Sowthistles (Sonchus L)
Sowthistles are annual, biennial or perennial herbs, with or without rhizomes. They are all in the genus Sonchus, and also known as Milkthistles.
The distribution map of Sowthistles in Australia is courtesy of The Atlas of Living Australia.
There are three Sowthistles you often find in lawns and turf areas in Australia. These are:
- Annual or Common Sowthistle (Sonchus oleraceus L).
- Perennial Sowthistle (Sonchus arvensis) and
- Spiny Sowthistle (Sonchus asper L.).
After you read this, you will be able to:
- Identify Sowthistles or Milkthistles.
- Know the habitat of Sowthistles or Milkthistles.
- Know the best cultural and chemical options to control Sowthistle or Milkthistle.
How to Identify Sowthistle.
All Sowthistles have soft, irregularly lobed leaves, that clasp the stem and form a basal rosette.The leaves are alternate, upright, hairless, and have hollow stems that sometimes branch towards the top.
Creeping Thistle is often mistaken for Prickly Sow Thistle.The easiest way to tell them apart is to cut the stem. Sowthistles exude a sap when cut, whilst thistles do not.
Annual Milkweed and Spiny Sowthistle are often mistaken for Prickly Lettuce, as they also have milky sap and similar foliage and flowers. However, Prickly lettuce leaves have raised spines along the midrib, that are not along the midribs of Spiny and Annual Sowthistle leaves.
Category: Dicot
Photosynthetic Pathway: C3 Weed.
Flower: The flower heads of Milkweeds or Sowthistles are yellow, and range in size from 12 to 25 mm in diameter. They have a ray type floret with no eye to the flower.
Height: Mature Sowthistle is 30 cm to 2 m tall.
Leaf length: The leaves are 6 to 35 cm long.
Leaf width:
Reproduction:
Sowthistles grow quickly in a wide range of conditions, and their wind-borne seeds allow them to spread rapidly. Germination is favoured by light, and seedling emergence is only in the top 1 cm of soil. The seed of Common Sowthistle is viable for up to 8 years.
Germination is good at day/night temperatures of 25 and 10°C respectively, and Annual Sowthistle needs wet soils for optimum germination.
- If you cut the Annual Sowthistle when it flowers it still produces viable seeds.
- Once Perennial Sowthistle establishes, it is difficult to control. This is because its perennial root system has the ability to spread vegetatively via small root fragments, and it is also reproduces via seed.
- Common Sowthistle produces between 4,700 and 8,000 seeds.
Comments: Sow Thistles are common hosts for aphids. Young plants are highly frost tolerant. Plants that emerge in the Autumn can over Winter as a rosette of leaves close to the ground, and withstand temperatures down to -18°C.
Habitat: A common weed of disturbed areas, gardens and waste places.
For more information on weeds check out our weed ID Chart.
Control of Sowthistle.
You can control Sowthistle or Milkweed by cultural and chemical means, but successful management of this weed is best if you adopt an integrated approach.
Cultural Control of Sowthistle:
- Maintain a dense turf cover as this limits the spread of this grass because it is a weak competitor. Milkthistles are easily uprooted by hand, and their soft stems present little resistance to slashing or mowing.
- Seed germination is favoured by light, and seedling emergence is limited to the top 1cm of soil.
- As they are in the rosette stage for quite some time, Sowthistles are susceptible to shade. So grow a thick and dense turf cover and you are half way there to managing this weed!
Chemical Control of Sowthistle.
Since the seed bank is relatively short-lived, pre emergents are a good option. Also post emergent herbicides work best when the weed is in the seedling stage.
Pre-Emergent Herbicides for Sowthistle.
Pre-emergent herbicides control Milkthistle, and the fact that this is a shallow germinating weed is the reason for this.
Currently only BASF Freehand is registered in turf grass as a pre-emergent for this weed. However, in agriculture prodiamine (Onset 10GR) , S-Metalochlor and Pendimethalin (Battalia 440) control Sowthistle.
If Milkthistle does break through your pre-emergent barrier, the next option is to use a selective post-emergent herbicide. Luckily there are a wide range of post emergent options to use.
Post Emergent Herbicides for Sowthistle.
Post-emergent Selective herbicide options include: 3-D herbicide, Dicamba 500, Qualipro Crest, Pylex, 2,4-D, and MCPA
Non Selective Control of Sowthistle.
You can use Glyphosate , Numchuk Quad or Cortex Duo as a non selective options to control this weed. If you use Glyphosate and water quality is an issue then we recommend the use of ProForce Manta Ray.
The following are non-selective but also have a long term residual and stop re-growth of Sowthistle.
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- Renegade. Renegade stops germination for up to 12 months, and reduces the need for multiple herbicide applications.
- Numchuk Quad. This gives effective post and pre emergent Sowthistle control for up to 12 months.
- Cortex Duo. Cortex Duo gives a rapid knockdown, and residual control for up to 3 months. It is also safe to use around trees.
