group G herbicides for marshmallow control (Table 4).

Table 3: The effect of adding a phenoxy spike to Goal or Hammer for the experimental control of marshmallow, Gooroc 2007

1 All treatments applied with 2 % liquid Ammonium Sulphate + 0.2 % LI700 as adjuvants unless otherwise stated.

2 Approximate cost, GST inclusive May 2010 includes adjuvants with Ammonium sulphate in crystalline form. LV Ester 600 is no longer on the market. Price adjusted for Estercide Xtra 680

3 EWRC Weed Control Scores: 1 = 100% weed control and 9 = no control, 4 = commercially acceptable.

Table 4: A comparison of phenoxy herbicide combination spikes applied for the experimental control of marshmallow, Gooroc 2007

1 All treatments applied with 2 % liquid Ammonium Sulphate + 0.2 % LI700 as adjuvants unless otherwise stated.

2 Approximate cost, GST inclusive May 2010 includes adjuvants with ammonium sulphate in crystalline form.           LV Ester 600 is no longer on the market. Price adjusted for Estercide Xtra 680

3 EWRC Weed Control Scores: 1 = 100% weed control and 9 = no control, 4 = commercially acceptable.

4 Indicates level of control (+,= or -) relative to a standard treatment (in bold) based on least significant difference.

Spray.Seed

Spray.Seed plus 2,4-D Ester can be effective on stressed marshmallow but there is a risk of regrowth if follow up rains occur. The level of control from the Spray.Seed LV Ester mix was not significantly different to that achieved by the standard glyphosate LV Ester Goal mix at Gooroc in 2007(Table 4) but is more expensive at current prices for glyphosate. Control of marshmallow with one application of Spray.Seed was worse than other treatments due to regrowth after the Spray.Seed was applied (Table 5).

24D Amine 625

24D Amine 625 1.5L/ha + 1% mineral oil shows promise for low cost marshmallow control and it was as effective as the glyphosate ester Goal mix at Gooroc in 2007 (Table 4) when applied to larged stressed marshmallow. Addition of mineral oil to 24D amine significantly improves control or marshmallow. This is a good option where the rest of the weed spectrum would be covered by this mix. Commercial experiences suggests that 24D amine is more effect on actively growing marshmallow than on stressed plants.

Double Knock

Spray.Seed applied as a second knock following glyphosate plus ester improves control of marshmallow (Table 5), however this is relatively costly. Commercial experience suggests that solid rates of paraquat have been effective on marshmallow when used to double knock ryegrass and could be substituted for Spray.Seed in these situations. The results in Table 5 indicate that there was no benefit in applying a Group G herbicide with mineral oil in a separate application following an application of glyphosate plus 2,4-D Ester. Control of marshmallow with a double knock with two applications of Spray.Seed was worse than other treatments due to regrowth where Spray.Seed was applied.

Table 5. EWRC scores for the experimental control of marshmallow using a “double knock” approach, Gooroc April 2007

1 Product costs GST inclusive May 2010, includes adjuvant with ammonium sulphate in crystalline form 3.

2Application cost estimated at $5/ha per application. 3 All treatments applied with 2 % liquid Ammonium Sulphate + 0.2 % LI700 as adjuvants unless otherwise stated. LV Ester 600 is no longer on the market. Price adjusted for Estercide Xtra 680 4 Indicates level of control (+,= or -) relative to a standard treatment (in bold) based on least significant difference.

Table 6: EWRC scores for the experimental control of marshmallow using different adjuvants added to as a glyphosate 450 1.5L/ha + LV Ester 600 0.4L/ha + Group G spike (Hammer or Goal), Gooroc April 2007

1 Approximate cost of adjuvant only, GST inclusive May 2010.

2 EWRC Weed Control Scores: 1 = 100% weed control and 9 = no control, 4 = commercially acceptable.

3 Indicates level of control (+,= or -) relative to a standard treatment (in bold) based on least significant difference.

Adjuvants

Limited testing of adjuvants suggested that mixes containing Goal achieved more rapid brown out when the adjuvant was Fulvic Acid or a vegetable oil derivative rather than Supercharge, a mineral oil based adjuvant (Table 6). Mixes containing Hammer achieved more rapid brown out when LI700 or Supercharge were the adjuvant compared with where vegetable oils were the adjuvant (Table 6). LI 700 was among the best adjuvants for mixes containing Goal or Hammer and is more cost effective than the other adjuvants tested. Use of Fulvic acid with mixtures containing Goal requires more work to evaluate its performance.

Cost Effectiveness

Glyphosate 450 1.5L/ha + LV Ester 600 400ml/ha + Goal 75ml/ha + 0.2% LI700 + ammonium sulphate is a useful broad spectrum lower cost mix for good suppression of 10-50cm diameter marshmallow and for good control of smaller marshmallow. Hammer has as an advantage over Goal where paddocks are to be grazed and may be an advantage where weeds are stressed.

Control with Selective Herbicides

Controlling marshmallow with selective herbicide is an important component of in crop management. There are typically two scenarios where marshmallow control is required. The first is where large plants that were present prior to sowing have survived non selective herbicide treatments and are present in the young crop and the second is where marshmallow emerges in the young crop.

Cereals Both scenarios are relatively easy to manage in cereals. Products which are registered in cereals and have activity on marshmallow include Affinity, Conclude, Midas (Clearfield wheat only), Precept, Tigrex, Broadstrike and 24D Amine. Affinity is very robust on large plants while rates of Tigrex 800 – 1000 ml/ha are also effective on large plants but plants take longer to die off. Both of these products can be used in relatively young crop. 24D Amine will control large plants that are actively growing but this treatment can not be used in young crop. The other products listed provide reasonable control of smaller, younger plants.

Triazine Tolerant Canola Triazine herbicides have good pre emergent activity on germinating marshmallow. Post emergent applications of atrazine plus oil have reasonable activity on emerged seedling marshmallow and provide some suppression of large marshmallow although this can be variable. Crop competition assists the suppression of larger plants where post emergent applications of atrazine are used but these plants can still set seed. Ideally, larger plants should be controlled prior to sowing.

Clearfield Canola On Duty and Intervix herbicides have good activity on seedling and larger marshmallow plants. Control of larger plants is improved in competitive crops.

Roundup Ready Canola Roundup Ready herbicide will provide reasonable control of seedling marshmallow but is similar to regular glyphosate on larger plants. Larger plants need to be controlled prior to sowing. Crop competition will help to suppress larger plants but they can still set seed.

Pulses Simazine, metribuzin and diuron have good pre emergent activity on marshmallow but have limited activity on emerged plants. It is essential that emerged plants are controlled with non selective herbicide prior to sowing. Broadstrike is registered for post emergent application in lentils, chickpeas, field peas and fenugreek and provides reasonable control of younger marshmallow but not older well established plants.

Conclusions

Poor control of summer and autumn germinating marshmallow lead to the presence of large plants prior to sowing or fallowing that are difficult to kill. Research and experience has shown that controlling young plants when growing conditions are favourable and selection of the appropriate non selective herbicide combinations can provide acceptable control which is cost effective. Marshmallow germinating in crop can be controlled but consideration needs to be given to crop type planned for known marshmallow paddocks and the control measures available for that crop. In crop marshmallow control needs to be given priority to prevent contamination of seed which may lead to downgrading and contribute to further spread of marshmallow around the farm.

References

John Stuchbery & Associates 2008 Summer Weed Control Final Report to Grains Research and Development Corporation

Michael P 2003 Small-flowered mallow in WA: What do we know? E Weed, Edition 7, 23 May, pp30-33

Michael Pippa J. Steadman Kathryn J. & Plummer Julie A. 2009 The biology of Australian Weeds 52. Malva parviflora L. Plant Protection Quarterly Vol 24 No 1 pp 2-9

Taylor S 2002 Marshmallow Control in reduced tillage systems Western Australia Crop Update February 21-22 2002

Contact

John Stuchbery

JSA Independent

PO Box 10 Donald Vic 3480,
jstuchb@bigpond.net.au  
P 03 54971956

M 0429 144475